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Evolution
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Evolution and the Origin of Life
Controversies That Have Divided America
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© W.P. Armstrong 3 March 2012
Note: Paragraphs are numbered in small case
in order to communicate with reviewers.
1. Introduction
- One of the problems in understanding evolution is that it may be defined in different ways. According to the Random House Webster's College Dictionary
(1999), biological evolution is a change in the genetic makeup of a
population from generation to generation. It may also be defined as the
development of species or other groups of organisms (genera and
families) from earlier forms by natural selection. Microevolution
refers to changes at or below the species level, while macroevolution
generally refers to large-scale changes over a long period of time
resulting in new species. In this article I have attempted to clarify
and summarize some of the major concepts and terms used by evolution
scientists. Depending on the author, evolution may be referred to as a
theory, hypothesis, law, truth, and fact. In fact, these terms are
often used interchangeably and sometimes incorrectly, even by
prestigious biologists. In my opinion, evolution is a well-established
scientific theory supported by facts. For science teachers, I have
included an extensive bibliography and a crossword puzzle containing
many of the terms defined in this article.
- The
noun evolution is derived from the verb evolve, meaning to gradually
change with time. Evolution can be defined in several different ways.
For example, Wayne's Word has evolved from a humorous, natural
history newsletter sent out to friends, to a large on-line, hyperlinked,
peer-reviewed textbook of natural history with millions of hits per
month. Another definition is organic (biological) evolution that
incorporates numerous facts and well-substantiated, tested hypotheses.
It is usually associated with natural selection based on the original
published work of Charles Darwin and Alfred Russel Wallace.
2. Is Evolution a Theory or a Fact?
Four Important Definitions Used In The Teaching Of Science:
Working Group on Teaching Evolution, National Academy of Sciences (1998)
- Fact: In science, an observation that has been repeatedly confirmed.
- Law: A descriptive generalization about how some
aspect of the natural world behaves under stated circumstances. Laws
can be very useful in supporting hypotheses and theories, but like all
elements of science they can be altered with new information and
observations.
- Hypothesis: A testable statement about the natural
world that can be used to build more complex inferences and
explanations, such as a scientific theory.
- Theory: In science, a well-substantiated
explanation of some aspect of the natural world that can incorporate
facts, laws, inferences, and tested hypotheses.
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It Is Obvious From The Following Quotations Regarding Evolution
That The Terms Theory & Fact Are Used In Several Different Ways
- In November 2004, suburban Atlanta biology textbooks included a warning sticker that said:
"This textbook contains material on evolution. Evolution is a theory,
not a fact, regarding the origin of living things. This material should
be approached with an open mind, studied carefully and critically
considered."
- On
November 19, 2004, the Dover School Board of Pennsylvania passed a
resolution requiring teachers to read a statement to students in 9th
grade science classes endorsing intelligent design as an alternative
explanation for the origin of life.
"The Pennsylvania Academic Standards require students to learn about
Darwin's Theory of Evolution and eventually to take a standardized test
of which evolution is a part. Because Darwin's Theory is a theory, it
continues to be tested as new evidence is discovered. The Theory is not
a fact. Gaps in the Theory exist for which there is no evidence. A
theory is defined as a well-tested explanation that unifies a broad
range of observations. Intelligent Design is an explanation of the
origin of life that differs from Darwin's view. The reference book, Of Pandas and People,
is available for students who might be interested in gaining an
understanding of what Intelligent Design actually involves. With
respect to any theory, students are encouraged to keep an open mind.
The school leaves the discussion of the Origins of Life to individual
students and their families. As a Standards-driven district, class
instruction focuses upon preparing students to achieve proficiency on
Standards-based assessments."
- The
resolution was challenged a year later in the United States federal
courts: Case No. 04cv2688, "Tammy Kitzmiller, et al v Dover Area School
District." The plaintiffs successfully argued that intelligent design
is a form of creationism, and that the school board policy thus violated
the Establishment Clause of the First Amendment to the United States
Constitution.
- The
argument whether evolution is a theory or a fact is an invalid
comparison. Evolution is a scientific theory that has explained the
factual evidence of scientists for more than a century. All
interpretations of facts in science are provisional and subject to
challenge. Under a strict definition, a scientific theory should not be
called a "fact" even though it explains all known facts and has
survived the test of time. There is always the possibility that a
scientific theory will be updated or changed as new evidence is
discovered. The theory of evolution assumes the existence of life and
is directed to an explanation of how life evolved. It does not deal
with the origin of life, and it does not presuppose the absence of a
creator or God. Although the origin of life is often included in
debates about evolution, it is a very different topic that does not have
all the empirical evidence of biological evolution. Scientific
explanations for the origin of life are more properly referred to as
hypotheses rather than scientific theories. Intelligent design is a
non-scientific argument or assertion that life "owes its origin to a
master intellect." Please refer to Origin of Life later in this this discussion.
- According to E.C. Scott (Evolution vs. Creationism,
Univ. of Calif. Press, 2004), most people consider facts more important
than hypotheses, theories and laws. In fact, they rank these terms in
order of importance as follows: Facts-Laws-Theories-Hypotheses.
Scientists rank these terms in the following order from most important
to least important: Theories-Laws-Hypotheses-Facts. Our interpretations
of facts are certainly not set in stone and are often changed in
science. Consider the number of chromosomes in a human somatic cell.
In the 1950s the number was determined to be 48. This fact was
published in all biology textbooks. With better techniques in staining
chromosomes, the number was later revised to 46, 23 from each parent.
Disagreement on chromosome numbers still occurs to this day,
particularly with plant species containing numerous chromosomes that
often overlap each other making accurate counts very difficult. For as
long as I can remember, Pluto has been listed as the ninth planet in our
solar system. Recent studies of its orbital patterns and other factors
indicate that Pluto is not a true planet compared with Mercury, Venus,
Earth, Mars, Jupiter, Saturn, Uranus and Neptune.
- The following image shows an example of a disputable interpretation of facts: The number of chromosomes in a species of Brodiaea
from coastal southern California. The stained microscope slide is my
factual evidence for the chromosome number of this species. My original
count was 36; however, another botanist prepared his own microscope
slides (factual evidence) and came up with 40+ chromosomes. In my
original microscope slide, there may have been some smaller chromosomes
that were obscurred by larger ones. Therefore, my orginal count of 36
may have been too low. My factual evidence (the prepared microscope
slide and photo image) have not changed, but my interpretation of the
factual evidence probably needs to be changed.
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Microsporogenesis in the San Marcos Coastal "BTK" showing the first
division of a pollen mother cell (microsporocyte). Cytoplasmic division
(cytokinesis) has not occurred yet. The two chromosome clusters (2
sets of chromosome doublets) contain at least 36 chromosomes, probably
more depending on how you count overlapping chromosomes. There are very
small chromosomes that are possibly obscured by the larger ones. [500
x]
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- Another
example of how our interpretation of tangible, factual evidence has
changed is the fossil remains of a petrified fig syconium named Ficus ceratops.
The 70 million-year-old Hell Creek Formation in the Montana badlands
is rich in fossil fig syconia and dinosaur fossils, including Tyrannosaurus rex, Triceratops and the amazing duck-billed Hadrosaurus. In fact, Ficus ceratops is named after the dinoaur Triceratops.
Recent excavations in T-rex fossil beds of Canada revealed that the
so-called fig syconia were actually covered with spines. Our
interpretation of these fossilized fruits has changed: It is now
thought that they are from of an extinct palm named Spinifructus antiquus. See Section 10 (K-T Boundary & Demise Of Dinosaurs).
- There
is compelling evidence to show that humans once had 24 pairs of
chromosomes (2n = 48) like present-day great apes (orangutans, gorillas
and chimpanzees). Like all other 23 chromosomes, chromosome #2 has a
terminal genetic marker called a telomere. Telomeres shorten prior to
chromosome replication and cell division. Shortened telomeres
eventually signal the cell to stop dividing. In cancer cells, an enzyme
called telomerase keeps telomeres at a constant length so that cancer
cells keep on dividing over and over again. Chromosome #2 has a second
telomere in the middle region, indicating that two chromosomes joined
together end-to-end and became permanently fused into a single
chromosome. The sister chromatids of a chromosome doublet are attached
in a constricted region of the chromosome called the centromere. Each
chromosome has a single centromere; however, chromosome #2 has two
centromeres, further evidence that it represents two fused chromosome
doublets, each with its own centromere. For more information about
telomeres, go to Section 5: Telomeres & The End Replication Problem.
- Some biologists say that evolution is a scientific theory and
a fact. According to T. Ryan Gregory of the Department of Integrative
Biology, University of Guelph, Ontario, Canada (2007), the notion that
species may change through time and that living organisms are related to
one another through common descent is a fact. He also states that
evolution is a well-established scientific theory. "That evolution is a
theory in the proper scientific sense means that there is both a fact
of evolution to be explained and a well-supported mechanistic framework
to account for it. To claim that evolution is "just a theory" is to
reveal both a profound ignorance of modern biological knowledge and a
deep misunderstanding of the basic nature of science."
- The eminent evolutionary biologist Ernst Mayr also considers evolution to be a fact. In his book What Evolution Is (2001), he gives the following explanation:
Is Evolution A Fact?
"Evolution is not merely an idea,
a theory, or a concept, but is the name of a process in nature, the
occurrence of which can be documented by mountains of evidence that
nobody has been able to refute. Some of this evidence was summarized in
Chapters 1-3. It is now actually misleading to refer to evolution as a
theory, considering the massive evidence that has been discovered over
the last 140 years documenting its existence. Evolution is no longer a
theory, it is simply a fact."
- The
Working Group on Teaching Evolution, National Academy of Sciences
(1998) defines a scientific theory as a well-substantiated explanation
of some aspect of the natural world than can incorporate facts, laws,
inferences, and tested hypotheses. Like "theory," the word "fact" has a
different meaning in science than it does in common usage. A
scientific fact is an observation that has been confirmed over and over.
However, observations are gathered by our senses, which can never be
trusted entirely. Observations also can change with better technologies
or with better ways of looking at data. Please refer to paragraph 6
which discusses the erroneous human chromosome number of 48 and the
planet Pluto. "Ironically, facts in science often are more susceptible
to change than theories--which is one reason why the word "fact" is not
much used in science."
- Eugenie C. Scott of the National Center For Science Education defines "facts" of evolution in the following paragraph:
"From the standpoint of
philosophy of science, the "facts of evolution" are things like the
anatomical structural homologies such as the tetrapod forelimb, or the
biochemical homologies of cross species protein and DNA comparisons, or
the biogeographical distributions of plants and animals. The "facts of
evolution" are observations, confirmed over and over, such as the
presence and/or absence of particular fossils in particular strata of
the geologic column (one never finds mammals in the Devonian, for
example). From these confirmed observations we develop an explanation,
an inference, that what explains all of these facts is that species have
had histories, and that descent with modification has taken place.
Evolution is thus a theory, and one of the most powerful theories in
science."
- Richard Dawkins presents his case for why evolution should be called a fact rather than a theory in his book The Greatest Show on Earth: The Evidence For Evolution
(2009). He refers to two kinds of theories: Well established
scientific theories and laymen theories or tentative hypotheses. In
fact, Dawkins compares a scientific theory with the mathematician's
theorem, and proposes that scientific theory be replaced with the word
"theorum." Two words with the same pronunciation and with different
meanings and slightly different spellings will require even more
clarification by writers and lecturers. In my opinion, capitalizing the
word Theory for scientific theory would serve the same purpose, or better yet, just say scientific theory.
- The following paragraph comes from Chapter 2 of Anarchy Evolution by Greg Graffin and Steve Olson (2010):
"Scientists believe that they can come closer and closer to something
that can be described as "the truth" through observation,
experimentation, and verification. They may never know if they have
achieved the absolute truth--to the extent that such a thing can be
defined. But if a statement has been tested so many times that there
are no longer any resaonable grounds to suspect that further testing
will reveal a discrepancy, scientists no longer refer to that statement
as a theory or hypothesis. They call it a fact."
- The following definition of a fact comes from the National Academy of Sciences "Definitions of Evolutionary Terms" (accessed January 2011):
"In science, a "fact" typically refers to an observation, measurement,
or other form of evidence than can be expected to occur the same way
under similar circumstances. However, scientists also use the term
"fact" to refer to a scientific explanation that has been tested and
confirmed so many times that there is no longer a compelling reason to
keep testing it or looking for additional examples."
Conclusion: Is Evolution Is A Theory Or A Fact?
- All
of these discussions of whether evolution is a theory or a fact depend
on how the terms "theory" and "fact" are defined. The "fact" that
living organisms change or "evolve" with time is undeniable. In this
sense evolution is certainly a fact. The complex mechanisms of
evolution are best explained as a well-established scientific theory
based on numerous facts. The position of Wayne's Word on teaching
evolution is summarized by the Steering Committee on Science and
Creationism, National Academy of Sciences (1999):
"The contention that evolution should be taught as a "theory, not as a
fact" confuses the common use of these words with the scientific use.
In science, theories do not turn into facts through the accumulation of
evidence. Rather, theories are the end points of science. They are
understandings that develop from extensive observation, experimentation,
and creative reflection. They incorporate a large body of scientific
facts, laws, tested hypotheses, and logical inferences. In a sense,
evolution is one of the strongest and most useful scientific theories we
have."
- The Merriam-Webster Unabridged Third International Dictionary of the English Lanuage
(1981) gives several definitions for the noun "fact:" (1) Something
that has actual (tangible) existence. (2) An assertion, or statement
containing something having objective reality. (3) Something proved by
the evidence to be or alleged to be of actual occurrence. (4) A verified
statement or proposition. (5) Something that makes a statement or a
proposition true or false. (6) The reality of events or things, the
occurrence or existence of which is to be determined by the evidence.
The evolution of life on earth is certainly proven by factual evidence
to be true; therefore, evolution can be called a fact under the
Merriam-Webster definition.
3. Evidence For Genetic Variability In Populations
- In
order to have changes in populations of organisms it is necessary to
have a source for the genetic variability. There are at least five
sources of genetic variability in populations: (1) DNA mutations or
changes in existing genes and the formation of new genes. (2)
Reshuffling of chromosomes and genes during meiosis and sexual
reproduction, including crossing over and transposons. (3) Natural
selection for favorable traits, and selection against undesirable genes.
(4) Interbreeding between genetically different populations, including
emigration and immigration. (5) Genetic drift in relatively small,
isolated populations.
Genetic Variability & Neutral Theory Of Molecular Evolution
- The
neutral theory of molecular evolution was proposed by the famous
Japanese geneticist Motoo Kimura in the late 1960s and early 1970s. It
basically states that the vast majority of evolutionary variations occur
at the molecular level and are caused by random genetic drift of
selectively neutral mutations not affecting fitness (Kimura, 1991). In
other words, most of the genetic variation in populations is the result
of DNA mutations and genetic drift and not selection. The theory
suggests that if a population carries several different versions of
genes, each of these versions is equally good at perforning its job and
variation is neutral: Whether you carry gene version A or gene version B
does not affect your fitness. When we see several versions of genes in
a population, it is likely that their frequencies are simply drifting
around. There are many examples of different versions of genes
(alleles) in a population. For example, in the A-B-O blood types and Rh
factor in humans there are several forms of genes that occur on
homologous chromosomes. Some advocates of the neutral theory use human
blood types as examples of the neutral theory; however, this theory is
easily misinterpreted. These blood traits occur in chimpanzees and are
very old. How can we be sure that they didn't convey some selective
advantage in ancestral populations.
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Left: Multiple allele inheritance. The diagram shows one pair of
homologous chromosomes, each with a single locus. Only one allele can
occur at each locus, but there are 4 possible alleles per locus. Since
the A1, A2, B and O alleles are located on one pair of loci on
homologous chromosome pair #9, the following 10 genotypes are possible:
A1A1, A1A2, A2A2, A1O, A2O, BB, BO, A1B, A2B and OO.
Right: Polygenic Inheritance. The Rh factor is an interesting example
of polygenic inheritance. Unlike the A-B-O blood types where all the
alleles occur on one pair of loci on chromosome pair #9, the Rh factor
involves three different pairs of alleles located on three different
loci on chromosome pair #1. In the following diagram, 3 pairs of Rh
alleles (C & c, D & d, E & e) occur at 3 different loci on
homologous chromosome pair #1. Possible genotypes will have one C or c,
one D or d, and one E or e from each chromosome. For example:
CDE/cde; CdE/cDe; cde/cde; CDe/CdE; etc.
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- In
order to determine how many different genotypes are possible, you must
first determine how many different gametes are possible for each parent,
then match all the gametes in a genetic checkerboard. Although the
three pairs of genes are linked to one homologous pair of chromosomes,
there are a total of eight different possible gametes for each parent:
CDE, CDe, CdE, Cde, cDE, cDe, cdE, and cde. This number of gametes is
based on all the total possible ways these genes can be inherited on
each chromosome of homologous pair #1. Since all three genes are
closely linked together on the same chromosome, each gamete must contain
one C (C or c), one D (D or d) and one E (E or e). The possible
different genotypes are shown in the following table:
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Gametes
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CDE
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CDe
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CdE
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Cde
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cDE
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cDe
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cdE
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cde
|
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CDE
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CDE/
CDE
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CDE/
CDe
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CDE/
CdE
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CDE/
Cde
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CDE/
cDE
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CDE/
cDe
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CDE/
cdE
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CDE/
cde
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CDe
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CDe/
CDE
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CDe/
CDe
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CDe/
CdE
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CDe/
Cde
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CDe/
cDE
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CDe/
cDe
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CDe/
cdE
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CDe/
cde
|
|
CdE
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CdE/
CDE
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CdE/
CDe
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CdE/
CdE
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CdE/
Cde
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CdE/
cDE
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CdE/
cDe
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CdE/
cdE
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CdE/
cde
|
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Cde
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Cde/
CDE
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Cde/
CDe
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Cde/
CdE
|
Cde/
Cde
|
Cde/
cDE
|
Cde/
cDe
|
Cde/
cdE
|
Cde/
cde
|
|
cDE
|
cDE/
CDE
|
cDE/
CDe
|
cDE/
CdE
|
cDE/
Cde
|
cDE/
cDE
|
cDE/
cDe
|
cDE/
cdE
|
cDE/
cde
|
|
cDe
|
cDe/
CDE
|
cDe/
CDe
|
cDe/
CdE
|
cDe/
Cde
|
cDe/
cDE
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cDe/
cDe
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cDe/
cdE
|
cDe/
cde
|
|
cdE
|
cdE/
CDE
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cdE/
CDe
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cdE/
CdE
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cdE/
Cde
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cdE/
cDE
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cdE/
cDe
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cdE/
cdE
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cdE/
cde
|
|
cde
|
cde/
CDE
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cde/
CDe
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cde/
CdE
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cde/
Cde
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cde/
cDE
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cde/
cDe
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cde/
cdE
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cde/
cde
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- You
can also plug into this neat little formula for calculating the number
of different genotypes based on the number of alleles per locus and the
number of loci per chromosome. The formula was actually devised by
several of my general biology students. It may occur somewhere in a
textbook, but the students came up with it independently.
- There
is ample evidence from field observations and DNA studies in
laboratories to show that genetic variation occurs within populations of
plants and animals. These are undeniable facts. The degree of
variability is reflected in different levels of organization (taxonomic
hierarchies), such as families, genera and species. DNA variations can
be induced in the laboratory using mutagenic agents, such as strong
chemical oxidizing agents and high energy radiation. Genetic variation
can be readily observed in pathogenic bacteria that defy medical science
by rapidly changing into new forms that are resistant to the latest
antibiotics. HIV disguises itself from our highly evolved immune system
and even acquires resistance to antiviral drugs, such as AZT. New
strains of influenza viruses develop each year In fact, our war on
pathogenic microbes has accelerated the process of microevolution in
these organisms. Genetic variability is the raw material for this
evolution. A thorough understanding of the genetics and evolution of
pathogenic microorganisms may be crucial to the survival of the human
race.
- DNA
sequences coding for essential components of cellular metabolism, such
as ribosomal subunits, are highly conserved, varying little between
major groups of organisms. Highly conserved genes are very stable
because changes in their DNA (mutations) are usually detrimental. Genes
that are not highly conserved are subject to mutations and are
important factors in natural selection and the evolution of new species.
Some isolated species populations have unique traits correlated with
geographic distribution. These populations are called subspecies, and
their unique genetic changes have occurred through long periods of
isolation. Varieties are similar to subspecies, but are not necessarily
geographically isolated. Sometimes variation within a species complex
makes identification very difficult. This is particularly true when the
variable traits are apparently not under selection pressure and cannot
be correlated with geographic distribution. One case in point is the
California species known as BTK (Brodiaea terrestris ssp. kernensis), native to grasslands from Kern and Santa Barbara counties to the Mexican border. Most Brodiaea
species have three sterile stamens called staminodes. Variants of BTK
have staminodes that are hooded (curved inwardly at apex), staminodes
that are flattened and strap-shaped or inrolled along upper margins, and
narrow staminodes that are tapered toward the apex. In addition, the
staminodes may be erect, leaning slightly outward or leaning inward.
Statistical data from Principal Components Analysis (PCA) thus far
indicates that BTK is one variable species complex; however, ongoing
cladistical studies using chloroplast DNA may prove that some of these
variants warrant segregation into species and subspecies or varieties.
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Staminode variation in BTK (Brodiaea terrestris ssp. kernensis):
A. Otay Mtn (San Diego County), B. San Marcos (San Diego County), C.
Sierra Nevada (Kern County), D. Gaviota Pass (Santa Barbara County), E.
Santa Rosa Plateau (Riverside County).
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- The
primary explanation for the astonishing variety of life on earth is the
amazing genetic molecule called DNA (deoxyribonucleic acid). DNA has
been metaphorically described as a long, twisted ladder with literally
millions of rungs composed of the base pairs: adenine (A) with thymine
(T) and guanine (G) with cytosine(C). Each rung has 4 possible
arrangements: A-T, T-A, C-G, and G-C. A DNA molecule with five billion
base pairs has 45,000,000,000 or 103,000,000,000
different possible base sequences. This astronomical number has three
billion digits and would fill about one million pages on a computer
printout (12 cpi). This incredible number helps to explain the enormous
diversity of life forms, from viruses and bacteria to complex plants
and people, all genetically programmed by DNA. Flowering plants alone
range from tiny wolffias less than one millimeter long to huge
eucalyptus trees over 100 meters tall. The potential variability and
mutability of DNA make it a perfect mechanism for evolution.
- DNA
bases are to proteins as letters are to words. Different
rearrangements and sequences result in numerous different proteins and
words. Even a mutation in the DNA involving only one misplaced base
(point mutation) may have a significant effect on the organism,
particularly if this gene results in a crucial protein, such as a vital
enzyme. A good analogy of a point mutation is a missplaced letter in a
word. Like a point mutation in DNA, a one-letter substitution in a word
can sometimes completely change the meaning of the sentence. This is
especially true when speaking or writing in another language. In a
store in Costa Rica I discovered that I had no Costa Rican currency
called colones, only U.S. dollars. In my meager attempt to communicate with the clerk in Spanish, I told her that I had no "cojones,"
inadvertently substituting a "j" for an "l." When she replied: "I am
sorry for you," I knew that my sentence was corrupted. In a vulgar
translation, I essentially told the woman that I had no testicles! In
this case, a one letter substitution (mutation) completely changed the
meaning of the sentence.
Table of DNA Base Triplets That Code For Specific Amino Acids
|
Amino Acid
|
DNA Base Triplets
|
M-RNA Codons
|
T-RNA Anticodons
|
|
alanine
|
CGA, CGG, CGT, CGC
|
GCU, GCC, GCA, GCG
|
CGA, CGG, CGU, CGC
|
|
arginine
|
GCA, GCG, GCT, GCC
TCT, TCC
|
CGU, CGC, CGA, CGG
AGA, AGG
|
GCA, GCG, GCU, GCC
UCU, UCC
|
|
asparagine
|
TTA, TTG
|
AAU, AAC
|
UUA, UUG
|
|
aspartate
|
CTA, CTG
|
GAU, GAC
|
CUA, CUG
|
|
cysteine
|
ACA, ACG
|
UGU, UGC
|
ACA, ACG
|
|
glutamate
|
CTT, CTC
|
GAA, GAG
|
CUU, CUC
|
|
glutamine
|
GTT, GTC
|
CAA, CAG
|
GUU, GUC
|
|
glycine
|
CCA, CCG, CCT, CCC
|
GGU, GGC, GGA, GGG
|
CCA, CCG, CCU, CCC
|
|
histidine
|
GTA, GTG
|
CAU, CAC
|
GUA, GUG
|
|
isoleucine
|
TAA, TAG, TAT
|
AUU, AUC, AUA
|
UAA, UAG, UAU
|
|
leucine
|
AAT, AAC, GAA, GAG
GAT, GAC
|
UUA, UUG, CUU, CUC
CUA, CUG
|
AAU, AAC, GAA, GAG
GAU, GAC
|
|
lysine
|
TTT, TTC
|
AAA, AAG
|
UUU, UUC
|
|
methionine
|
TAC
|
AUG
|
UAC
|
|
phenylalanine
|
AAA, AAG
|
UUU, UUC
|
AAA, AAG
|
|
proline
|
GGA, GGG, GGT, GGC
|
CCU, CCC, CCA, CCG
|
GGA, GGG, GGU, GGC
|
|
serine
|
AGA, AGG, AGT, AGC
TCA, TCG
|
UCU, UCC, UCA, UCG
AGU, AGC
|
AGA, AGG, AGU, AGC
UCA, UCG
|
|
stop
|
ATG, ATT, ACT
|
UAA, UAG, UGA
|
AUG, AUU, ACU
|
|
threonine
|
TGA, TGG, TGT, TGC
|
ACU, ACC, ACA, ACG
|
UGA, UGG, UGU, UGC
|
|
tryptophan
|
ACC
|
UGG
|
ACC
|
|
tyrosine
|
ATA, ATG
|
UAU, UAC
|
AUA, AUG
|
|
valine
|
CAA, CAG, CAT, CAC
|
GUU, GUC, GUA, GUG
|
CAA, CAG, CAU, CAC
|
4. Origin Of Asexual Species: Populations With No Males
- The
adaptive advantage of genetic variability through meosis and sexual
reproduction has been well demonstrated, but what about the evolution of
species without meiosis or sex? One of these remarkable cases is
Fuller's rose weevil (Naupactus cervinus), a small flightless,
broad-nosed weevil introduced into California from South America in the
late 1800s. This beetle feeds on many cultivated plants and is
especially troublesome in citrus groves where the adults eat new growth
on young trees. The larvae feed on roots and make furrows in the bark.
Eggs are laid on citrus fruits under the green calyx, and are
transmitted during the shipment of infected fruits. Males have never
been found in this species, so the females must produce viable eggs
without fertilization, a phenomenon known as parthenogenesis. Each
generation is composed of only females (thelytokous) that come from the
same mother. According to Benjamin Normark (1996), parthenogenetic
weevils are apomictic; meiosis does not occur and all female offspring
are genetically identical to their mothers, except for mutations.
Traditionally, hybridization and polyploidy were the main explanations
for the origin of asexuality in weevils; however, Marcela Rodriguero, et
al. (2010) suggests another possible explanation: the parthenogenesis
inductor bacterium Wolbachia pipientis. The endosymbiont
bacterial genome can produce drastic consequences on the evolution of
its host species, such as extinction or sex role reversal. Assuming
this species is haploid with one set of chromosomes, it certainly would
not have the genetic variability and selection advantage of diploid
populations with sexual repruduction. Nontheless, Fuller's rose weevil
is a very successful insect. All mature adults can readily lay eggs,
resulting in a serious agricultural pest in California.
- In
a PBS TV broadcast about honey bees, the narrator referred to drone
bees as "clones" of each other. Since clones are usually defined as
genetically identical individuals (usually derived asexually), Wayne's
Word strongly disagrees with the accuracy of this statement. Although
the haploid drone comes from an unfertilized egg with only one set of
maternal chromosomes, they are certainly not all genetically identical.
The diploid queen bee undergoes normal meiosis (oögenesis) producing
haploid eggs. During this meiotic process her 16 pairs of homologous
chromosomes become altered by crossing over and reshuffled through
random assortment, resulting in haploid eggs that are not chromosomally
identical. In fact, with 16 pairs of homologous chromosomes, there are 216
or 65,536 different chromosomal combinations possible. Haploid drone
bees produce haploid sperm in their testes through a mitotic
spermatogenesis without a meiotic reduction division.
- Drone
honey bee larvae develop within larger cells of the hive in order to
accommodate the larger adult male. Worker bees develop in smaller cells
from fertilized eggs and are essentially sterile females. The size of
the cells is determined by workers who construct and mold the hexagonal
wax cells with their mandibles; however, honeycomb cell size in sex
determination is contradicted by K. Sasaki and Y. Obara (Zoological Science
Volume 16, 1999) who reported fertilized eggs laid by queens in open
areas outside honeycomb cells. Certain diploid worker larvae develop
into fertile queens in much larger cells only if they are fed a special
hormonal-nutrient mixture known as "royal jelly." The additional random
combination of gametes during fertilization insures that worker bees
are more genetically diverse than drones. With 216 or 65,536 different chromosomal combinations in the gametes, there are (216)2
or 4,294,967,296 different ways for the egg and sperm to combine.
Since drone bees possess only maternal genes, a sister worker bee cannot
share any paternal genes with her brother. Worker bees can inherit
paternal genes from one set of drone chromosomes and maternal genes from
two sets of queen chromosomes. Since the available gene pool is much
greater for worker bees compared with drones, their genetic variability
is greater. Therefore, the female workers contribute more to the
Darwinian fitness of the species through natural selection. Although
Darwin did not know about the genetics of honey bees, he was fascinated
by their honeycomb cell-making instinct and devoted a dozen pages to the
subject in Chapter 7 of The Origin of Species.
|
Worker female honey bees (Apis mellifera) on their wax honeycomb.
The honeycomb is composed of two layers of hexagonal cells. One layer
of cells can be accessed from the front side, and another layer can be
accessed from the back. This ingenious construction of the two layers
of cells provides for the maximum utilization of space. The cells are
used to store honey and larvae. Larger cells are constructed by the
worker bees to accommodate the male drones which develop from
unfertilized eggs. Extra large cells are used for larvae of fertilized
eggs which are fed "royal jelly." These special females develop into
sexually mature queens.
|
- Entomologist
Robert Page and his research team at the University of California,
Davis have discovered the sex-determining "gender gene" (csd gene) in
honeybees. Haploid drones have only one set of this gene per cell,
while females have two sets. There are at least a dozen different forms
of this gene (e.g. cds1, cds2, cds3, etc). Queen bees mate with many
males, perhaps to insure that they get a good mix of different forms of
the csd genes. If a queen mates with a male carrying her identical
version of the csd gene, half of her fertilized eggs will develop into
phenotypic sterile males, even though they are genetically diploid
females with two sets of the same csd gene. These larvae are destroyed
by worker bees. Apparently, two sets of identical csd genes will not
function together.
|
Queen (cds1/cds2) X Drone (cds1)
|
|
50% cds1/cds2
|
50% cds1/cds1
(destroyed by workers)
|
|
Fire ants that are currently invading the southern United States also
have a similar sex-determining (csd) gene. They suffer from high levels
of sterility, probably because they brought only a few versions of csd
genes with them from South America. For beekeepers, inbreeding
honeybees is a method of selecting docile insects that produce copious
honey and reproduce rapidly; however, inbreeding also results in a high
percentage of sterile males that are destroyed by workers. Pinpointing
the precise "gender gene" in the sperm of males will enable beekeepers
to produce disease-resistant, female honeybees containing different
combinations of csd genes.
|
- In
addition to male bees and wasps (order Hymenoptera) and short-nosed
weevils (tribe Naupactini), there are other examples of parthenogenetic
animals with no males. The virgin whiptail lizard (Cnemidophorus neomexicanus)
of the western United States has only females in its population. One
female mounts and clasps another female, presumably to induce ovulation.
Because the genetic information has already been recombined in
meiosis, the offspring are not identical clones of each other. Many
species in the large insect order Homoptera (aphids & scale insects)
also exhibit parthenogenesis with only females in their populations.
5. Gene Duplication: The Formation of New Genes
- In
the previous section, mutations were listed as a source of genetic
variability by the formation of new genes. Gene duplication plays such
an important role in evolution that I have included a separate section
about it. Gene duplication may involve single genes, chromosomes or the
entire genome. It may be caused by translocations on the same
chromosome, or by an error in the pairing of homologous chromosomes
during meiosis where sister chromatids are out of alignment. Unequal
crossing over can result is cells receiving extra genes and cells with
deleted genes. Gene duplication in the laboratory is accomplished by
the polymerase chain reaction (PCR). Using the PCR technique a single
gene can be amplified or cloned into millions of duplicate copies.
Plants are the most prolific gene duplicaters because they commonly form
polyploids with multiple sets of the original base number of
chromosomes. Most of our fruits and vegetables are polyploids, not to
mention numerous wildflower species throughout the state of California.
|
Simplified explanation for chromosome duplication during mitosis of a
typical plant cell. In normal mitosis, the chromosome doublets separate
during anaphase so that each of the daughter cells (far right) receives
the exact same chromosome number as the original mother cell, i.e. four
single chromosomes: two red and two blue. In this case, the anaphase
cell divided into two daughter cells after the chromosome doublets had
already separated into single chromosomes (i.e. chromatids separated).
In other words, the spindle dissolved, leaving the cell with eight
single chromosomes. These became doublets and the cell divided into two
daughter cells each containing eight single chromosomes: four red and
four blue. This phenomenon can be induced by the alkaloid colchicine.
|
- According
to Soltis, et al. (2003), old genomes that are polyploid with respect
to the base number and amount of genetic material, may function as
diploids with respect to the level of gene expression and chromosomal
characteristics. These "old polyploids" may have become "diploidized"
by the loss, mutation or suppression of duplicate genes. Other causes
for diploidization may include genomic rearrangements and transposons.
This can drastically change the chromosome properties of a species. For
example, an odd polyploid with a base number of six might have a
sporophyte number of 5n = 30. This pentaploid would tend to be sterile
because of an odd chromosome set at synapsis of prophase I. However, if
this plant behaves as a diploid with 2n = 30, it would be fertile with
two sets of 15 chromosomes during meiosis. It would simply have a
diploid (sporophyte) number of 2n = 30 and a haploid (gametophyte)
number of n = 15. Diploidization has apparently occurred in the
California genus Brodiaea. In fact, populations of B. terrestris ssp. kernensis in Kern County have a diploid chromosome number of 48, higher than a human somatic cell.
- Athough
polyploid plants often have more chromosomes than people, the
morphological complexity of an organism is certainly not reflected by
the number of genes or DNA base pairs. For example, according to Gerald
Tuskan of Oak Ridge National Laboratory, Tennessee (2006), the black
cottonwood (Populus trichocarpa) genome has now been determined.
His team identified more than 45,000 putative protein-coding genes.
The Human Genome Project is a worldwide endeavor to map the DNA base
sequence of every gene in the human genome. The total number of
functional genes is considerably less than expected, about 30,000 genes
per cell compared with previous estimates of 100,000 genes per cell. It
has been estimated that a human somatic cell contains about 5 billion
base pairs. If the average gene contains 1500 bases, then 30,000
functional genes is only about one percent of the total DNA per cell.
It seems that some of this extra DNA came from gene duplication.
- The
two genes that exist after gene duplication are called paralogs and
usually code for proteins with a different function and/or structure.
The second copy of the gene may be under less selective pressure because
mutations of it have no deleterious effects on the host organism.
Therefore, it mutates faster than a functional single-copy gene over
generations of organisms. It should be noted here that gene duplication
may not be passed on if it occurs in somatic cells. In addition, the
duplication of oncogenes are known to be a common cause of certain types
of cancer. The overall advantage of gene duplication is greatly
increased genetic variability, the raw material for evolution. One of
the truly remarkable examples of gene duplication is in the antibody
mediated immune system of animals. How can we produce a seemingly
endless array of germ-fighting antibody proteins in response to an
infection?
|
Immune (IgG) antibody model composed of four polypeptides: Two heavy (H)
chains (longer green), and two light (L) chains (shorter blue). The
two combining sites where the antibody "arms" attach to antigens are
shown in red. Using this model, a separate gene for every antibody
protein is not necessary. 1,000 genes could produce 1,000 different L
chains and 1,000 genes could produce 1,000 different H chains. With
2,000 genes 1,0002 or 1,000,000
different antibodies could be produced, simply by using different
combinations of H chains and L chains. This may explain how organisms
can produce antibodies against different antigens, even synthetic
antigen proteins that animals have never been exposed to. Using this
model, animals would not need separate genes for every antigen that they
will ever encounter, they simply manufacture millions of different
possible antibodies from a given number of genes for L chains and H
chains. The reservoir of L and H chains comes from gene duplication.
|
- Another
example of gene duplication is the evolution of color vision
(trichromacy) in higher primates. [See G.H. Jacobs and J. Nathans Scientific American
300 (4): 56-63, April 2009.] This remarkable property of human vision
is possible because the retina (the layer of nerve cells in the eye that
captures light and transmits visual information to the brain) uses only
three types of light-absorbing pigments for color vision. Each
individual cone cell in the retina contains genes for all three color
pigments, but randomly selects only one of the three to activate and
shuts down the other two. The M and L pigment genes reside on the X
chromosome, and a third S pigment gene is located on chromosome #7.
Each of the three pigments absorbs light from a particular region of the
spectrum and collectively produce a wide range of color vision. The
pigments absorb light of different wavelengths that we perceive as
yellow, green and blue. This phenomenon is similar to the the mixing of
red, green and blue pixels to generate a full spectrum of color in
computer monitors. The following table shows 216 color combinations
produced from three colors using a six character hexadecimal HTML code:
Different Colors Produced By 6 Character HTML Hexadecimal Code
216 Color Combinations From 000000 (black) to ffffff (white)
|
000000
|
000033
|
000066
|
000099
|
0000cc
|
0000ff
|
006600
|
006633
|
006666
|
006699
|
0066cc
|
0066ff
|
00cc00
|
00cc33
|
00cc66
|
00cc99
|
00cccc
|
00ccff
|
|
003300
|
003333
|
003366
|
003399
|
0033cc
|
0033ff
|
009900
|
009933
|
009966
|
009999
|
0099cc
|
0099ff
|
00ff00
|
00ff33
|
00ff66
|
00ff99
|
00ffcc
|
00ffff
|
|
330000
|
330033
|
330066
|
330099
|
3300cc
|
3300ff
|
336600
|
336633
|
336666
|
336699
|
3366cc
|
3366ff
|
33cc00
|
33cc33
|
33cc66
|
33cc99
|
33cccc
|
33ccff
|
|
333300
|
333333
|
333366
|
333399
|
3333cc
|
3333ff
|
339900
|
339933
|
339966
|
339999
|
3399cc
|
3399ff
|
33ff00
|
33ff33
|
33ff66
|
33ff99
|
33ffcc
|
33ffff
|
|
660000
|
660033
|
660066
|
660099
|
6600cc
|
6600ff
|
666600
|
666633
|
666666
|
666699
|
6666cc
|
6666ff
|
66cc00
|
66cc33
|
66cc66
|
66cc99
|
66cccc
|
66ccff
|
|
663300
|
663333
|
663366
|
663399
|
6633cc
|
6633ff
|
669900
|
669933
|
669966
|
669999
|
6699cc
|
6699ff
|
66ff00
|
66ff33
|
66ff66
|
66ff99
|
66ffcc
|
66ffff
|
|
990000
|
990033
|
990066
|
990099
|
9900cc
|
9900ff
|
996600
|
996633
|
996666
|
996699
|
9966cc
|
9966ff
|
99cc00
|
99cc33
|
99cc66
|
99cc99
|
99cccc
|
99ccff
|
|
993300
|
993333
|
993366
|
993399
|
9933cc
|
9933ff
|
999900
|
999933
|
999966
|
999999
|
9999cc
|
9999ff
|
99ff00
|
99ff33
|
99ff66
|
99ff99
|
99ffcc
|
99ffff
|
|
cc0000
|
cc0033
|
cc0066
|
cc0099
|
cc00cc
|
cc00ff
|
cc6600
|
cc6633
|
cc6666
|
cc6699
|
cc66cc
|
cc66ff
|
cccc00
|
cccc33
|
cccc66
|
cccc99
|
cccccc
|
ccccff
|
|
cc3300
|
cc3333
|
cc3366
|
cc3399
|
cc33cc
|
cc33ff
|
cc9900
|
cc9933
|
cc9966
|
cc9999
|
cc99cc
|
cc99ff
|
ccff00
|
ccff33
|
ccff66
|
ccff99
|
ccffcc
|
ccffff
|
|
ff0000
|
ff0033
|
ff0066
|
ff0099
|
ff00cc
|
ff00ff
|
ff6600
|
ff6633
|
ff6666
|
ff6699
|
ff66cc
|
ff66ff
|
ffcc00
|
ffcc33
|
ffcc66
|
ffcc99
|
ffcccc
|
ffccff
|
|
ff3300
|
ff3333
|
ff3366
|
ff3399
|
ff33cc
|
ff33ff
|
ff9900
|
ff9933
|
ff9966
|
ff9999
|
ff99cc
|
ff99ff
|
ffff00
|
ffff33
|
ffff66
|
ffff99
|
ffffcc
|
ffffff
|
- Most
nonprimate mammals exhibit dichromacy, with color vision based on just
two kinds of visual pigments. These animals do not have full color
vision. Although the mechanism of gene inactivation is slightly
different in Old and New World primates, the evolution of trichromacy
enabled these mammals to see a technicolor world of flowers, fruits and
insects. This is somewhat analogous to the evolution of high resolution
color monitors from the drab monochrome green and amber progenitors.
One obvious advantage of trichromacy is being able to clearly
distinguish the subtle shades of ripening fruit from surrounding
vegetation.
- Color
blindness or the inability to differentiate between certain color
variations is more common in men because they have only one X
chromosome. If defective M and L pigment genes reside on their single X
chromosome, men will exhibit this trait. Women have two X chromosomes
and therefore can be homozygous or heterozygous for the color blind
trait. The phenomenon of X inactivation complicates the expression of
color blindness in heterozygous females since only one X is functional
and the other remains inactive as a Barr body. The inactivation of X
chromosomes appears to be a random event.
6. Telomeres: A Major Molecular Fix To A Chromosome Replication Problem
- The
structure and function of DNA are certainly two of the most significant
discoveries that have revolutionized the science of biology. Even
though DNA appears to be a perfect storage molecule for genetic
information, it has a serious replication problem. Chromosomes of
eukaryotic cells are composed of linear DNA. In order for cell division
to take place, the DNA molecule must replicate. In other words, the
single chromosome must become a doubled chromosome composed of two
chromatids. The problem is that each time DNA replicates the new
molecules get slightly shorter. After a number of consecutive
divisions, this degradation could result is serious gene loss at the
ends of the chromosomes. Both Alexey Olovnikov and James Watson
independently described this phenomenon called "end replication problem"
in the early 1970s. In fact, Olovnikov's "A Theory of Marginotomy"
predicted that the loss of terminal sequences resulting from end
replication problem would lead to senescence (Olovnikov, 1973). [James
Watson and Francis Crick discovered the structure of DNA in 1953 and
received the Nobel Prize in Medicine in 1962.]
- To
cope with the devastating end problem replication problem, eukaryotic
cells have evolved protective "caps" on the ends of chromosomes called
telomeres. For their discovery of how chromosomes are protected by
telomeres and the enzyme telomerase, Elizabeth Blackburn, Jack Szostak
and Carol Greider were awarded the Nobel Prize in Medicine in 2009.
With their ingenious genetic research and meticulous biochemical
studies, they not only solved a fundamental problem in biology but also
opened a new field of research and initiated the development of
potential therapies against the aging process and cancer. It should be
noted here that more than 60 years earlier, Barbara McClintock was
studying telomeres in corn. In the early 1940s she turned her attention
to the study of transposable elements (transposons) in corn, another
remarkable genetic phenomenon with important medical implications in
people. For her lifelong research on transposons, she received the
Nobel Prize in Medicine in 1983.
- Telomeres
are repetitive strands of DNA (sequences of repetitive bases) at the
terminal ends of linear chromosomes. They play an essential role in
maintaining the integrity of the chromosome by protecting it from
degradation and from end-to-end fusion with other chromosomes.
Telomeres are essentially protective "end caps" of non-coding DNA at the
extreme ends of chromosomes. Telomeres have been metaphorically
compared with the tips of shoelaces that keep the laces from unraveling.
Each time a cell divides, the telomeres lose a small amount of DNA.
Eventually, when all of the telomere DNA is gone, the cell can no longer
divide and dies. End replication problem is not an issue in
prokaryotic cells because they have circular DNA molecules without ends.
- The
number of times a population of normal cells can divide is called the
Hayflick limit, named after its discoverer Leonard Hayflick. In 1961,
Hayflick demonstrated that normal human fetal cells in a culture divide
between 40 and 60 times. It is now clear that cell division occurs
until the telomeres reach a critical length. An estimated length for
human telomeres ranges from 8,000 base pairs at birth to 3,000 as people
age, and as low as 1,500 in elderly people. Starting with 8,000 base
pairs, a loss of 100 to 200 with each division would completely erode
away the telomeres in 40 to 80 divisions.
|
Prerequisites For DNA Polymerase:
1. When DNA Unzips, DNA Polymerase Must Attach To 3' End of Mother Strand.
2. It Must Add Nucleotides (Synthesize Daughter Strand) In The 5' to 3' Direction.
3. An RNA Primer Must Attach First To Give DNA Polymerase A Place To Start.
|
- Chromosome
duplication starts with the unzipping of the double stranded DNA into
two strands (mother strand #1 & mother strand #2). These
complementary mother strands serve as templates to build two DNA
molecules. An RNA primer attaches to the 3' end of mother strand #1,
thus giving DNA polymerase a place to start. The primer attaches just
before the initial attachment of DNA polymerase. DNA polymerase moves
in the 3' to 5' direction along mother strand #1, adding nucleotides to
form a continuous complementary daughter strand of DNA the entire length
of the mother strand #1 template. This complementary strand is called
the "leading strand" and it is synthesized in the 5' to 3' direction.
[5' and 3' refer to specific carbon atoms of deoxyribose sugar in DNA
building blocks called nucleotides.] When 5' and 3' directions are
mentioned, it is important to specify whether you are referring to the
mother strand or the complementary daughter strand.
- As
the original mother DNA unzips, DNA polymerase cannot attach to the top
of mother strand #2 at the 5' position (top right in following
diagram). Even if it could attach to the top of mother strand #2 at the
5' position, it could not move down the mother strand and synthesize a
daughter strand in the 3' to 5' direction. Therefore, DNA polymerase
attaches farther down on mother strand #2 and produces a series of DNA
sections in the 5' to 3' direction. These sections are named "Okazaki
fragments" after the Japanese scientist Reiji Okazaki who discovered
them. The sections collectively form a daughter strand called the
"lagging strand" to the top of mother strand #2. This is nicely
explained by R. Ohki, T. Tsurimoto and F. Ishikawa (Molecular and Cellular Biology Vol. 21, 2001). Short RNA primers must attach ahead of each DNA section in order to form a starting point for DNA polymerase.
- There
is a problem at the 3' end of mother strand #2. When the last RNA
primer reaches this end, there is no more DNA template for it to keep
ahead of DNA polymerase. The last primer attaches to the 3' end, but
DNA polymerase cannot add the last section of the lagging strand,
leaving a gap where the primer was attached. Therefore, the 5' end of
each newly synthesized lagging strand is cut short. About 100 base
pairs are shaved off with each round of replication, thus shortening the
telomere. In the following diagram, mother strand #2 has a gap at the
5' end of the newly formed lagging strand.
|
The following animated gifs show replication of DNA in six consecutive
divisions without any shortening, compared with the end replication
problem on lagging strand and the gradual shortening of DNA.
|
- Telomeres
can be restored by the enzyme telomerase. This enzyme lengthens
telomeres in germ cells (cells that produce eggs and sperm), thus
restoring telomeres to their maximum length in the zygote. It is also
present in other cells that must continually divide, including bone
marrow stem cells that produce large numbers of generations of red blood
cells necessary to sustain life, the epithelium of skin, and cells
lining the intestine. Telomerase is generally not active in normal
somatic cells. This enzyme adds noncoding DNA sequence repeats TTAGGG
in vertebrates to the 3' end of DNA strands in the telomere region of
eukaryotic chromosomes. The presence of active telomerase in cancer
cells may be useful in the diagnosis and treatment of some cancers with
telomerase inhibitors.
- The following paragraph comes from Science and Technology
(9 November 2007): Sharks have telomerase in all of their cells.
Their telomeres don't shorten and sharks do not have a genetically
programmed life span like humans. In fact, sharks keep growing
throughout their life. The limit to their life span is the fact that
they must keep moving in order to circulate air through their gills for
the uptake of oxygen. Sharks are exceptionally genetically stable,
having changed very little in hundreds of millions of years. In
addition, sharks rarely get cancer.
- Telomeres and telomerase also occur in plant cells. Plant telomere biology is summarized by T.D. McKnight and D.E. Shippen in The Plant Cell
Vol. 16: 794-803 (2004). In most flowering plants, telomeres consists
of the DNA base repeats TTTAGGG. Like the somatic cells of animals,
there is little or no active telomerase in vegetative tissue, although
it is reactivated during flowering, probably to ensure that gametes and
embryos inherit telomeres restored to their maximum length. Like cancer
cells in in animals, telomerase is fully functional in cells of plant
tissue cultures, as might be expected for cells with an unlimited
capacity for proliferation. The monocot order Asparagales that contains
about 27,000 species (roughly10 percent of all angiosperms) has 6-base
repeats of TTAGGG, the same sequence found in mammalian telomeres. This
order includes many familiar plant families, such as orchids, iris,
amaryllis, agave, onion and asparagus. Since plants and mammals evolved
into multicellular organisms along completely separate pathways, this
appears to be yet another example of parallel evolution (homoplasy).
- Telomeres
do not prevent the shortening of DNA, they just postpone the erosion
process. The telomere shortening mechanism normally limits cells to a
fixed number of divisions. Eventually, when all of the telomere is
gone, the cell can no longer divide, thus terminating the cell cycle.
Most cancer's are the result of "immortal" cells which have evaded
programmed cellular death due to erosion of telomeres. Chromosomes of
malignant cells usually do not lose their telomeres, thus resulting in
uncontrolled cell division. Animal studies suggest that telomere length
may be related to the aging process on the cellular level and the life
span of animals. There are even studies suggesting that regular
exercise and stress reduction may help to minimize telomere erosion. In
fact, a study published in the May 3, 2005 issue of the American Heart
Association journal Circulation found that weight gain and increased insulin resistance were correlated with greater telomere shortening over time.
- It
is interesting to speculate on the origin of telomeres. If another
version of DNA polymerase existed that attached to the 5' end of mother
strand #2 and added nucleotides in the 3' to 5' direction, then
theoretically a continuous strand could be synthesized to the end of the
mother strand template without the end replication problem. This
theoretical version has never been found and therefore telomeres are
essential to prevent the gradual shortening of DNA and erosion of genes.
Why is there only one form of DNA polymerase that synthesizes daughter
strands in the 5' to 3' direction? This is like asking why living
systems only have L-form (left handed) amino acids and D-form (right
handed) sugars. Did the evolution of telomeres solve a replication
problem inherent in the original DNA, or were telomeres present in the
original DNA of the first eukaryotic cells?
Evolutionary Significance of End Problem Replication & Telomeres
- When
I first wrote this section about end replication problem, I concluded
that it was a defect in DNA that literally shortened the life of a cell
by limiting the number of divisions. Telomeres serve as mitotic time
clocks that prolong life by a certain number of consecutive erosions.
However, there is another side to this story where limiting the life
span of organisms could actually be beneficial. In a rapidly changing
environment, survival of a species depends on genetic variability
through DNA mutations and the ability to pass these genes on to future
generations. A species with exceedingly long generation times may not
be able to compete because adaptive mutations can't keep up with
environmental changes; however, longer generation spans could also slow
population growth as long as fecundity (number of offspring per female)
remains constant. To an individual, immortality may seem good; however,
this may not be good for the species. This logic is mentioned in Star
Trek 2: "The Wrath of Khan" when Spock said: "The good of the many
outweighs the good of the few, or the one." Actually, this logic is
mentioned two thousand years earlier in John 11:49-50.
Of course, one caveat to the benefit of end replication loss is the
shark, which apparently has active telomerase in all of its cells and
telomeres lengths that don't decline significantly with age. Sharks
(class Chondrichthyes) are a very successful group and they have been
around for more than 200 million years. In fact, some species have age
estimations of 100 years or more. Undoubtedly, environmental changes in
the ocean have not been as rampant as on land.
7. Evolution & Irreducible Complexity
- Coagulation
or the clotting of blood is a complex process which evolved from
repetitive gene duplication. It involves platelets and the clotting
protein fibrin plus a series of enzymatic reactions called the
coagulation cascade. Disorders of coagulation can lead to an increased
risk of bleeding (hemorrhage) and/or clotting (thrombosis). So crucial
is this elaborate cascade, that one failed protein can disrupt the
entire process. Intelligent design advocate Michael Behe (1996) has
argued that coagulation is an example of irreducible complexity, where
less complex models simply could not exist. In other words, each and
every element of the complex cascade of enzymes and cofactors must be in
place for blood clotting to work. According to Behe (Darwin's Black Box,
1996), an irreducibly complex system cannot be produced by Darwinian
natural selection, and must have been "designed." However, Kenneth
Miller ( Finding Darwin's God, 1999) discusses Russell
Doolittle's pioneering work on protein evolution and the mechanism of
blood coagulation. Miller describes a simpler blood clotting system in a
lobster whose step-by-step evolution is relatively easy to account for.
Behe also uses irreducible complexity in his description of the common
mousetrap, an oversimplified argument that has been clearly refuted by
other scientists. In fact, please refer to the following web pages by
Keith Robison (1996) and John H. McDonald (2002).
- Another
example of irreducible complexity that is often used by advocates of
intelligent design is the remarkable bombardier beetle (Brachinus).
How could such a complex and potentially lethal mechanism for
repelling predators be produced by natural selection? This suborder of
beetles known as Adephaga secrete a number of chemicals for a variety of
purposes, only one of which is defense. Bombardier beetles inject an
explosive mixture of hydroquinone, hydrogen peroxide plus several potent
catalysts into a reaction chamber in the abdomen. Catalase breaks down
the hydrogen peroxide into water and oxygen gas. Peroxidase oxidizes
hydroquinone into benzoquinone. The mixture of chemicals and enzymes
volatilizes instantly upon contact with the air, generating a puff of
"smoke"" and an audible popping sound. This caustic flatulence is
totally controlled by the beetle, otherwise it might accidentally blow
up its rear end. The explosive discharge apparently discourages
predators, either by chemical irritation, heat or repugnance. The
temperature of the explosive mixture of gasses and fluids is over 100
degrees Celsius, the boiling point of water. This astonishing chemical
defense mechanism is discussed by D.J. Aneshansley and T. Eisner (1969)
in Science Vol. 165: 61-63.
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Bombardier beetles of the genus Brachinus, a member of the large
ground beetle family (Carabidae). These small beetles are about 13 mm
long (1/2 inch). They are fairly common in southern California,
particularly near streams, lakes and marshy areas. The wing covers
(elytra) are dark blue-brown with a contrasting reddish-orange head and
prothorax.
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- Other
arthropods also produce some of the same chemicals found in bombardier
beetles. Like bombardier beetles, these chemicals are used for defense
or make the animal distasteful to predators; however, the mechanisms are
not as sophisticated as bombardier beetles. Starting with these
simpler mechanisms, a plausible step-by-step microevolutionary pathway
culminating in bombardier beetles can be constructed. In fact, Mark
Isaak (2003) discusses this in his on-line article entitled: "Bombardier Beetles and the Argument of Design."
8. Scientific Theory vs. Common Theory
- A
scientific theory is a testable (verifiable & falsifiable)
explanation about the cause or causes of a broad range of related
phenomena. It remains open to tests, revision, and tentative acceptance
or rejection. It should not be confused with a common layman theory or
proposition that has not been scrutinized by the scientific method. In
the scientific method, a hypothesis or tentative explanation is
formulated to explain an observation or phenomenon. This is a good
example of inductive reasoning where a general conclusion (hypothesis)
is based on specific observations or data. Then an attempt is made to
prove or disprove the hypothesis through detailed research and
experimentation. Successful results by one scientist does not
automatically turn a hypothesis into a theory. "One scientist cannot
create a scientific theory; he or she can only create a hypothesis."
Only after repeated tests by other scientists who arrive at similar
conclusions does a hypothesis become a scientific theory. Evolution is
not merely a common theory or "only a theory" as some people state in
their commentaries. It is a scientific theory based on more than a
century of research by thousands of dedicated scientists from throughout
the world.
- Biological
evolution has considerable empirical evidence, and it is testable and
verifiable. It fits the definition of a true scientific theory. Under a
strict definition, a scientific theory should not be called a "fact"
even though it explains all known facts and has survived the test of
time (more than a century in the case of evolution). There is always
the possibility that a scientific theory will be updated or changed as
new evidence is discovered. The sticker placed in biology textbooks in
Atlanta is poorly worded. It implies that evolution is just another
"common theory," a tentative statement that attempts to explain
something without any factual proof. A common theory has a popular
meaning, which is roughly equivalent to an "educated guess" or "hunch."
The label should say "scientific theory" rather than theory. In
addition, the dichotomous comparison of a scientific theory with the
word "fact" is incorrect.
- As
stated above, all interpretations of facts in science are provisional
and subject to challenge. Physicists never refer to any theory as a
"fact," and always leave open the possibility that any theory will be
found to be incomplete or needing revision, even though it fits all
known facts and has passed all tests so far. It is still Einstein's
theory of relativity, even though it is probably as close to a fact as
anyone can get in science. It has been tested every day in many ways
and has survived. Some day we may find a more complete theory that
explains and predicts relativity even better than Einstein's theory.
The noun "theory" associated with time-tested explanations, such as
evolution, relativity and plate tectonics, should be modified by the
adjective "scientific" in order to distinguish it from a "common" or
"layman" theory that is essentially a tentative explanation or untested
hypothesis. In fact, it would be better to capitalize the word theory
when it refers to a scientific theory. [For example, gram calories and
dietary Calories are spelled the same, except that a dieter's Calorie is
capitalized and actually refers to a kilocalorie!]
- It
should be noted here that a hypothesis does not always become a
scientific theory. Here is an example: A botanist discovers a
wildflower population that appears different from all other known
species of a particular genus. His hypothesis states that this is a new
(undescribed) species unknown to science. To prove this hypothesis it
is necessary to collect numerous detailed measurements of the floral and
vegetative parts and to conduct an extensive search of the literature
and herbaria. A type specimen (holotype) of the plant is deposited in
an internationally recognized herbarium. Duplicated specimens
(isotypes) are deposited in other herbaria. The hypothesis is confirmed
by statistical analysis of the data, including PCA (principal
components analysis). The results are published in a peer-reviewed
botanical journal. Generally, this hypothesis is not elevated to the
level of a scientific theory. It can be reviewed by another botanist
who may accept or reject the species status, perhaps by performing
additional tests, such as DNA sequencing. This was essentially the
method used by this author in coauthoring a new species of Brodiaea in 2007 named Brodiaea santarosae; however, in our particular case, the hypothesis for a new species came after our preliminary statistical investigation.
9. Explanations For Moving Rocks Of Racetrack Playa: Theories or Hypotheses?
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The sliding (sailing) rocks on Devil's Racetrack move in different directions.
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- The
mysterious moving rocks of Racetrack Playa in Death Valley National
Park have baffled scientists for decades. Although several plausible
explanations have been proposed, no one has actually filmed this
remarkable phenomenon in motion or observed it in real time. A few
explanations are controversial and not agreed upon by all scientists,
and a thorough verifiable and well substantiated proof appears to be
lacking. Unfortunately, most references refer to these explanations as
theories. As of January 2010 the explanations for this remarkable
phenomenon appear to be scientific hypotheses and have not become a
comprehensive scientific theory. One thing is certain: The rocks
definitely change position with time, and this multidirectional movement
has been verified by GPS measurements.
- Several
hypotheses have been proposed to explain the sliding ("sailing") rocks
across the lakebed. Most authorities agree with at least two conditions
necessary for the movement of rocks. (1) Occasional heavy rains and
runoff from nearby slopes producing a slick surface on the fine clays of
the lakebed. (2) Gail force, multidirectional winds of at least 80-100
miles per hour, strong enough to push the rocks across the surface in
different directions. (3) A third hypothesis builds upon the previous
ones and appears to be necessary for larger rocks weighing up to 700
pounds. Freezing nighttime winter temperatures that produce a floating
ice sheet on the muddy clay surface. As the lakebed dries, the clay mud
shrinks and cracks into a mosaic of interlocking polygons. When the
playa fills with water, the fine clay imbibes water and the polygonal
cracks coalesce into a sticky surface. Another hypothesis describes
colonies of cyanobacteria living in the surface clay that also imbibe
water and become mucilaginous, possibly contributing to the slippery
surface. Filamentous cyanobacteria secrete a mucilaginous sheath that
helps to bind soil particles together in microbial communities known as
cryptobiotic crust.
- There
are additional hypotheses to explain the movement of rocks, however,
most of these can be discounted. Gravity can be ruled out since the
north end of the lakebed is 1.5 inches (4 cm) higher than the southern
end, and most of the rocks traveled slightly uphill. Pranksters during
wet years is a possibility, except they left no footprints in the soft,
muddy clay surface adjacent to the rocks. Another hypothesis involves
aliens from another planet who visited this playa during exceptional wet
years.
10. The K-T Boundary & Demise Of The Dinosaurs
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About 70 million years ago, eastern Montana was a vast subtropical
swampland with a lush forest that supported numerous dinosaurs,
including Tyrannosaurus rex. Today this area is an arid badlands
composed of sedimentary strata containing the fossilized remains of a
diverse flora and fauna. The above painting is on the outside wall of
the Makoshika Dinosaur Museum in downtown Glendive, Montana.
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- The
K-T (Cretaceous-Tertiary) boundary is a dark, narrow band of sediments
and carbonized plant material (coal) that separates the Cretaceous and
Tertiary periods about 65 million years ago. In Makoshika Stae Park the
tan strata above the K-T band is called the Fort Union Formation. It
is younger than 65 million years and does not contain dinosaur fossils.
The term Paleocene ("early-recent") refers to a time period when
dinosaurs were replaced by smaller mammals, long before modern mammalian
orders emerged. Below the K-T band is the older brownish-gray Hell
Creek Formation that is rich in dinosaur fossils, including Tyrannosaurus rex, Triceratops and the amazing duck-billed Hadrosaurus. It also contains the fossilized fruits of an extinct palm (Spinifructus antiquus), once thought to be an extinct fig.
- The
best explanation (scientific theory) for the mass extinction of
non-avian dinosaurs is an enormous 10 km (6 mile) diameter asteroid that
collided with the earth about 65 (65.5) million years ago causing a
global dust cloud that blotted out the sun for many months. In 1980, a
team of researchers consisting of Nobel prize-winning physicist Luis
Alvarez, his son, geologist Walter Alvarez, and chemists Frank Asaro and
Helen Michels discovered that sedimentary layers found all over the
world at the K-T boundary contain a concentration of iridium many times
greater than normal. Iridium is a rare earth element that is abundant
in most asteroids and comets. It is the second densest element after
osmium and the most corrosion-resistant metal. The Alvarez team
suggested that an asteroid struck the earth at the time of the K-T
boundary.
- This
explanation for the rapid demise of non-avian dinosaurs is based on a
lot of research and published data from many independent scientists from
around the world. It is much more than a mere hypothesis. Estimates
as high as 85 percent of all species disappeared from the face of the
earth at this time. This catastrophic event forever changed the
direction of the evolution of life on earth.
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Badlands of Makoshika State Park near Glendive, Montana.
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11. Scientific Theory & Scientific Law
- As
I stated previously, the Working Group on Teaching Evolution, National
Academy of Sciences (1998) defines a scientific theory as a
well-substantiated explanation of some aspect of the natural world than
can incorporate facts, laws, inferences, and tested hypotheses. They
define a scientific law as a descriptive generalization about how some
aspect of the natural world behaves under stated circumstances. Laws
can be very useful in supporting hypotheses and theories, but like all
elements of science they can be altered with new information and
observations. Scientific theories and scientific laws are both derived
from carefully formulated hypotheses that have been scrutinized and
repeatedly tested by scientists. In general, they are both accepted to
be true by the scientific community and they are both used to make
predictions of events. The notion that scientific theories eventually
become laws is incorrect. Scientific theories are generally more
complex and dynamic than scientific laws; they have many components, and
may be changed as the body of available experimental data and analysis
develops. In addition, scientific theories explain a whole series of
related phenomena. Examples of scientific theories include the chaos
theory in mathematics, the theory of relativity in physics, and the
theory of plate tectonics and continental drift in geology.
- Scientific
laws are strictly empirical and explain a single action or set of
actions. They can sometimes be expressed in terms of a single
mathematical equation. Examples of scientific laws include the laws of
thermodynamics, Newton's law of gravity, Hook's law of elasticity, Ohm's
law, and the gas laws (Boyle's law and Charles' law). Scientific laws
may be components of scientific theories. For example, Newton's law of
gravity is contained within Einstein's theory of relativity. Mendel's
laws of segregation and independent assortment refer to specific
mechanisms of the inheritance of genes. They were once hypotheses used
by Gregor Mendel in 1865 to explain the inheritance of specific traits
in garden peas, such as round vs. wrinkled and yellow vs. green peas.
His original hypothesis explained the observed 9:3:3:1 ratios obtained
from his dihybrid crosses. Mendel's laws are essential components of
the modern theory of evolution; however, there are exceptions to these
laws. When Mendel completed his research on genetic crosses with garden
peas, he assumed that the individual traits were assorted independently
of each other. One of his hypotheses became known as the Law of
Independent Assortment. Today we can explain this law because the
traits Mendel studied just happened to occur on separate chromosomes.
Since the garden pea has 7 pairs of chromosomes, it is obvious that all
the hundreds of genes in peas cannot occur independently of each other,
and must be located on 7 pairs of homologous chromosomes. Depending on
the exact genes you are studying, chromosomal linkage may result in
ratios of offspring that are far different from the 9:3:3:1 predicted by
Mendel. Laws and theories are the foundations of scientific knowledge;
together they explain our complex natural world. They can be modified
or changed as more information is available.
- In
1908 English mathematician Godfrey Hardy and German physicist Wilheim
Weinberg independently came up with an algebraic expression that
describes how genotype frequencies in populations are related to allele
frequencies. Known as the Hardy-Weinberg Law, it states that gene
frequencies will remain constant generation after generation in large,
randomly-mating populations. Although it is a law, it is only
applicable under a strict set of conditions, including no mutations, no
selection, no migrations between populations (immigration and
emigration), and no genetic drift. In a 2-allele system, such as round
and wrinkled peas, it is based on the square of a binomial (A + B). In a
3-allele system, such as the A-B-O blood types, it is based on the
square of a trinomial (A + B + O). Although it is beyond the scope of
this discussion, the Hardy-Weinberg Law is useful for studies in
population genetics, particularly the determination of genotype
frequencies in populations.
Percentages Of Blood Genotypes In Hypothetical Population
There Are 4 Phenotype Percentages Including: 32% Type A (Red),
15% Type B (Green), 4% Type AB (Blue) and 49% Type O (Brown)
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Alleles
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0.2 A
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0.1 B
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0.7 O
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0.2 A
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AA 4%
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AB 2%
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AO 14%
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0.1 B
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AB 2%
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BB 1%
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BO 7%
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0.7 O
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AO 14%
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BO 7%
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OO 49%
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6 Genotypes In Above Table Appear In The Trinomial Expansion (A + B + O)2 =
A2 (4%) + 2AB (4%) + B2 (1%) + 2AO (28%) +2BO (14%) + O2 (49%)
12. Evidence For Evolution Based On Fossils & Cladistics
- A fascinating article about evidence for evolution was written by David Quammen in the November 2004 issue of National Geographic.
Evidence for evolution comes from many disciplines, including
paleontology, biogeography, anatomy, embryology, physiology,
biochemistry and cladistics. Most of Darwin's arguments for evolution
in his original Origin of Species (1859) were based almost
exclusively on evidence from living organisms (Prothero, 2007). In
modern cladistical analysis, computers create elaborate phylogenetic
trees or cladograms from DNA sequences containing thousands of base
pairs. The cladogram "trees" are organized from the most primitive
organisms to the most advanced. The number of shared characteristics
between any one species and another indicates how recently these two
species have diverged from a shared lineage. Sometimes the cladograms
fit the existing models based on fossil evidence and sometimes they do
not. Whale paleontologist Peter D. Gingerich collected fossil specimens
of early whales from remote areas of Egypt and Pakistan. His research
traced the ancestry of whales back to a group of Eocene carnivorous
mammals called mesonychids. Evidence from DNA comparisons suggests that
whales descended from artiodactyls (even-toed, hooved mammals, such as
antelopes, pigs and hippos). Then in 2000, a 47 million-year-old
anklebone (astragalus) from a four-legged whale was discovered in
Pakistan. This bone closely matched the homologous anklebone in an
artiodactyl. The biochemists were right, whales are indeed related to
hippos and antelopes!
- In
2006, a team of scientists, including Edward B. Daeschler, Neil H.
Shubin, and Farish A. Jenkins, Jr., unearthed an extinct fossil fish in
the Canadian Arctic that dates back to the Devonian Period (360 million
years ago). Their discovery appeared in the journal Nature 440 (6 April 2006: 757-763). The fossil fish is named Tiktaalik roseae.
This animal has the characteristics of a fish, including gills and
scales, but also has the flattened head of a small crocodile and unusual
fins used for walking on land. Its fins have thin bones for paddling
like most fishes, but they also have sturdy interior bones that would
have allowed Tiktaalik to prop itself up in shallow water and use
its limbs for support as most four-legged animals do. This remarkable
fossil is truly a "missing link," an evolutionary transition between
swimming fish and their descendents, a major phylogenetic branch (clade)
giving rise to all four-legged vertebrates (tetrapods), including
amphibians, dinosaurs, birds, and mammals.
- Although
cladograms are generated from comparative DNA samples, some are
produced repeatedly and have higher "bootstrap" values. Cladograms with
the highest bootstrap values are considered the most accurate and
reliable. For example, if one thousand cladogram "trees" are generated
from a comparative DNA sample and the same pattern comes out 900 times,
this cladogram would have a bootstrap value of 90 percent. Evolution is
a scientific theory because it is based on an abundance of empirical
data, even though the precise mechanisms remain open to tests, revision,
and tentative acceptance or rejection. There is substantial factual
evidence to show that the genetic makeup of populations changes from
generation to generation, and that these genetic changes are reflected
in a bewildering array of different species of plants and animals.
- A
taxonomic group that represents a single branch (clade) in a cladogram,
and having a common ancestor, is termed monophyletic. For example, all
birds and reptiles are thought to have descended from a single common
ancestor and are monophyletic. DNA evidence also indicates that Humans (Homo) and chimpanzees (Pan)
are monophyletic. These phylogenetic studies are not always based on
an actual "missing link" which may never be found, or has vanished from
the fossil record. Cladograms are typically based on detailed computer
analysis of the DNA of extant organisms. Cladograms are also based on
fossil evidence, particularly in the field of vertebrate paleontology;
however, these are sometimes supplemented with evidence from DNA.
13. Monophyletic Groupings: All Descendants From A Common Ancestor
Duckweeds Placed In Arum Family (Araceae) & Genus Acacia Split
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Phylogenetic
studies by D.H. Les, et al. (2002), G.W. Rothwell, et al. (2004) and
L.I. Cabrera, et al. (2008) indicate that duckweeds belong to the arum
family (Araceae). Their cladograms are based on sequences of the
trnL-trnF intergenic spacer region of the chloroplast genome.
Consequently, the Lemnaceae will no longer appear as a separate family
in the latest edition of the Jepson Flora of California.
This is especially noteworthy to me since I wrote the section on
Lemnaceae for the previous 1993 and 1996 printings! It is interesting
to note that duckweeds belong to the same plant family as the titan arum
(Amorphophallus titanum). This remarkable plant has a 2.4 m
erect spadix that protrudes from a vase-shaped, pleated spathe 4 m in
circumference. Many other traditional plant families will also be
consolidated as molecular biologists create phylogenetic trees based on
consistent monophyletic groupings. Maintaining Lemnaceae and Araceae as
distinct families would make the arum family paraphyletic, with a
common ancestor but without all of its descendants (i.e. duckweeds are
excluded).
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- Based
on derived characteristics over time, modern phylogenetic trees
(cladograms) of animal and plant groupings show all taxa descending from
a common ancestor. This grouping is termed monophyletic. Starting
with a common ancestor all the branching is typically in 2's
(dichotomous), with every new branch (clade) giving rise to a pair of
closely related sister clades. Each of these clades in turn gives rise
to another pair of sister clades, and so on. Evolutionary relationships
displayed in cladograms are not always dichotomous. Three or more
branches may arise from a node (polytomy) when closely-related taxa
cannot be completely resolved into dichotomies. This is clearly seen in
the cladogram for Acacia (see below). In monophyletic groupings
all descendants have a common ancestor and share one or more derived
characters. See the following simplified cladogram.
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In the above simplified monophyletic cladograms, the grouping depends on
the derived characters that the taxa share. A, B, and C are all seed
plants because they all share the derived character for seeds that also
occurs in the common ancestor. In the above right cladogram, only A
& B are truly flowering plants because they share the derived
character for flowers that also occurs in their recent common ancester
(where A & B arise). This character came later in the evolutionary
timeline, so (C) is not a flowering plant.
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A modern representation of the phylogeny of gymnosperms based on
chloroplast DNA. Dichotomous (paired) sister branches (clades) with a
common ancestor are said to be monophyletic and are more closely
related. For example, the conifer division Pinophyta and ginkgo
division (Ginkgophyta) have a common ancestor in the cycad division
(Cycadophyta). The pine family (Pinaceae) and a sister branch leading
to six additional families have a common ancestor within the division
Pinophyta. In other words, the seven major families of cone-bearing
trees and shrubs all evolved from the division Pinophyta. The araucaria
and podocarpus families (Araucariaceae and Podocarpaceae), which have
their greatest diversity in the southern hemisphere, are monophyletic
and occur side-by-side on sister clades.
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- Many
traditional phylogenetic groupings of species within families and
genera are not monophyletic and are inconsistent with modern cladistical
analyses based of DNA. In other words, the groupings are paraphyletic
or polyphyletic, and do not show all species within a group descending
from a common ancestor. Monophyly is the natural evolutionary pattern
in which all species are descended from a common ancestor. In order to
have consistent computer-generated, monophyletic cladograms, it is
sometimes necessary to change paraphyletic and polyphyletic groupings by
moving species into different genera, and by moving genera into
different families. Many of the taxonomic revisions in the Jepson
Manual 2nd Edition (2012) make more sense if you understand the terms
monophyletic, paraphyletic and polyphyletic. For example, why was the
duckweed family (Lemnaceae) reduced to a subfamily (Lemnoideae) within
the arum family (Araceae)? Why were non-phyllode acacias with prickles
(e.g. cat's claw acacia) removed from the genus Acacia and placed in a separate genus Senegalia? Hopefully, the following chart will shed some light on these significant changes.
- As
I stated above, maintaining the duckweeds (Lemnaceae) and arums
(Araceae) as distinct families would make the arum family paraphyletic,
with a common ancestor but without all of its descendants (i.e.
duckweeds are excluded). In order to have a monophyletc,
computer-generated cladogram for the arum family, the duckweeds are now
placed in the Araceae. Modern reptiles is a paraphyletic grouping that
contains a common ancestor, but does not contain all descendants of that
ancestor (i.e. birds are excluded). Dinosaurs would also be a
paraphyletic grouping because it does not contain the bird descendants.
- According to Seigler, D.S., Ebinger, J.E., & J.T. Miller (2006) Phytologia 88 (1): 38-94, morphological and genetic studies indicate that the genus Acacia
is polyphyletic with more than one recent ancestor. As shown in the
simplified diagram above, the most recent ancestor of "C" is not part of
the grouping within the blue square. For example, it has derived
traits that are not shared by the recent common ancestor of A & B.
Therefore, it cannot be part of the group with A & B. In order to
maintain Acacia as a monophyletic genus, "C" must be placed in a
different group. If "C" and "D" share the same derived traits and
common ancestor, then "C" must be a sister clade with "D." This is
essentially why some previous members of the genus Acacia, such as the "cats claw acacia" (Acacia greggii) are now placed in the genus Senegalia.
A grouping of warm-blooded animals would include birds and mammals and
is called polyphyletic because the members of this group do not include
the most recent common ancestor.
Cat's Claw Acacia Now In Genus Senegalia
- Most members of the genus Senegalia differ from Acacia by the presence of prickles and the absence of phyllodes. A naturalized acacia in San Diego County called "sweet acacia" (A. farnesiana var. farnesiana) has been placed in the genus Vachellia. Members of the genus Senegalia can be distinguished from Vachellia
by the absence of stipular spines and the presence of prickles. The
origin of stipular spines vs. prickles is quite different, and molecular
taxonomists have concluded that the separation of these species into
separate groups is warranted. Some of the morphological subdivisions in
my following Acacia article are now placed in separate genera.
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A. Prickles of cat's claw acacia (Senegalia greggii). B. Stipular spines of sweet acacia (Vachellia farnesiana var. farnesiana).
Unlike stipular spines at the bases of leaves, prickles arise from the
cortex and epidermis of plant stems. The classic thorns of roses are
actually prickles.
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This
simplified dichotomous flow chart is not a computer-generated,
monophyletc cladogram. It simply shows how the traditional genus Acacia
was subdivided into groups in order to key out different species. Some
of these species with prickles and stipular spines have been removed
from the genus Acacia and placed in new genera. The original
1350 species now comprise 5 genera, with 960 (mostly Australian) spp.
still retained in Acacia.
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Computer-generated cladogram of Acacia sensu lato (in the broad sense) showing five major monophyletic lineages (genera) in red. The group containing Mariosousa, Acaciella and Faidherbia is Polytomous. I.e. It doesn't resolve into dichotomies. Faidherbia is a monotypic genus that was formerly classified as Acacia albida. Original cladogram published in: Maslin, Miller & Seigler (2003), Australian Systematic Botany 16 (1): 1-18. The updated generic names follow B.R. Maslin (2006).
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Phylogeny of the Rafflesiaceae
Click on image to see slightly simplified polyphyletic version.
14. Coexistence Of Grasses & Dinosaurs
- The
earliest fossils of flowering plants date back approximately 130
million years, to a time when dinosaurs walked the earth. Exactly which
ancestral seed plants gave rise to the flowering plants has been a
hotly debated topic for more than a century. In fact, in a letter to
Joseph Hooker in 1879, Charles Darwin referred to the sudden appearance
of flowering plants in the fossil record as "an abominable mystery."
Grasses are considered relatively advanced flowering plants, and most
macrofossils and pollen from grasses appear long after the demise of
dinosaurs at the end of the Cretaceous Period (65 million years ago).
Dioramas in museums have long depicted large sauropod dinosaurs grazing
on conifers, cycads and ferns in landscapes without grasses. In the
November 18 issue of Science, Caroline Strömberg of the
Swedish Museum of Natural History and her Indian colleagues Vandana
Prasad, Habib Alimohammadian and Ashok Sahni report phytoliths from
grasses in the fossilized dung of sauropods that lived in central India
about 65 to 71 million years ago.
- Phytoliths
are microscopic silica bodies found inside the cells of stems and
leaves of grasses and other plants. Depending on the species of plant,
they range from 5 to 100 micrometers in length. Because they are made
of a crystalline form of silica called opal, they are very durable and
retain their characteristic shapes over millions of years. Like
microscopic pollen grains and diatoms, the phytoliths remain perfectly
preserved in spaces between soil particles. Different genera of grasses
have phytoliths with unique shapes, including square, rectangular,
oblong, bilobed, wavy with undulate margins, and butterfly-shaped.
Grasses belonging to the subfamily Panicoideae typically have phytoliths
that are shaped like a dumb-bell. I examined the leaf blade of
crabgrass (Digitaria sanguinalis), a member of the Panicoideae, and the phytoliths are indeed shaped like a dumb-bell.
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Magnified view of a row of phytoliths within the leaf epidermis of crabgrass (Digitaria sanguinalis).
The dumb-bell shaped phytoliths are 32 micrometers in length. Compare
this with an average cuboidal grain of table salt in which each side is
300 micrometers long. More than 800 of these crabgrass phytoliths
could fit into a box the size of a grain of table salt! Photo taken at
400x and 1000x magnifications with a light microscope.
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- It
is remarkable how much information has been determined about the
distant geologic past with new and improved methods of chemical analysis
and sophisticated digital instruments. Each day the scientific theory
of evolution is becoming more complete, as scientists uncover new facts
and piece them together like a complex jigsaw puzzle. DNA, the genetic
blueprint of all creatures, provides a stunning, detailed record of
evolution, from primitive unicellular bacteria to complex vertebrate
animals. This is eloquently described by Sean B. Carroll in his book The Making of the Fittest (2006).
15. Origin Of The First Land Plants On Earth
- Based
on fossil evidence and cladistic analysis, the general evolutionary
trend for land plants was increasing levels of complexity, from the
earliest algal mats, through bryophytes, lycopods and ferns, to the
complex gymnosperms and angiosperms of today. Fossil evidence of land
plants (embryophytes) is scarce prior to the Silurian Period, although
liverwort cryptospores have been found in Argentina dating back to early
Ordovician about 470 milion years ago (C.V. Rubinstein, et al. 2010).
The spore walls contained sporopollenin, one of the most stable
biological polymers known and a characteristic of well-preserved pollen
grains throughout the plant kingdom. The earliest land plants were
probably similar to liverworts or hornworts with a dominant thalloid
gametophyte generation and a reduced sporophyte. Fossils of ancient
land plants are well represented in the early Devonian Rhynie Chert beds
in Aberdeenshire in the north of Scotland. This time period is
approximately 398-416 million years ago, long before the age of
dinosaurs. Two of the abundant fossil plants at Rhynie, Horneophyton and Rhynia,
are possible relatives of modern day hornworts. These were small,
leafless plants only one or two feet tall (30-60cm) with rhizoids
instead of true roots. The following statement regarding Horneophyton and Rhynia fossils at Aberdeenshire comes from my old Historial Geology
textbook by Carl Dunbar (1960): "The wonderfully preserved plants
found there represent almost the simplest possible type of structure a
land plant could have, and suggest the steps whereby an aquatic alga
adapted itself for land life." This statement has stayed with me all
these years and is especially fascinating after studying the relatively
simple structure of hornworts and their single large chloroplasts. The
lack of any fossil wood prior to the Devonian suggests that this
important period of geologic time is when land plants truly colonized
the earth. This is truly one of the most important evolutionary
breakthroughs in earth's history. It irreversibly changed climates and
biogeochemical processes on a global scale, and it enabled eukaryotic
life to evolve and invade the continents of the world.
- The
life cycle and morphology of hornworts (division Anthocerotophyta) has
characteristics in common with liverworts and mosses (divisions
Marchantiophyta and Bryophyta). In fact, older references reduced all
three groups to classes within the division Bryophyta. New evidence
from DNA sequencing and cladistic analysis reveals that they represent
three distinct divisions (phyla). Unlike mosses and liverworts, the
hornwort sporophyte continues to grow throughout its life from a
meristematic region near the base and not from the tip as in other
plants. Unlike liverworts, most hornworts have true stomata on their
sporophytes as in mosses. The slender, erect sporophyte has a
multicellular outer layer, a central rod-like columella, and a layer of
tissue in between that produces meiospores and pseudoelaters. At
maturity the sporophyte splits open just below the apex to release the
spores inside. This dehiscent region containing spores is essentially a
sporangium. The pseudoelaters are multicellular, unlike the elaters of
liverworts. Helical thickenings cause the pseudoelaters to change
shape and twist as they dry out, thereby helping to disperse the mass of
trilete spores. When trilete spores separate from the common tetrad,
each spore shows 3 lines radiating from a central pole. In monolete
spores there is a single line on the spore surface indicating that the
mother cell split into 4 along a vertical axis. The early nonvascular
land plant Horneophyton also had a central columella and trilete spores.
- Like
liverworts, the haploid gametophyte (thallus) of hornworts is
dorsiventrally flattened. It often becomes slimy due to mucilage-filled
cavities when groups of cells break down. These cavities are invaded
by colonies of cyanobacteria (Nostoc) giving the thallus a
blue-green color. One of the most unusual characteristics is the single
large chloroplast per cell. A single chloroplast is common in green
algae (division Chlorophyta) but it is quite uncommon and unusual for
true multicellular plants (kingdom Plantae). Like mosses and liverworts
the motile sperm are biflagellate and swim to the egg via water.
Cladistic analyses suggest that hornworts (Anthocerotophyta) originated
much earlier in the history of land plants, possibly before the
Devonian. Hornworts may even be one of the earliest lineages of land
plants. Some of their morphological characteristics are similar to
sporophytes of the ancient land plant Horneophyton that lacked true vascular tissue. In fact, some paleobotanists have suggested that Horneophyton may be the "missing link" between hornworts and the Rhyniopsida, an extinct class of early vascular plants that includes Rhynia. Some of these characteristics are shown in the following image.
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A hornwort compared with the extinct 400 million-year-old land plants Horneophyton and Rhynia.
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Moss Life Cycle Has Animal-Mediated Fertilization!
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Mosses belong to the division Bryophyta characterized by
nonvascular plants with embryos that develop within multicellular female
sex organs called archegonia. The dominant (conspicuous) part of the
life cycle is the haploid, leafy gametophyte. The diploid sporophyte
consists of a sporangium-bearing stalk that grows directly out of the
gametophyte. Spore mother cells within the sporangium undergo meiosis,
producing numerous haploid spores that fall to the ground like tiny
particles of dust. Since the sporophyte is without chlorophyll, it is
completely dependent on the autotrophic (photosynthetic) gametophyte for
its water, minerals and carbohydrate nutrition. Consequently, the
sporophyte of the moss is heterotrophic and parasitic on the
gametophyte. About half of all moss gametophytes are dioecious, with
separate male and female individuals in the population. The
gametophytes are produced by "male" and "female" spores. Mosses have a
primitive method of fertilization that involves a motile, biflagellate
sperm that swims through water to reach the egg on female plants.
For more than a century, textbooks stated that moss sperm needed water
to swim or splash to a female archegonium. According to Lils Cronberg
and his colleagues at Lund University, Sweden (2006), mosses have a
carrier system for sperm that is similar to pollen transfer in flowering
plants. This explains how moss plants spaced too far apart for
swimming sperm produce sporophytes following fertilization. From a
simple lab experiment it has now been shown that fertile moss shoots
attract minute arthropods (collembolans and mites) that passively carry
moss sperm, similar to bees with pollen in flowering plants. The role
of collembolans in moss fertilization is undoubtedly much older due to
the antiquity of mosses. Mosses and collembolans are extant members of
ancient organisms that originated and radiated after the early
colonization of land about 440-470 million years ago.
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- Collembola
are also known as springtails because they have a forked springing
device or furcula at their posterior end. It is folded beneath the tail
under tension which can be released suddenly, catapulting the
springtail into the air and hopefully out of harm's way. They are
classified as hexapods along with insects; however, DNA analysis has
shown that they do not belong to the class Insecta. They represent a
separate evolutionary line (class) that includes Protura and Diplura.
They are considered the most abundant hexapod on earth with over 6,000
known species. In just one handful of grassland soil there can be
literally thousands of individuals representing hundreds of different
species. They feed primarily on detritus and microscopic life,
including fungal hyphae, bacteria and algal cells. Along with
nematodes, collembola are one of the main biocontrol agents on microbial
populations. There are also parasitic species associated with
dermatitis in humans. One of the most interesting collembola are called
"snow fleas" (Achorutes nivicolus) that feed on the unicellular alga (Chlamydomonas nivalis) that colors the snow red.
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Left: Snow fleas (Achorutes nivicolus) in the hexapod class
Collembola. Center: A boot print in pink snow. Right: Bright red
resting cells (aplanospores) of snow algae (Chlamydomonas nivalis)
and a pine pollen grain. Snow fleas can jump by means of a springing
device or furcula (red arrow). Unlike other colorless soil springtails
(see below), snow fleas are dark-colored. This allows them to absorb
heat from the sun. They also produce a glycine-rich protein that works
like anti-freeze, allowing them to function in sub-zero environments.
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Left: Soil springtails compared with the "eye" of an ordinary sewing
needle. They are actually colorless and about 1.0 to 1.5 mm long;
however, backlighting makes them appear darker. With their springing
device or furcula they can jump about 100 times their body length or
about 7-8 inches (18-20 cm). Although they are abundant in some soils,
they are barely visible with the naked eye. In just one handful of
grassland soil there can be literally thousands of individuals
representing hundreds of different species.
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Left: Magnified view of a soil springtail taken with a Bausch & Lomb
dissecting microscope and Sony W-300 camera. The springing device
(furcula) is clearly visible on the ventral side of posterior end. A
handful of soil can contain literally hundreds (or thousands) of
springtails. Because of their small size and colorless bodies they are
difficult to see with the naked eye. In order to acheive a more
accurate rendition of its colorless body, I used backlighting and then
"invert" with Adobe Photoshop.
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16. Origin Of Flowering Plants (Angiosperms)
- Although
considerable evidence has been compiled since the time of Darwin, the
precise origin of flowering plants in the early Cretaceous (140 million
years ago) remains an enigma. As I stated above, Darwin mentioned this
controversy in 1879 when he referred to the origin of flowering plants
as an "abominable mystery." There are at least four hypotheses to
explain the origin of flowering plants.
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The following cladograms illustrate four different hypotheses for the
origin of flowering plants. They show common ancestry between
angiosperms and more ancient gymnospermous groups, including cycads,
gnetophytes and other conifers. A. According to the Anthophyte Hypothesis,
Gnetophyta (Mormon tea & allies) and angiosperms are monophyletic
and share a common ancestor. This is based (in part) on the presence of
vessels and double fertilization in some gnetophtyes. B. According to
the Gnetifer Hypothesis, gnetophytes are most closely related to other conifers and not angiosperms. C. In the Gnepine Hypothesis, gnetophyes are monophyletic with the pine family Pinaceae. D. The Cycad Hypothesis
has angiosperms monophyletc with cycads. The latter hypothesis is
based (in part) on the superficial resemblance of a female "cone" of Cycas revoluta to a large angiosperm flower. See flowerlike "cone" of female Cycas revoluta in image to the right.
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Cladograms showing four different hypotheses for the monophyletic origin of flowering plants.
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Molecular phylogenetic studies indicate that the first split within
modern angiosperms is between a lineage that includes a single species (Amborella trichopoda) and all the rest of the extant angiosperm species. In other words, Amborella is monophyletic with all the rest of the angiosperms (see right cladogram: Origin of Amborella & Angiosperms). Amborella trichopoda is a rare flowering shrub that grows in the rain forest understory in New Caledonia. Unlike practially all other angiosperms, Amborella xylem has only tracheids, supporting the modern view that the first angiosperms lacked vessels. Amborella
gametophytes are also unusual in having three, rather than two,
synergid cells with the egg cell at the micropylar end (egg apparatus),
hence a total of nine nuclei and eight cells in the embryo sac.
According to W.E. Friedman (Nature 441, 2006), this extra
cell in the egg apparatus could provide evidence of a critical link to
the gymnospermous ancestors of flowering plants. The gametophyte of the
vast majority of angiosperms has three cells in the egg apparatus, and a
total of seven cells and eight nuclei in the embryo sac (see angiosperm
life cycle below).
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Origin of Amborella & Angiosperms.
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Underside of flowering branch of male Amborella trichopoda.
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Close-up view of male (staminate) flowers of Amborella trichopoda.
Each flower is aprroximately 4 to 5 mm in diameter with a dozen or
more bract-like tepals (perianth segments undifferentiated into petals
& sepals). The flowers have a dozen or more spirally arranged
stamens, which become progressively smaller toward the center.
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Water Lily (Nyphaea odorata)
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Amborella trichopoda (above): A rare, sprawling, understory
shrub from New Caledonia, and the most primitive living flowering plant.
It belongs to the monotypic family Amborellaceae in the monotypic
order Amborellales. Most authorities consider it to be the living
descendent of a line of primitive flowering plants without vessels that
diverged near the base of the main clade of vessel-bearing angiosperms
about 130 million years ago. The younger growth (inset) has bright
green, shiny leaves. The flowers have many tepals (perianth parts not
differentiated into calyx & corolla) and numerous stamens. They are
similar in appearance to miniature versions of the closely-related
water lily clade (order Nymphaeales), see left image.
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- Williams, J.H. 2009. "Amborella trichopoda (Aborellaceae) and the Evolutionary Developmental Origins of the Angiosperm in Programic Phase." American Journal of Botany 96: 144-165.
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Left: Vessels in midvein of petal from Brodiaea terrestris ssp. kernensis.
The spirally-thickened secondary cell walls appear like coiled
springs. This provides strength as well as flexibility to these strands
of tubular, water-conducting cells that compose the xylem (vascular)
tissue. Right: Two types of water-conducting cells: tracheid and
vessel (vessel element). Technically a vessel is composed of many
hollow vessel elements joined end-to-end like sections of PVC pipe.
With the exception of the Gnetophyta, most gymnosperms lack vessels.
Vessels are characteristic of all flowering plants, except for the
earliest ancestral sister clade (Amborella) that have only tracheids like most gymnosperms.
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17. The Remarkable Angiosperm Life Cycle
- From
their first appearance in the fossil record of early Cretaceous,
flowering plants evolved into the dominant land plants on earth in a
relatively short period on the geologic time scale. There are a number
of tentative hypotheses to explain this astonishing explosion of
species. Did the development of vessels and vascularization of leaves
with more efficient water conduction systems give them a significant
photosynthetic advantage? Perhaps pollination & fertilization
followed by seed and fruit development is more efficient in flowering
plants. The evolution and diversity of flowering plants certainly
coincides with amazing insect diversity. When flowering plants began to
dominate the landscape, they edged out the conifers, tree ferns and
cycads that the long-established sauropod dinosaurs depended on. The
competitive advantage of flowering plants is probably a lot more
complicated, and undoubtedly is related to changing climatic conditions
(and perhaps an enormous asteroid that collided with the earth causing a
global dust cloud that blotted out the sun for months). At any rate,
flowering plants possessed many adaptive traits that made them
particularly resistant to drought and extreme cold. Some of these
sophisticated advancements included more efficient water-conducting
cells, advanced photosynthetic patterns adapted to drought conditions,
including C4 and CAM photosynthesis, advanced leaf and stem anatomy that
could withstand environmental extremes, cold and drought-deciduous
foliage (which is rather uncommon in today's gymnosperms), perennials
with dormant underground rootstocks capable of surviving severe winters,
extremely tough, resistant seed coats that protect the dormant embryo
inside for prolonged unfavorable periods, an enormous chemical defense
arsenal against disease organisms and herbivores, and an efficient
pollination and dispersal system involving complex interactions with
animals. Flowering plants have literally colonized every conceivable
habitat on earth, including the extensive fast-growing, fire-adapted
grasslands of today that support huge herds of grazing mammals, and yet
many of these vast vegetation types were not around when dinosaurs
reached their peak.
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Typical angiosperm life cycle: Flowering plants (angiosperms) belong to
the vascular plant division Anthophyta. Like ferns and conifers, the
diploid sporophyte consists of a herbaceous or woody plant with roots,
stems and leaves. Unlike ferns and conifers, flowering plants produce
reproductive organs called flowers and seed-bearing fruits. The term
angiosperm is derived from angio (vessel-like container) and sperm
(seed), referring to the seed-bearing containers called fruits. The
female gametophyte of Amborella is very unusual because it has
three synergid cells rather than the usual two found in most
angiosperms. In addition, the typical water conducting cells called
vessel elements (vessels) found in practically all flowering plants are
absent in Amborella. The variation in size, color, number and
arrangement of floral parts in blossoms of the 300,000 different plant
families is absolutely staggering.
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silver sword
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18. Adaptive Radiation On The
Hawaiian & Galapagos Archipelagos
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alula
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- The
Hawaiian archipelago has been isolated from continental land masses
during the past 30 million years, and yet the 1,000 species of
indigenous Hawaiian angiosperms are believed to stem from natural
introduction by long-distance dispersal of 280 ancestral plant colonists
(Manual of the Flowering Plants of Hawaii by Wagner,
W.L., Herbst, D.R. and S.H. Sohmer, 1990). It appears that seeds were
carried thousands of miles to these islands, possibly by rafting or
within protective capsules and pods. For example, an ancestral
California tarweed of the sunflower family (Asteraceae) traveled at
least 3,000 miles to the Hawaiian Islands where it gave rise to a
remarkable group of endemics known as the "Silver Sword Alliance." The
small seeds from ancestral members of the lobelia family (Campanulaceae)
also reached these islands from the American mainland giving rise to an
unusual group of endemic Hawaiian lobelioids. This phenomenon where
ancestral species colonize a new habitat and evolve into different
species is called adaptive radiation. The new species evolve in
response to different selection pressures that enable them to fill
unique ecological niches. The story of Darwin's Finches on the
Galapagos Islands is a classic example of adaptive radiation. The
Hawaiian silver swords and lobelioids are truly unusual in appearance.
They are strikingly different in appearance compared with California
tarweeds or members of the lobelia family on the mainland of North and
South America. Their taxonomic affinities with ancestral species are
based on chromosome comparisons, hybridization studies and comparative
chloroplast DNA. The unscientific hypothesis that these bizarre plants
were placed here in their present form by a creator is untenable.
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Alula (Brighamia insignis), a rare member of the lobelia family
(Campanulaceae) endemic to steep sea cliffs on the island of Kauai.
Alula is perfectly adapted for living on vertical volcanic cliffs. A
single rosette of leaves arises from the top of a thick, succulent stem,
like a cabbage head on a baseball bat. The rosette varies in size,
depending on the availability of moisture. Roots penetrate the cliffs
horizontally, and the base of the plant is rounded, permitting the plant
to rock slightly in the wind. Water stored in the stem enables the
plant to survive periods of drought which may last days or weeks. The
flower is very different from members of the lobelia family on the
mainland of North America. Another rare species with white flowers (B. rockii)
grows on sea cliffs along the windward coast of Molokai. Like Hawaii's
endemic silver sword alliance that evolved from an ancestral tarweed
(Asteraceae), the alulu is another example of adaptive radiation.
According to Sherwin Carlquist (Hawaii: A Natural History,
1980), the Hawaiian lobeliads evolved from several ancestral
introductions rather than a single original colonization; however,
molecular data from Thomas J. Givnish of the University of Wisconsin (Evolution on Islands, 1998) indicate that they are monophyletic in origin and represent the product of a single introduction.
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18. Adaptive Radiation On The Galapagos Archipelago
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Giant prickly pear cactus (Opuntia echios var. gigantea) on Santa Cruz Island and Galapagos marine iguana (Amblyrhynchus cristatus)
on Hood Island. The remarkable giant Galapagos prickly pear evolved a
tall woody trunk resembling a ponderosa pine to protect it from browsing
by giant tortoises. The marine iguana is unique amoung modern lizards
because it dives and forages in the sea. There are also endemic land
iguanas on the Galapagos Islands that feed on prickly pear cactus and
other vegetation. These are excellent examples of adaptive radiation
where ancestral species colonized a new habitat and evolved into
different species. Many thousands of years ago, cactus and iguanas
floated out to these volcanic islands from the mainland of Ecuador on
rafts of debris. The new species evolved in response to different
selection pressures that enabled them to fill unique ecological niches.
The story of Darwin's Finches on the Galapagos Islands is another
classic example of adaptive radiation.
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19. Homoplasy: Parallel & Convergent Evolution
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A small mantispid and a preying mantis. Although they differ greatly in
size, these two insects are remarkably similar in appearance. They
both have triangular heads with large eyes and a pair of raptorial
(grasping) front legs. Their other two pairs of legs are used for
walking. They belong to two very different insect orders. Although
mantids were once placed in the order Orthoptera along with
grasshoppers, crickets and cockroaches, they are now placed in the
separate order Mantodea. Mantispids belong to the order Neuroptera,
along with lacewings, snakeflies and antlions. Their remarkable
adaptive similarity is an example of convergent evolution.
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- Sometimes
evolutionary change follows a common pathway in two or more unrelated
or distantly-related organisms because of similar environmental
pressures. It culminates in unrelated organisms with similar
morphological characteristics even though they did not have a common
ancestor. This phenomenon is called parallel evolution. There
are many examples of parallel evolution in plants, including
distantly-related plant families that have evolved from an autotrophic
to a parasitic mode of existence. Some plants have evolved
independently into a mycotrophic mode of existence where they obtain
nutrients from mycorrhizal soil fungi, which in turn, are parasitic on
the roots of nearby forest trees and shrubs. Photosynthetic pathways,
such as CAM (crassulacean acid metabolism) and C-4 photosynthesis, have
also evolved independently in distantly-related plant families.
- Another
example of parallel evolution is the appearance of xylem vessels in the
vascular tissues of very distantly-related plants, such as Ephedra
in the gymnospermous division Gnetophyta and flowering plants in the
angiospermous division Anthophyta (Magnoliophyta). In addition, species
of Ephedra have double fertilization, where two sperm are involved in the fertilization process. Double fertilization
was once thought to be a strictly angiosperm characteristic. Some
older references have suggested that the Gnetophyta may represent a
"missing link" in the evolution of flowering plants, but others say that
vessels and double fertilization are examples of parallel evolution,
and the Gnetophyta are more closely related to conifers than
angiosperms. The current consensus among authorities (as of 2010) is
that Amborella trichopoda (a primitive angiosperm without
vessels) and all other flowering plants represent sister clades derived
from an common ancester without vessels. This latter conclusion
supports the independent (parallel) evolution of vessels in the
Gnetophyta.
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Left: Ephedra viridis in the Panamint Range overlooking Death
Valley National Monument. Right: Wildflowers in Anza-Borrego Desert
State Park. Although Ephedra belongs to an entirely different
plant division (Gnetophyta), it has several characteristics of flowering
plants (division Anthophyta) including vessels and double
fertilization. These traits are considered to be homoplastic because
they evolved independently through parallel evolution in distantly
related ancestors. Some authors would call this convergent evolution.
If Ephedra and flowering plants shared a common ancestor with
vessels and double fertilization, the appearance of these traits would
not be an example of parallel evolution. [Wildflowers include Encelia farinosa, Cylindropuntia bigelovii, Lupinus arizonicus and Mimulus bigelovii.]
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- The
laurel family (Lauraceae) includes about 2,000 species of trees and
shrubs in 50 genera. Familiar genera in the family include Cinnamomum (cinnamon and camphor), Laurus (European bay), Umbellularia (California bay), Sassafras and Persea (avocado). One genus in this family (Cassytha)
has broken away from the other trees and shrubs and has evolved into a
leafless, twining parasitic herb that absorbs nutrients from it host by a
specialized modified root called a haustorium. The distantly-related
morning-glory family (Convolvulaceae) also has about 50 genera of trees,
shrubs and vines. Like the laurel family, one of these genera (Cuscuta)
has evolved into a twining, leafless parasitic herb with a specialized
organ of absorption (haustorium). Some botanists place this genus in
its own family, the Cuscutaceae, although the floral morphology is very
similar to some members of the Convolvulaceae. Both Cassytha and Cuscuta
are remarkably similar in appearance, even though they live in
different parts of the world and do not share a common ancestor. The
radical modification of these two unrelated plants into specialized
parasites is a marvelous example of parallel evolution, or convergent
evolution if you prefer the latter term.
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Left: Cassytha filiformis (Lauraceae) on the Caribbean Island of Grand Cayman. Right: Cuscuta californica
(Convolvulaceae or Cuscutaceae). These twining, parasitic plants are
without chlorophyll and absorb nutrients from their host plants be means
of specialized roots called haustoria.
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- When
parallel evolution under similar environmental conditions in
distantly-related organisms results in plants and animals that are
morphologically very similar in overall appearance, this is usually
called convergent evolution. It should be noted here that some
authors use these two terms interchangeably. North American cactuses
(family Cactaceae) and South African euphorbias (family Euphorbiaceae)
belong to different plant families and are distant relatives in the
phylogeny of flowering plants; however, they both have succulent, thick
stems that store water, they both have spines for protection, and the
both are adapted for survival in arid desert regions with low rainfall.
Without flowers, some African euphorbias are practically
indistinguishable from their North American counterparts.
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Homoplasy: Which of these xerophytes is a cactus and which one is a euphorbia?
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- The supplemental biology text Of Pandas and People
(2nd. Edition, 2004) that is endorsed by advocates of intelligent
design includes a different definition for parallel evolution.
According to the authors of the latter text, "If two organisms are
judged to be related through a "recent" common ancestor, their
similarities are said to result from parallel evolution." They also
state that if the ancestor is distantly related, convergent evolution
has occurred. This definition of parallel evolution is incorrect. In
parallel evolution, the two organisms don't necessarily share a common
ancestor; however they do experience similar kinds of environmental
pressures and survive by developing similar adaptations.
Why Differentiate Parallel From Convergent Evolution?
- The following paragraph is from an article by Jeff Arendt and David Reznick in Trends in Ecology and Evolution Vol. 23 (1): 26-32, 2007:
"Biologists often distinguish
'convergent' from 'parallel' evolution. This distinction usually
assumes that when a given phenotype evolves, the underlying genetic
mechanisms are different in distantly related species (convergent) but
similar in closely related species (parallel). However, several
examples show that the same phenotype might evolve among populations
within a species by changes in different genes. Conversely, similar
phenotypes might evolve in distantly related species by changes in the
same gene. We thus argue that the distinction between 'convergent' and
'parallel' evolution is a false dichotomy, at best representing ends of a
continuum. We can simplify our vocabulary; all instances of the
independent evolution of a given phenotype can be described with a
single term - convergent."
"If the use of the terms 'parallelism'
and 'convergence' cannot be associated with a clear dichotomy, either at
a phylogenetic level or a molecular level, then
their continued use is not justified and can even be misleading. They
are relics of a time when we could not evaluate the underlying causes of
phenotypic similarity and were confined to inferences based on
comparative anatomy. These terms are also relics of a time when there
was not an appreciation of the complexity of genetic and developmental
networks that underlie the determination of simple phenotypic traits,
such as coloration. We argue that this might be a good time to simplify
our vocabulary. We need only one term to describe the independent
evolution of phenotypic similarity. 'Convergent evolution' will do
nicely."
- In
Australia there are many examples of marsupials that resemble our North
American placental mammals. For example, Australia's flying phalanger
is remarkably similar to the North American flying squirrel. Both
tree-dwelling mammals glide through the air with their parachute-like
fold of furry skin between the front and hind legs. These are excellent
examples of convergent evolution.
Homology: Similarity Of Characteristics Due To Origin From A Common Ancestor.
Homoplasy: Similarity Due To Independent Origin From Distantly Related Ancestors
- I
prefer to use the terms homology and homoplasy when dicussing the
evolution of similar characteristics. Homology refers to similarity due
to a common ancestor. Characteristics derived from a common ancestor
are termed homologous. Homologous organs are similar in structure and
embryonic origin but are not necessarily similar in function. Cactus
spines are homologous to bud scales of an axillary bud. Seed-bearing
carpels of flowering plants are homologous to leaves because of their
similarity in form, anatomy and development. The bone structure in the
wings of a bat is homologous to the forelimbs of humans and other
mammals. For example, a bat's wing and whale's flipper both originated
from the forelimbs of early mammalian ancestors, but they have undergone
different evolutionary modification to perform radically different
tasks of flying and swimming. The presence of homology is evidence that
organisms are related.
- Homoplasy
(ho-MOP-la-see) means similarity due to independent origin that is not
from a common ancestor. Using the term homoplasy avoids the confusing
distinction between parallel and convergent evolution. Homoplastic
characteristics, such as the spines of cacti and stem-succulent
euphorbias, evolved independently from each other. Cactus spines arise
from an axillary growth center called an areole. Euphorbia spines are
derived from modified stipules. Homoplasy includes parallel and
convergent evolution. Similarity of appearance in unrelated or
distantly-related organisms is often the result of similar evolutionary
pathways under similar environmental conditions. For example, the wings
of birds and insects are used for flight. They are analogous but not
homologous because their structure, embryonic development and genetics
is very different. In addition, they do not have a common ancestral
origin at the beginning of their evolutionary development. Sometimes it
is unclear whether similarities in structure in different organisms are
analogous or homologous. For example, the wings of birds and bats are
homologous in that they are both modifications of the forelimb bone
structure of early reptiles. However, bird's wings differ from those of
bats in the number of digits and in having feathers for flight while
bats have no feathers. Bat wings involve the bones that in humans make
up the hands, while bird wings lack many of these bones entirely, and
instead include only the bones that in humans make up the arms. In
addition, the power of flight arose independently in these two very
different classes of vertebrates. The two lineages had a long
evolutionary separation before they independently became fliers. Wings
in birds evolved from early reptiles, and in bats they evolved after
their mammalian ancestors had already completely differentiated from
reptiles. Thus, the wings of birds and bats can be viewed as analogous
rather than homologous upon a more rigorous scrutiny of their
morphological differences and evolutionary origins.
- Homoplasy
is a characteristic shared by members of a phylogenetic tree
(cladogram), but not present in their nearest common ancestor. It
arises independently by convergent evolution in more than one clade.
For example, both mammals and birds are able to maintain a high constant
body temperature (warm-blooded). However, the ancestors of each group
did not share this character, so it must have evolved independently.
Mammals and birds should not be grouped together on the basis of whether
they are warm-blooded.
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A. Equisetum (Division Sphenophyta). B. Casuarina
(Division Anthophyta). Two unrelated plants in different divisions of
the plant kingdom. They both have jointed stems with whorls of
scale-like leaves at the nodes. One is a flowering tree and the other
is a non-flowering plant with an apical spore-bearing cone (strobilus).
Is this an example of homology or homoplasy?
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A hummingbird and hawkmoth, a classic example of convergent evolution
(homoplasy). These two distantly related animals are adapted to
hovering over flowers and sipping nectar with their long tongues. The
hummingbird sips sugary nectar from the tubular flowers of the Mexican
coral tree (Erythrina coralloides). As the hummingbird feeds,
pollen adheres to its head and bill and is transferred from one flower
to another. If the pollen is transferred between flowers of different
trees, then cross pollination is accomplished. The tree supplies the
hummingbird with sucrose-rich nectar for this valuable pollination
service. The nocturmal hawkmoth (Manduca sexta) has a long proboscis which reaches the nectar in long, tubular flowers of jimsonweed (Datura) and Petunia species.
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- Another
example of homoplasy is the evolution of similar glistening bodies in
the flowers of distantly related plant families. According to Peter K.
Endress (Diversity and Evolutionary Biology of Tropical Flowers,
Cambridge University Press, 1994), some flowers display glistening
bodies to attract insects. They are interpreted as mimicking nectar
drops. They are best known in the temperate genus Parnassia
(Saxafragaceae), where they have been termed "pseudonactaries." Similar
structures also occur in genera of tropical and subtropical regions,
such as Solanum (Solanaceae), Lopezia (Onagraceae) and Nemacladus
(Campanulaceae). Although their origin and morphology may be
different, they appear to have the same function. They are completely
dry, and there is no sign of secretory activity; however, they are often
situated near the real nectaries, which are in a more hidden position.
Pollinators may be led to the nectar source by these pseudonectaries.
"The gadgets of Nemacladus are bizarre. Two stamens have a
protruding socket at the filament base. Each socket bears three or more
reflexed clavate giant cells, which cause the glistening effect."
- Aaron
Schusteff (personal communication, 2012) photographed insects from
three different families (two bee flies and a wasp) on three different
species of Nemacladus. See thumbnail images at the following link: Schusteff Images. Two of the visitors (the bee fly Lepidanthrax and a chalcid wasp) were apparently taking nectar. A second bee fly (Mythicomya) spent most of its time probing the anthers and stigma of N. rubescens.
According to Schusteff: "It seems plausible to me that they all may
have been attracted by glistening of the rods...simulating well-charged
nectaries."
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Left: Nemacladus rubescens of the bellflower family
(Campanulaceae). Right: Magnified view of translucent, glistening cells
attached at base of filaments on upper side of gynoecium. These cells
may serve to attract insect pollinators like "pseudonectaries" in the
saxifrage family (Saxifragaceae).
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- The recently discovered cerambycid beetle Onychocerus albitarsis
in Peru is truly one of the most remarkable examples of convergent
evolution (homoplasy). It is described by A. Berkov, N. Rodriguez and
P. Centeno in Naturwissenschaften Vol. 95, March 2008.
Venom-injecting structures have arisen independently in unrelated
arthropods, including spiders, centipedes and antlions. The venom is
injected through hollow fangs (poison jaws), or in the case of
centipedes, through modified forelegs. Among insects only wasps, bees
and ants of the order Hymenoptera are known to possess true stingers.
Microscopic examination of the newly discovered beetle has revealed that
the tip of each antenna is truly a stinging device. In fact, Mr.
Centeno discovered this fact first hand. As he grabbed the beetle, the
insect jerked back its antennae and pricked his finger, which swelled as
if stung by a bee. This is the first example of a stinger in the
enormous beetle order Coleoptera.
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A hypothetical cladogram showing the origin of a similar stinging device
in two distantly related groups of arthropods that is not present in
their nearest common ancestor. These stinging organs are not
homologous. Although they are remarkably similar in appearance and
function, they are structurally quite different. One is the modified
terminal segment of a tail, and the other is from the terminal segment
of a beetle antenna.
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- The terminal antennal segment of Onychocerus albitarsis
has two pores opening into channels leading to the pointed tip through
which the venom is delivered. The delivery system is almost identical
to that found in the stinger of certain scorpions. Since beetles and
scorpions belong to entirely different arthropod orders and are only
distantly related, this is a dramatic example of homoplasy: similarity
due to independent origin that is not from a common ancestor. In this
case the homoplastic characteristics (stinging devices) evolved
independently from each other. Although the article in Naturwissenschaften
uses the term "convergent evolution," one might argue that this is
"parallel evolution." The term homoplasy makes this confusing
distinction unnecessary.
Tail of Arizona desert scorpion (Hadrurus arizonensis)
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Beetles, Stinging Organs,
and Fang-Like Jaws
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20. Homoplasy: Carrion Flowers That Attract Flies & Beetles For Pollination
- Some
of the world's largest and truly bizarre flowers emit odors that
attract flies and beetles. They are often called "carrion flowers"
because they have the overpowering scent of rotting flesh or excrement.
Some even have the color and harriness of a dead animal. Blow flies
lay their eggs in the flowers, but the maggots often perish without a
food supply. They belong to a variety of distantly related families and
are quite different from each other based on morphology and DNA. Since
they have evolved independently they are excellent examples of
convergent evolution, specifically homoplasy. I have dedicated an
entire page to these remarkable flowers at the following link on Wayne's
Word.
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The remarkable starfish flower (Stapelia gigantea), a member of
the family Apocynaceae (subfamily Asclepiadoideae). The carrion stench
attracts green bottle flies of the blowfly family Calliphoridae. The
flies have laid small white eggs on the hairy flower surface; however,
there is no hope of survival for the tiny maggots that will soon hatch.
Although the flower gives off the exact odor of rotting flesh, it
provides no food for the blowfly larvae. The benefit of this clever
carrion masquerqade to the flower is pollination by blowflies, ideally
from another starfish plant.
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Three unrelated families of flowering plants all with bizarre flowers that emit the scent of carrion. Rafflesia arnoldii
is a parasitic flowering plant native to Sumatra. It lives inside the
stem of its host vine, only breaking through the stem surface as a large
bud when it is time to flower. Hydnora africana is a root parasite on shrubby species of Euphorbia in South Africa. The Malaysian-Indonesian) Amorphophallus titanum
grows from an enorous tuber up to 6 feet (2 m) in circumference and
weighing up to 120 pounds (54 kg). The massive "flower" is technically
an inflorescence composed of numerous small, unisexual male and female
flowers. The minute flowers are clustered around the base of the erect
spadix, within the showy, vase-shaped, pleated spathe. The 8 foot
phallus-like spadix of A. titanum rivals the size of a blue whale
penis. It is sometimes called the "stinking corpse lily," although
this descriptive common name is more commonly applied to Rafflesia arnoldii.
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- Flower
"scent mimics" that lure carrion insects into their putrid blossoms are
some of nature's most fascinating and successful experiments in
evolution. They certainly represent some of the most fantastic examples
of floral diversity. Although they include some of the largest and
showiest blossoms in the world, they will probably never be used in a
floral arrangement on your dining room table.
21. Origin Of Eyes In Distantly Related Animals: Is This An Example of Homoplasy?
- In Chapter 6 of The Origin of Species,
Charles Darwin expressed concern over how a complex organ, such as the
eye, could evolve by natural selection. "... if numerous gradations
from a perfect and complex eye to one very imperfect and simple, each
grade being useful to its possessor, can be shown to exist; if further,
the eye does vary ever so slightly, and the variations be inherited,
which is certainly the case; and if any variation or modification in the
organ be ever useful to an animal under changing conditions of life,
then the difficulty of believing that a perfect and complex eye could be
formed by natural selection, though insuperable by our imagination, can
hardly be considered real." In fact, some creationists have taken the
previous quotation out of context and used it in their argument against
evolution be means of natural selection. Like the wings of birds, bats
and insects, many biologists have used the term homoplasy to explain the
independent origin of eyes in diverse groups of animals. In fact,
because the anatomy of the camera-like vertebrate eye is so different
from the simple eyes of limpets and the compound eyes of insects,
scientists thought that eyes evolved independently numerous times.
- Homoplasy
may not adequately explain the origin of eyes in different animal
phyla. Recent evidence indicates that eye-forming genes evolved only
once in a distant ancestor. These primordial eye genes provided the
ancestor of present-day animals with photoreceptor cells and the ability
to detect light. Mutations and evolution brought other genes under the
control of these primordial genes, and together they produced a variety
of eyes from simple to complex. With photoreceptor cells at the base
of a simple eye cavity, mutations and modifications leading to a more
advanced eye are plausible. An explanation for the development of eyes
in different present-day species of mollusks is presented by M.F. Land
and D.-E. Nilsson (2002) in their book Animal Eyes. This fascinating hypothesis is summarized by C. Zimmer in the November 2006 National Geographic.
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Three examples of mollusks in the diverse phylum Mollusca.
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- All
of the following mollusks share a common ancestral trait: A layer of
light-sensitive cells at the base of an eye cavity. In the limpet, this
layer of photosensitive cells in a shallow depression can detect light
but the eye cannot produce an image. Beyrich's slit shell has a deeper
eyecup that provides more information about the direction of the light
source, but creates no image. In the chambered nautilis, a small gap at
the top of the eye cavity acts as a pinhole pupil, focusing light on
photosensitive cells that serve as a rudimentary retina. In Murex,
fluid in a fully enclosed eye cavity functions as a primitive lens,
focusing light on the retina to create a slightly sharper image. In the
common octopus, advanced vision is created by a more complex eye
equipped with a protective cornea, colored iris, and focusing lens.
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Light Gathering Propeties Of The Barn Owl
The barn owl (Tito alba) is remarkably adapted for
night vision. The large, forward-facing eyes give owls the best
stereoscopic (3D) vision of all birds. This is essential for judging
distances, particularly when diving to catch small rodents on the
ground. Because of their tubular shape, the eyes are in a fixed
position; therefore, the owl must move its head rather than its eyes.
In fact, some owls can rotate their head almost 270 degrees in either
direction. The beak is positioned relatively low on the face, keeping
out of the owl's field of view. The retina is very large compared with
other birds and is packed full of light sensitive rods, about 56,000 per
square mm in the Tawny Owl (Strix aluco). These rods are far more sensitive than cones at low light levels. According to World Owl Trust:
"Tawny Owls would appear to have the best developed eyes of all owls,
indeed of all vertebrates, being probably about 100 times more sensitive
at low light levels than our own." The phenomenal light gathering
properties of the owl are further enhanced by the large, reflective,
mirror-like layer (tapetum lucidum) behind the retina. Even though my
camera was about 75 feet away, this reflective layer glows in the flash
image, a phenomenon known as eyeshine.
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22. Punctuated Equilibrium & Natural Selection
- The
term evolution is very broad and has numerous subdivisions,
particularly explanations for the mechanisms of change. Charles
Darwin's On the Origin of Species by Means of Natural Selection
was a testable scientific hypothesis that explained one mechanism of
evolution called natural selection. Alfred R. Wallace also came up with
this hypothesis at the same time period (see "The Man Who Wasn't
Darwin" by David Quammen in December 2008 National Geographic). In fact, Darwin and Wallace coauthored a paper on natural selection for Journal of the Proceedings of the Linnean Society in 1858. The following year, Darwin published his Origin of Species
and his name is usually associated with the theory of evolution.
Natural selection may explain some aspects of evolution; however, there
are other explanations. In the first edition of Origin of Species,
Darwin was careful to acknowledge the limits of natural selection,
writing, "I am convinced that natural selection has been the main but
not the exclusive means of modification." Nevertheless, he was
misinterpreted as claiming that natural selection was entirely
responsible for evolution. In fact, evolution and natural selection
have been used interchangeably by some authors. Other scientific
hypotheses to explain certain mechanisms of evolution include punctuated
equilibrium and symbiogenesis. Punctuated equilibrium was proposed by
Niles Eldredge and Steven Jay Gould in the 1970s. It postulates that
speciation may occur relatively quickly in geologic time, with long
periods of little change (equilibria) in between. Punctuated
equilibrium explains the evolutionary patterns of species as observed in
the fossil record, particularly the sudden appearance of new species in
a geologically short period of time.
- In his book On Naval Timber and Arboriculture
(1831), Patrick Matthew gives precisely the same view as Darwin and
Wallace on the origin of species by natural selection. His main premise
was that Britain's navy needed sturdy ships made from first-class
timber: Nurserymen's artificial selection of seeds would eventually
result in varieties that would not survive well in the wild, thus
weakening the quality of timber. Some scholars believe that Darwin was
aware of Mr. Matthew's work prior to the publication of his first
edition of Origin of Species (1861). Matthew's letter to Darwin stating his claim to natural selection was published in Gardeners' Chronicle: 7 April 1860. In his reply to Matthew ( Gardeners' Chronicle: 21 April 1860), Darwin apologized for his entire "ignorance" of Matthew's publication. In his third edition of the Origin of Species,
Darwin cites Matthew's work under the section entitled "An Historical
Sketch of the Recent Progress of Opinion of the Origin of Species."
"In 1831 Mr. Patrick Matthew published his work on 'Naval Timber and
Arboriculture,' in which he gives precisely the same view on the origin
of species as that (presently to be alluded to) propounded by Mr.
Wallace and myself on the 'Linnean Journal,' and as that enlarged in the
present volume. Unfortunately the view was given by Mr. Matthew very
briefly in scattered passages in an Appendix to a work on a different
subject, so that it remained unnoticed until Mr. Matthew himself drew
attention to it in the cellent history of opinion on this subject ... He
clearly saw, however, the full force of the principle of natural
selection. In answer to a letter of mine (published in Gard. Chron.,
April 13th), fully acknowledging that Mr. Matthew had anticipated me, he
with generous candour wrote a letter (Gard. Chron. May 12th) containing
the following passage:----"To me the conception of this law of Nature
came intuitively as a self-evident fact, almost without an effort of
concentrated thought. Mr. Darwin here seems to have more merit in the
discovery than I have had; to me it did not appear a discovery. He
seems to have worked it out by inductive reason, slowly and with due
caution to have made his way synthetically from fact to fact onwards;
while with me it was by a general glance at the scheme of Nature that I
estimated this select production of species as an à priori recognisable fact----an axiom requiring only to be pointed out to be admitted by unprejudiced minds of sufficient grasp."
23. Symbiogenesis: Genomer Mergers
- Another scientific hypothesis known as symbiogenesis, is described in the book entitled Acquiring Genomes: A Theory of the Origins of Species
by L. Margulis and D. Sagan (2002). According to the authors, genomic
mergers are a major source of genetic variability leading to the
evolution of species. Instead of relying on the hit-or-miss method of
random mutations, symbiogenesis provides advantageous genetic
combinations through the fusion of entire genomes from two or more
organisms. This phenomenon may have been a major factor in the
evolution of land plants from lichen-like ancestors. In 1981, Lynn
Margulis also proposed the endosymbiont hypothesis to explain the origin
of organelles within eukaryotic cells. According to endosymbiosis,
cellular organelles (mitochondria and chloroplasts) originated from
bacteria and cyanobacteria that became incorporated within living cells.
When these original hypotheses have been repeatedly tested and
scrutinized by different scientists, they may become scientific
theories.
- The
arise of photosynthetic organelles called chloroplasts represented a
major step in the evolution and diversification of plant life on earth.
This event had a significant effect on the evolution of animal life
that depended on the plants for food, either directly in the case of
herbivores, or indirectly in the case of carnivores. Chloroplasts
exhibit remarkable similarities with cells of cyanobacteria, and may
have shared a common prokaryotic ancestry. Indeed, the outer membrane
structure and circular DNA molecules of chloroplasts and mitochondria
are very similar to individual prokaroytic cells. According to the
endosymbiont hypothesis (or endosymbiont theory for those who are less
skeptical), ancient photosynthetic prokaryotic cells became incorporated
within the cells of algae or ancestral plants, forming stable
mutualistic symbionts known as chloroplasts. Mitochondria may have had a
similar origin. Without chloroplasts and oxygen-producing
photosynthesis, the amazing diversity of today's plants and animals may
have never evolved.
- According to Margulis and Sagan (2002), there is even a green photosynthetic animal named Elysia viridis,
a minute slug (saccoglossan opisthobranch) that never feeds throughout
its adult life. Instead, it obtains carbohydrate-rich molecules by
bathing in the sunlight. This slug evolved from algae-eating ancestors,
only the algal cells entered the slug's tissue and remained their as
photobionts (photosynthetic symbionts). According to some invertebrate
zoology textbooks, the chloroplasts from algae cells are sucked into the
slug's gut and incorporated within digestive gland cells where
photosynthesis occurs.
- There
are numerous examples of plants and animals that contain microbial
symbionts within their tissues, including bacteria, cyanobacteria,
protozoans and algal cells. Cycads, water ferns (Azolla), legumes and the tropical flowering plant Gunnera
contain nitrogen-fixing bacteria in their tissues; sea anemones and
corals contain photosynthetic unicellular algae (zooanthellae and
zoochlorellae); the rumen of cattle contain cellulose-digesting
bacteria; termite guts contain flagellated protists which contain
wood-digesting bacteria; and human intestines contain bacteria that
produce essential B vitamins.
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The water fern Azolla filiculoides contains colonies of filamentous cyanobacteria (Anabaena azollae)
within cavities in its leaves. [See magnified inset.] These two
organisms once formed an intimate symbiotic marriage many millions of
years ago and have remained together ever since. Azolla provides a protected place for the Anabaena to survive at the surface of ponds and streams in full sunlight. Like other nitrogen-fixing organisms, colonies of Anabaena convert inert atmospheric nitrogen into ammonia, a form of nitrogen that is available to Azolla and other plants. Nitrogen-fixation occurs in oval cells called heterocysts (red arrow).
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- Lichens
are one of the best examples of symbiogenesis involving the fusion of
algal and fungal genomes (kingdoms Protista and Fungi). Some lichens
include the genome of a third kingdom Monera because they contain
prokaryotic cells of cyanobacteria. In the case of lichens, this
genomic merger has enabled these organisms to survive in some of the
most inhospitable environments on earth, where neither symbiont could
survive on its own. In fact, lichens are an excellent example of
synergism because the whole is truly greater than the sum of its parts.
The algal (photobiont) and fungal (mycobiont) components develop into a
unique body form with morphological features quite different from
either symbiont.
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Center: British soldiers (Cladonia cristatella), a soil lichen
with upright stalks (podetia) bearing bright red spore-bearing apothecia
at the tips. At the bottom of the centrifuge tube (A), the fungal
component of this lichen (also named C. cristatella) has grown into a white, amorphous blob without its algal symbiont. In the right test tube (B), the algal symbiont (Trebouxia erici)
has grown into a mass of bright green cells. Only when the genomes of
the two symbionts form the "marriage" known as lichen is the unique
structure of "British soldiers" formed. In true synergistic fashion,
this lichen is truly more than the sum of its parts. For example, the
podetium is a unique lichen structure that is not produced by the algal
or fungal components.
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Some Other Examples Of Synergism In Lichens: Lichens Produce Structures
That Are Not Found In Either Of Its Fungal or Photosynthetic Algal Symbionts
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Podetia are asexual reproductive structures produced by lichens, but not
the algal or fungal symbionts. The podetium bears microscopic granules
called soredia. Each soredium contains fungal hyphae wrapped around
algal cells. The soredia become airborne and carry the lichen symbionts
to different locations where a new colony can grow. This mechanism
appears to be more efficient than starting a new lichen by the chance
encounter of algal cells and fungal spores.
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Soralia are small breaks (pustules) in the lichen thallus where masses
of soredia erupt. The microscopic soredia become airborne and travel to
new locations. Isidea are pimple-like protuberances from the lichen
thallus containing fungal hyphae and algal cells. They readily become
detached and develop into new colonies. Soralia, soredia, podetia and
isidia are unique lichen characteristics.
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- Both
Darwin and Wallace are credited with the scientific hypothesis of
evolution by means of natural selection. This hypothesis has been
tested repeatedly by scientists during the past century and it is now
elevated to the status of a scientific theory. Our knowledge of the
evolution of life on this planet has grown considerably during the past
147 years since the time of Darwin. In fact, the term "Darwinist"
repeatedly used in the intelligent design textbook Of Pandas and People
is misleading. Does Darwinist refer to one who believes in the
teachings of Darwin, or one that follows the modern scientific theory of
evolution that encompasses many new concepts since the time of Darwin?
The latter interpretation of evolution includes substantial information
from the fields of genetics and molecular biology and is often referred
to as neoDarwinism.
24. Coevolution Between The Fig & Its Wasp
A Complex and Remarkable Life Cycle That Defies Oversimplifications and Generalizations
- One
of the most remarkable examples of coevolution between a plant and an
insect is the fig tree and its symbiotic wasp. Minute male and female
fig wasps are borne inside hollow, fleshy, flower-bearing structures
called syconia. [The syconium is what most people associate with the
tasty fruit of a fig, but technically it is not a true fruit.] The
syconium is lined on the inside with hundreds of tiny, apetalous,
pollen-bearing male flowers, long-style, seed-bearing female flowers,
and short-style female flowers. Fig wasps develop from eggs laid inside
the ovaries of the short-style female flowers (one egg per flower). In
about half of the fig species (referred to as monoecious), male flowers
and the long and short-style female flowers occur in the same bisexual
syconium; but in all other fig species (referred to as dioecious or
gynodioecious), the seed-producing, long-style female flowers only occur
in unisexual syconia on female trees, while pollen-bearing male flowers
and wasp-bearing, short-style female flowers occur in the same syconia
on male trees. In the common edible fig (Ficus carica) the male
trees are called caprifigs. [The prefix capri refers to goat, and these
figs were apparently fed to goats.] Wasp eggs are not laid in the
ovaries of long-style flowers because the wasp's ovipositor cannot reach
the ovary; therefore, the ovary develops a seed rather than a wasp
(assuming it is pollinated). Without the pollinator wasps transferring
pollen from one syconium to another, the female flowers inside would not
get pollinated and no seeds would be produced (a catastrophe for the
fig tree). This remarkable floral dimorphism (heterostyly) is how the
fig tree produces seeds (on female trees) while still maintaining its
vital, "in-house" population of symbiotic wasps on male trees. There
are approximately 830 species of figs (genus Ficus), mostly
distributed throughout tropical regions of the world. They all have one
or more pollinator wasps in the family Agaonidae that enter their
syconia through a small opening (called an ostiole) to pollinate the
female flowers inside. Although pollinator wasps are often host
specific, one pollinator can have more than one host (J.M. Cook and
J.-Y. Rasplus, 2003). In addition, the syconium may contain one or more
non-pollinating wasps in different wasp families. Like the symbiotic
pollinator wasps, the nonpollinators have flattened heads and bodies,
and are perfectly adapted to squeeze between the ostiolar bracts of
receptive syconia. This is yet another example of convergent evolution.
Depending on the fig species, pollen is transferred to the female
flowers, either passively or purposively. In the latter case the wasp
actually transfers pollen from pollen baskets (corbiculae) on the
underside of her thorax to the stigma.
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Life cycle of the common fig (Ficus carica). Style length is
genetically determined and it is vital that syconia on seed-bearing
female trees have styles longer than the female wasp's ovipositor.
Unable to reach the ovaries of these flowers, she does not lay eggs
(oviposit). Therefore, a seed develops inside the ovary rather than a
wasp larva. She can only oviposit in the short-style female flowers on
"male" trees called caprifigs. Caprifig trees produce pollen and the
crucial pollinator wasps (Blastophaga psenes). In some common
figs termed "caducous" or early deciduous, the immature female syconium
drops from the tree if the flowers inside are not pollinated. There are
many cultivated "parthenocarpic" varities of the common fig in which
the syconia develop on female trees wthout wasp pollination
(caprification). The ripe syconia are fleshy and edible; however, the
numerous ovaries (drupelets) inside are hollow and seedless.
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- In
short-style female flowers, the wasp inserts her ovipositor down the
stylar canal and lays an egg in the ovary of the flower. The subsequent
larva feeds on endosperm tissue initiated by the ovipositing pollinator
wasp. Since the endosperm in some figs may be initiated
parthenogenetically (without pollination and fertilization), possibly by
a mechanical or chemical stimulus during oviposition, food tissue for
the developing larva functions like a minute gall. Style length is
genetically controlled, and it is important for the tree to have style
lengths longer than the wasp's ovipositor in long-style flowers so that
seeds can develop in these ovaries. It is also important to have
flowers with short styles so that female wasps can lay eggs (oviposit)
in the ovaries. "Bogus" fig wasps (parasitoids and inquilines) with
extra long ovipositors present a formidable problem to figs. They can
readily lay eggs in long-style flowers, and can even penetrate the
syconium wall without pollinating the female flowers inside. Some
dioecious figs can counter this problem by simply aborting unpollinated
syconia, thus ridding itself of seedless syconia. This strategy does
not work on monoecious figs with multiple style lengths in the same
syconium. Dioecious figs may represent an advanced (further evolved)
species.
Overwintering mame crop of syconia.
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A fig wasp larva inside the ovary of a short-style female flower in an
overwintering mamme syconium. This is the leafless dormant season for
the caprifig (Ficus carica). Since the previous mammoni crop of
syconia do not produce pollen, the larva fed on parthenogenetic
endosperm tissue that developed without pollination. The initiation of
endosperm tissue after oviposition by the female wasp meets the
criterion for gall formation. The larva consumes the galled tissue
within the ovary wall. After metamorphosis, the adult male wasp chews a
hole through the ovary wall and exits the female flower. He crawls to
another short-style female flower that contains a mature female wasp.
He climbs up on the ovary of the flower, bites a fertilization hole in
the ovary wall, and inserts his long, slender abdomen into the opening,
thus inseminating the female. After being inseminated, the female
crawls out of the fertilization hole through the ovary wall initially
made by the male. Wasp larva photographed in January 2009.
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- It
should be noted here that some fig species have two or more species of
symbiotic wasp pollinators. In fact, the classic one-fig/one-wasp
partnership has been challenged by D. Molbo, et al. (Proceedings of the National Academy of Sciences 100,
2003). S. G. Compton, et al. (African Entomology, 2009) found three or
more species of pollinator agaonid wasps in the syconia of Ficus natalensis.
Fig wasp species may be closely related sister taxa, or may be quite
different from each other. This indicates both long-term coexistence on
shared hosts and relatively recent colonization of fig species. Fig
syconia may also contain wasps who do not pollinate the female flowers
inside. It is clear that the fig/fig wasp scenario is far more
complicated than originally described. In fact, Richard Dawkins
(personal communication, 1996) agrees that it is a difficult subject to
adequately cover in laymen publications about evolution. There are
literally thousands of articles on this subject with numerous exceptions
and contradictions to any universal fig wasp life cycle pattern.
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A. Close-up view of a male and female fig wasp (Pleistodontes imperialis) that inhabits the syconia of the Australian rustyleaf fig (Ficus rubiginosa).
The slender ovipositor on female wasp is too short to penetrate the
ovary of long-style flowers; therefore she does not lay eggs in these
flowers. The smaller, wingless male has large mandibles and a greatly
reduced body which has two primary purposes: (1) Inseminating the female
and (2) Chewing exit tunnels through the syconium wall through which
the females escape. The "eye" of an ordinary sewing needle is shown for
a size comparison.
B. A non-pollinator "bogus" fig wasp collected from the syconium of the Baja California wild fig (Ficus palmeri or possibly Ficus brandegeei).
The ovipositor is much longer than the symbiotic pollinator wasp. In
fact, some non-pollinator wasps can penetrate the entire syconium wall
from the outside. They can also lay eggs in long-style fig flowers
reserved for fig seeds. Consequently, no seeds are produced in these
flowers. In addition, these "bogus" fig wasps do not pollinate fig
flowers. Although they do not benefit the fig tree, non-pollinator
wasps of the families Torymidae and Eurytomidae are common inhabitants
of New World monoecious fig syconia. Their coexistence with natural fig
pollinator wasps is a complex and perplexing coevolutionary problem in
fig biology.
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- According to Carole Kerdelhue and Jean-Yves Rasplus (Oikos
Vol. 77, 1996), dioecious figs may have evolved from monoecious
ancestral fig species due to selection pressure by non-pollinating fig
wasps. Although these non-pollinator wasps belong to the same order
Chalcidoidea as pollinators, many of them belong to different families.
They do not benefit the fig and may even be harmful, especially when
they compete with and/or parasitize the beneficial pollinator wasps.
The non-pollinator, parasitic wasps never occur in the long-style
flowers of syconia on female trees, and non-pollinator wasps are
uncommon in the syconia of male trees. Therefore, seed production in
syconia on female trees and pollinator wasp production in syconia on
male trees are not diminished, as in the syconia of monoecious figs
containing detrimental non-pollinator wasps. Kerdelhue and Rasplus,
1996) state that no gall-makers that lay eggs through the syconium wall
(after pollination by beneficial pollinator wasps) have ever been found
so far in dioecious figs; however, agaonid wasps of the genus Philotrypesis that oviposit through the syconium wall are well known in the dioecious edible fig (Ficus carica).
In general, having separate male and female trees in the population
may be an adaptive advantage with regards to pollination and seed
production.
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Left: A dioecious fig showing short-style and long-style female flowers
inside syconia of male and female trees. Non-pollinator wasps typically
do not inhabit these syconia.
Right: The monoecious syconium of the South African Ficus sur
contains long-style and short-style female flowers densely packed
together in a layer that lines the inner cavity of the syconium.
Although the styles all form a relatively continuous stigmatic layer
called a synstigma (i.e. all stigmas in the same plane) within the
syconium, the ovaries may be deep or shallow relative to the synstigma
depending on the length of their flower stalks (pedicels). There are
four style lengths (heterostyly) and four ovary positions designated by
different colored ovaries. Generally, the deep-seated black ovaries (on
short pedicels) with long styles each contain a seed, while the shallow
yellow ovaries (on long pedicels) with short styles each contain a wasp
larva. A pollinator wasp walking on this "bed" of styles (synstigma)
can insert her ovipositor down the short style and easily penetrate the
ovary where she lays an egg. The deep-seated, long-style ovaries are
out of reach for her ovipositor (style longer than her ovipositor), and
consequently these ovaries develop seeds rather than wasp larvae. If
the non-pollinating wasps are very numerous, the medium layers 1 and 2
(yellow and green ovaries) will be occupied entirely by exploiters and
these occupied flowers will not produce seeds or pollinator wasps.
According to Kerdelhue and Rasplus (1996), this probably represents a
high cost to the fig with regard to seed production.
The above syconium structure is not the case for all monoecious figs. According to Zhang, et al. (Annals of the Entomological Society of America Vol. 102, 2009), the Asian fig F. curtipes
and relatives (subgenus Urostigma, subsection Conosycea) lack a
synstigma, which is replaced by an irregular mass of elongate stigmas.
The stigmas are unbranched, slightly curved, and only slightly broader
than the style. Instead of walking on a mat of stigma tips (synstigma),
the female wasps frequently stumble and fall between different stigmas
as they attempt to find oviposition sites. The lack of a fused
synstigma enables the wasps to insert their ovipositors at the basal end
of the stigmas, not at the top of stigma as described in other species,
which reduces the length of ovipositor that has to be inserted. Thus,
the ovipositor of the pollinator Eupristina sp. is sufficiently long enough to reach all the ovules. In other figs with unbranched stigmas (subgenus Sycomorus) and figs with branched stigmas (subgenera Urostigma and Pharmacoscea),
oviposition occurs at the top of stigma. Oviposition behavior in fig
wasps is therefore not universal, and is responsive to variation in
floral structure within their host figs. There may indeed be factors
other than style length preventing oviposition in long-style flowers.
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25. Vicarious Selection In Dioecious Figs
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A plausible explanation why pollinator
wasps don't evolve longer ovipositors so they can oviposit in the
ovaries of long-style female flowers. Male and female syconia are
virtually indistinguishable in external appearance. Selection takes
place in male syconia of caprifig.
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- In
his book "Climbing Mount Improbable (1996), Richard Dawkins devotes
Chapter 10 to the fig/fig wasp coevolution and the model for vicarious
selection proposed by Grafen and Godfray (Proceedings of the Royal Society, 1991). In vicarious selection of the dioecious fig subgenus Urostigma,
morphology (style and ovipositor length), and wasp behavior (purposive
loading and unloading of pollen) is taking place in wasps who enter and
leave male syconia containing short-style female flowers on male trees.
This selection is crucial for the perpetuation of fig trees when wasps
enter female syconia on female trees (which superficially resemble male
syconia). Female syconia produce seeds (the vital genetic link for fig
trees) and are a genetic graveyard for wasps because they cannot
oviposit in the long-style female flowers. The female wasps die in
these syconia. For wasps in female syconia, mutations for a longer
ovipositor that could reach the ovary of long-style flowers would not be
passed on. For wasps in male syconia, there is no selective advantage
for longer ovipositors because they are perfectly adapted for laying
eggs in the ovaries of short-style flowers. Vicarious selection does
not explain the evolution of ovipositor length in all figs, particularly
the numerouas monoecious species. The fig/fig wasp scenario is much
more complicated, with many variations in the life cycles depending on
the different subgenera. Unfortunately, it is beyond the scope of this
discussion.
26. Coevolution Between Swollen Thorn Acacia & Acacia Ant
- Another
remarkable example of coevolution between a tree and an insect is the
acacia and its symbiotic ant. Some species of Central American
swollen-thorn acacias lack the chemical defenses of most other acacias
to deal with their predators and competition. Without bitter alkaloids,
ravaging insects and browsing mammals eat the leaves and branches,
slowing the growth of the acacias and allowing fast-growing, competing
vegetation to shade them out. Symbiotic stinging ants have taken over
this vital defense role, protecting the acacia from hungry herbivores
and pruning away competing plants. The ants live inside inflated thorns
(stipular spines) at the base of the leaves. The ants will even clear
away seedlings of fast-growing competing plants around the base of the
acacia. The acacia tree provides housing for the ants in the form of
hollowed-out thorns. It provides nourishment in the form of
carbohydrate-rich nectar from glands on the leaf stalks (petioles). It
also provides protein-lipid Beltian bodies from its leaflet tips. There
is no other known function for these tiny, yellowish morsels other than
to provide food for its ants. Swollen thorn acacias of lowland Central
America include Acacia cornigera, A. collinsii, A. hindsii, A. sphaerocephala, and A. cookii. The ant colonies inhabiting these trees typically belong to the genus Pseudomyrmex.
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Three species of Acacia with swollen stipular spines that are hollowed out and occupied by symbiotic ants. Left: The bullhorn acacia (Acacia cornigera), a swollen-thorn acacia native to Mexico and Central America. In its native habitat, colonies of stinging ants (Pseudomyrmex ferruginea)
occupy the hollowed-out thorns and fiercely defend the tree against
ravaging insects, browsing mammals and epiphytic vines. In return, the
host supplies its little guardian ants with protein-lipid Beltian bodies
from its leaflet tips (yellowish granules in photo) and
carbohydrate-rich nectar from a gland on its petiole just above the pair
of spines (white arrow). Center: Another Central American swollen thorn acacia (A. collinsii) with an acacia ant (P. ferruginea) sipping nectar from the petiolar nectary. Right: The African whistling thorn acacia (A. drepanolobium).
The common name comes from the whistling sound that is produced when
wind blows across the large hollowed-out thorns. Since the "thorns" on
these trees are technically pairs of modified stipules, they are more
correctly referred to as stipular spines. In order to have consistent
monophyleic cladograms based on chloroplast DNA, these three Acacia species of have been moved to the genus Vachellia (See section 12).
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- Nests of at least a dozen bird species have been reported in swollen thorn acacia trees that contain Pseudomyrmex
ants (Janzen, 1969). The relationship between birds nesting in acacias
inhabited by ants appears to be commensal, because ants that protect
acacias against herbivores also offer protection against avian nest
predators (Janzen, 1969, 1983). On the other hand, the birds do not
seem to provide any benefit to the acacias or ants. A symbiotic
relationship in which one organism is benefited while the other is
neither benefited nor harmed is called commensalism. Unlike the
relationship between the fig and fig wasp, the acacia and acacia ant is
probably not a good example of mutualism. Daniel Janzen also reported
predation on the ants by certain birds, primarily by splitting open the
thorns to access larvae and workers. In the latter case, the
relationship between the acacia ant and bird could be considered
parasitism (at least in part), since one member is actually harmed by
the relationship.
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Whistling Thorn Symbiosis May Be One-Sided:
Additional complexities about Africa's whistling thorn (Acacia drepanolobium) are discussed in a recent article by Maureen Stanton and Truman Young in Natural History Volume 108 (November 1999). Studies by Stanton and Young reveal that four species of stinging ants live symbiotically on A. drepanolobium: Crematogaster mimosae, C. nigriceps, C. sjostedti, and Tetraponera penzigi.
Their studies also reveal that the symbiotic relationship between some
of these ant species and their host acacia may not be equally
beneficial to both partners. Since these different species of ants are
rival enemies, they occupy separate trees. If acacia branches
containing rival ant colonies make contact, the different species of
ants will fight each other, with the loser being evicted from its tree.
Unlike the Central American thorn acacias that provide their ant
warriors with protein-lipid Beltian bodies on leaflet tips, the
whistling thorn provides no such service. This forces the ants to leave
the tree to forage for insects and other protein-rich foods which they
bring back to the developing ant larvae living inside the swollen
thorns. According to Stanton and Young, refuse pushed out of the thorn
nests may help to fertilize the tree.
The relationship
between some of these ant species and their acacia is not completely
mutualistic because it may harm the acacia tree. Crematogaster mimosae and C. sjostedti
both tend scale insects that feed on the tree's vascular system,
milking the aphid-like insects for their nutritive honeydew secretions.
In fact, C. sjostedti pays little attention to herbivores that
attempt to feed on the tree. The latter species also nests in hollow
spaces within dead and dying branches, rather than in the swollen
thorns. Colonies even thrive in the stumps of dead trees. To make
matters worse, this species of ant often comes out the winner in battles
with other ant species over the possession of a tree. According to
Stanton and Young, the balance in a once mutualistic relationship has
shifted in favor of one partner (the ant) at the expense of the other
(the tree).
Although not as extreme as Crematogaster sjostedti, the relationship between C. nigriceps
and its acacia host is also one-sided in favor of the ant. This ant
species chews off the tips of growing shoots, including leaf and flower
buds, thus pruning and sterilizing the tree. New branches are allowed
to grow mainly in the proximity of swollen thorns, thus ensuring
nectar-rich petiolar nectaries (glands) on new leaves easily accessible
to worker ants inside the thorns. Over time, this pruning by ants
changes the growth rate and shape of the tree canopy, compared with
trees occupied by other ant species. In addition, pruning is more
radical on sides adjacent to rival ant trees, thus reducing the chance
of contact with branches of rival trees containing more aggressive ant
species.
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27. Hybridization Between 2 Species Of Desert Harvester Ants
Harvester Ant Workers: Hybrids Between Pogonomyrmex rugosus & P. barbatus:
Another Truly Remarkable Example Of Genetic Diversity, Selection & Coevolution
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Sex determination
in the more than 12,000 species of ants is typical of the enormous
insect order Hymenoptera, including bees and wasps. The method is
called "haplodiploidy." Males develop from unfertilized eggs and are
haploid with one set of maternal chromosomes. They are not identical
clones of their queen mother because of crossing over and random
assortment of chromosomes during meiosis (oogenesis). Deleterious
(unfavorable) recessive genes are quickly weeded out in haploid males
because they are expressed and cannont be masked by dominant genes.
Females develop from fertilized eggs and are diploid with two sets of
chromosomes. Some references say that larvae destined to become
sexually mature queens are "well-nurtured," presumably similar to royal
jelly in honey bees; however, other reputable authoritees state that
selection of a queen in some ant species is a lot more complicated and
may involve special eggs destined to become queens (see next paragraph).
In zones of hybridization, Pogonomyrmex harvester ant workers of the southwestern U.S. are hybrids between P. rugosus and P. barbatus.
They possess the best genetic traits of two species. The queen of each
species mates with the males of opposite species. Sexually mature ants
(queens and winged males) are purebreds: They are offspring of queen
and males of the same species. Young queens need to mate with their own
species to produce more purebred queens. They need to mate with the
other species to produce "superorganism" workers. This strategy appears
to be evolutionarily advantageous to both species. For more
information, please refer to articles in bibliography by S.H, Cahan, L.
Keller and T. Schwander (2003-2007).
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Dark harvester ants (Pogonomyrmex rugosus) or possibly hybrids with P. barbatus.
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Comparison Of Pogonomyrmex Hybrid Workers With A Mule
If the workers of
an ant nest can be thought of as the superorganism's body, and the
sexuals (queens & males) can be thought of as the superorganism's
genetic material, it is as though an animal with the body of a mule has
the genetic make-up of a horse and donkey!
Although Pogonomyrmex
hybrid workers form a colony and the mule is a single organism, they
make an interesting comparison. They both involve a cross between two
species that forms a stronger hybrid offspring with the best traits of
its parents. The ant colony of hybrid workers functions as a unit that
could be described as a "superorganism." This is similar to the "Borg
Collective" in Star Strek: The Next Generation.
The female horse
(mare) mates with a male horse (stallion) to produce more male and
female horses. If she mates with a male donkey (jackass) she can
produce a male or female mule. The mule is a sterile hybrid with the
body size of a horse and the sure-footedness and endurance of a donkey.
That is why the mule is essentially a "superorganism" used as a
powerful pack animal. In the case of Pogonomyrmex, the hybrid
"super-ants" are the workers. An original 20 mule team wagon train was
used in 1885 to haul borax from Death Valley to Mojave, a distance of
165 miles. The borax weighed 24 tons and the entire wagon train weighed
36.5 tons (gross weight). The last wagon carried water for the mules
during the hot, 10 day journey across the Mojave Desert. Today, a load
of this size would be pulled by a 600 horse power Kenworth T-2000
tractor with an air conditioned cab!
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In a previous
paragraph I compared ants with the "Borg" of Star Trek, a fictional
cybernetic race controlled by drones. This is probably not an accurate
comparison because the Borg apparently have centralized control by the
"Borg Queen." Ant colonies exhibit "swarm intelligence" with the
collective behavior of a decentralized system. The individual units
(workers & soldiers) are sterile females that operate without a
power hierarchy or permanent leader. The queen's role is basically an
egg machine to reproduce and perpetuate the colony. The individual
worker ants are like cells of an organism, and they are able to
communicate with each other by the release of pheromones. This social
unity is evident in extreme warfare between different colonies, and
explains how colonies recognize "self" from "alien" ants. Recognition
between cells of an organism is also controlled by chemicals.
One of the worst ecological disasters in southern California is the introduction of the Argentine ant (Linepithema humile).
They are a serious pest that tends aphids and scale insects, kills off
the native harvester ants that our coast horned lizard depends upon,
and destroys baby birds in their nest. On a recent trip to Maui I
discovered that Argentine ants in Haleakala National Park are
threatening the native pollinators of the endemic, rare and endangered
silver sword (Argyroxiphium sandwicense ssp. macrocephalum).
Many of our native ant species have been completely eliminated from
urbanized areas of southern California by the Argentine ant. This is
particularly true in areas that are regularly watered, providing damp,
cool habitats for Linepithema. Although these ants are only 3 mm
long, they are very aggressive and quickly annihilate other ant
species, even larger ants with powerful jaws and stings. They overpower
other species by their sheer numbers. Argentine ants in the U.S. are
descendants of original colonizers that entered Louisiana in the late
1890's, as coffee ships from Brazil unloaded their cargo in New Orleans.
U.S. populations are so closely related that different colonies with
multiple queens can literally merge together into supercolonies.
According to Mark Moffett (2011), the biggest of these supercolonies
ranges from San Francisco to the Mexican border and may contain a
trillion individuals!
Winter Ant
(Prenolepis imparis)
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Just when I thought
there was no hope of survival for native California ants within the
range of the Argentine ant supercolony, I came across a little native
called the winter ant (Prenolepis imparis). Workers of this
species may be able to survive the attacks of Argentine ants. I first
noticed these shiny black ants in December 2012 near Palomar College,
hence the common name of "winter ant." According to Trevor R.
Sorrells, et al., Department of Biology, Stanford University (2011),
winter ants secrete a hydrocarbon mixture that is lethal to Argentine
ants, thus providing an effective defense against this invasive species
from South America. Please refer to following link in the
peer-reviewed journal PLOS ONE Volume 6 (4) 2011.
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28. Natural Selection & Survival of the Fittest
- There
are many remarkable examples of natural selection in animals where a
particular adaptation has survival value. Some of these include
camouflage (protective form & coloration), warning coloration and
mimicry. The main evolutionary advantage for these adaptations is to
avoid being eaten by predators long enough to pass on your genes to
future generations, thus perpetuating the species. Consider Batesian
mimicry in a harmless little snake with a banding pattern similar to a
deadly Arizona coral snake (Micruroides euryxanthus). A predator
might not attack this snake because of its coloration. Similar banding
patterns among different species of poisonous coral snakes is termed
Mullerian mimicry.
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Only one of these snakes has the banding pattern of a deadly Arizona coral snake (Micruroides euryxanthus).
Helpful rhyme: "Red bordered by yellow, kills a fellow." If you
guessed correctly on the banding pattern of the poisonous species,
please do not feel overconfident in identifying all coral snakes and
their mimics. The classic rhyme does not apply to the poisonous coral
snake (Micrurus frontalis) native to Brazil.
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A striking South American lanternfly (Phosphora lanternaria).
The enlarged head extension mimics the head of a small alligator. Some
authorities have suggested that the reptilian head may ward off an
attack by potential predators of this harmless, plant-eating insect. It
has also been suggested that the head resembles an unshelled peanut;
however, it is doubtful that any adaptive advantage can be gained by
mimicking a peanut. These mimics are the derivation of the common names
"gator" lanternfly and "peanut" lanternfly.
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Left: A Japanese giant hornet (Vespa mandarinia japonica). With their powerful mandibles, several dozen of these giant hornets can annihilate 30,000 European honey bees (Apis mellifera)
in a few hours by quickly decapitating them. The hornets feed on the
honey bee larvae. European honey bee stings do not phase the giant
hornets and they simply have not evolved any defense against this large
predator. The native Japanese honey bee (Apis cerana japonica)
has evolved a clever strategy for killing this predator if it invades
their nest. The bees do not attack the scout hornet, but instead allow
it to enter their hive. Then a mob of several hundred worker bees
envelop the invader and vibrate their flight muscles, thus raising the
hornet's body temperature by 2 degrees Celsius. The large hornet body
is vulnerable to an increase in temperature and 2 degrees is lethal.
See the following two YouTube videos:
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Leafy sea dragon (Phycodurus eques), one of the most remarkable
examples of camouflage in the animal kingdom. Native to southern
Australia, this fish is difficult to distinguish from leafy seaweeds.
In fact, at first glance it is hard to tell that it is a fish. Sea
dragons belong to the order Solenichthyes, along with sea horses and
pipefish.
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29. Selection In Tardigrades: Are They Over-equipped?
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Tardigrades belong to a remarkable phylum of minute multicellular animals.
They are adapted to extreme conditions, some of which are more severe
than any earth environment. Does their origin defy natural selection?
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- Tardigrades
are microscopic, water-dwelling, segmented animals with eight legs.
Depending on the species, they range in size from 0.1 to 1.5 mm long.
They are called "water bears," in reference to their stumpy legs tipped
with claws, and especially their lumbering gait. More than 1,000
species have been described. They occur throughout the world in some of
the most inhospitable places, from high peaks in the Himalayas (above
20,000 ft) to boiling water near heat vents on the ocean floor, and from
the Arctic tundra to vast ice fields of Antarctica. During severe
environmental conditions that would kill most creatures on earth,
tardigrades roll up into little dehydrated balls called "tuns" where
they survive for extended periods of time. This dormancy phenomenon is
called cryptobiosis (or anabiosis). Their ability to survive in this
desiccated state for long periods is largely dependent on high levels of
the non-reducing sugar trehalose that protects their tissues and
preserves the integrity of intracellular organelles, membranes and DNA.
Tardigrade physiology has led to the discovery of "dry vaccines" that
don't require refrigeration and thus can be delivered and stored at room
temperature. Some species can survive temperatures near absolute zero
(-273° C) where liquids and gasses freeze solid; some can survive
temperatures up to 151° C (304° F); some can live without water for 10
years; some can survive 1,000 times more gamma radiation than other
animals (tardigrades can withstand 570,000 rads of X-ray radiation while
500 rads would kill a human); some can withstand pressures up to 6,000
atmospheres (more than the deepest ocean trench), and some can actually
live for a while in a vacuum. In fact, tardigrades were taken into
earth orbit on the Russian robotic spacecraft FOTON-M3 and survived 10
days in space. Upon returning to earth and hydrated, many actually laid
eggs that hatched normally!
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Lateral (side) view of the exoskeleton of an aquatic tardigrade (Hypsibius
sp.) containing 5 eggs. There are 4 pairs of stout, stumpy legs, each
tipped with several slender claws. The name "water bear" refers to its
deliberate "pawing" sort of locomotion. Thomas Huxley, English
naturalist and good friend of Charles Darwin, gave tardigrades this name
in 1869. Photographed with a Sony W-300 digital camera through an
Olympus laboratory grade compound microscope (400x magnification).
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Trehalose is a disaccharide sugar formed from two
glucose units joined by a 1-1 alpha bond. The bonding makes trehalose
very stable even under extreme conditions of temperature, pressure,
dehydration and pH (acidity). Since it is a nonreducing, it doesn't
react with amino acids or proteins. During long periods of desiccation,
trehalose maintains the integrity of intracellular organelles,
including mitochondria and plasma membranes. Rehydration then allows
normal cellular activity to be resumed without the lethal damage that
normally follows the dehydration/rehydration cycle.
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- Trehalose: The Remarkable Sugar Of Tardigrades
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When tardigrades
dry out, the glucose in their bodies changes into trehalose as they
enter the dormant state of cryptobiosis. It apparently forms a "gel
phase" when cells dehydrate, thus maintaining the integrity of
intracellular organelles (e.g. mitochondria and membranes) by splinting
them in position. The resurrection plant (Selaginella) also
contains trehalose. This remarkable plant of arid desert regions
becomes cracked and desiccated, but rapidly revives and turns green
after a rain. Trehalose also explains how certain crustose rock lichens
can survive for months without water and then quickly revive and become
metabolically active when they receive water.
There are
currently three main hypotheses to explain the mechanisms by which
trehalose sugars stabilize living systems during extreme cycles of
freeze-thaw, heat-cooling and dehydration-rehydration (Pereira, et al.
2004). The Water-Replacement Hypothesis suggests that during
drying, sugars can substitute for water molecules (by forming hydrogen
bonds) around the polar and charged groups present in phospholipid
membranes and proteins, thereby stabilizing their native structure in
the absence of water and preventing ice formation. The Water Entrapment Hypothesis,
in contrast, proposes that sugars concentrate residual water molecules
close to the biostructure, thereby preserving its solvation and native
properties. The Vitrification Hypothesis suggests that trehalose
sugars are good vitrifying agents and protect biostructures through the
formation of amorphous glasses (non-crystalline solids), thereby
preventing denaturation or mechanical damage to cells and tissues.
The enzyme
trehalase breaks trehalose into two glucose molecules. This is the
primary glucose source in insects for the rapid energy requirements of
flight. Humans also posses the enzyme trehalase, although it is not
abundant in most people. Several on-line sites suggest that replacement
of dietary sucrose with trehalose may help to reduce the accumulation
of malformed, clumping proteins in the brain and spinal cord associated
with neurodegenerative diseases, such as Alzheimer's, Parkinson's and
Huntington's. In addition, trehalose has a low glycemic index and
reportedly does not spike the glucose level in diabetics. I have not
verified all these medical claims in reputable medical journals. Could
this be a sweetener that is actually good for you?
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- Advocates
of intelligent design believe that tardigrades defy evolution by
natural selection because they are adapted to extreme conditions, some
of which are more severe than any earth environment. Creationists
maintain that natural selection can only select characteristics
necessary for immediate survival. Consequently, evolution cannot be
expected to "over-equip" species for a host of environments that they
have never faced. Advocates of the panspermia hypothesis believe that
tardigrades had an extraterrestrial origin, although DNA sequencing data
indicate that the phylum Tardigrada (tardigrades) is a sister clade
with the phylum Arthropoda (arthropods), and both phyla (Tardigrada +
Arthropoda) form a sister clade with the phylum Onychophora (velvet
worms).
- A
strong case to support the evolution of "over-equipped" species is
"gene duplication" in diploid organisms that greatly increases genetic
variability. One of the truly remarkable examples of gene duplication
is the antibody mediated immune system of animals. Gene duplication is a
plausible explanation for how organisms can produce antibodies against
different antigens, even synthetic antigen proteins that animals have
never been exposed to. Using this model, animals would not need
separate genes for every antigen they will ever encounter. This
mechanism goes way beyond the simple evolution of adaptations based on
Darwin's theory of evolution by means of natural selection.
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An aquatic tardigrade of the genus Hypsibius. Its length is approximately 184 micrometers (microns), about the same length as the hair follicle mite Demodex brevis.
It is much smaller than a grain of common table salt (NaCl). The
image was enhanced with Photoshop to bring out detail of the claws.
Magnification 400x.
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30. Faith & The Existence Of Coconut Pearls
- An
example of a common theory based on faith is the existence of coconut
pearls, beautiful calcareous stones that allegedly form inside coconuts.
Most records of coconut pearls are second-hand accounts where the
owner never actually saw the pearl within its original coconut.
Published first-hand accounts have been shown to be fraudulent. All
tested coconut pearls have been shown to be pearls and polished shells
from giant clams (Tridacna) of Malaysia. They were carefully
inserted into coconuts to fool the owners, or simply came with
fictitious stories about their origin. In fact, I almost purchased one
from Singapore until I discovered its price of $60,000 U.S. dollars!
Although millions of coconuts are harvested annually, there is no
documented coconut pearl that has survived scientific analysis by
authorities. Without empirical evidence, the existence of coconut
pearls appears to be a myth (Armstrong, 2005 & 2007). Completely
independent of my research, Dr. J.V. Veldkamp of the National Herbarium
of the Netherlands and editor of the prestigious journal Flora Malesiana Bulletin, has also been studying coconut pearls called "mestica calappa" and published a paper on this subject for Flora Malesiana Bulletin in 2002. Like myself, he is now convinced that they are a hoax and is updating his original article in a forthcoming issue of Flora Malesiana Bulletin.
- Some
authors still maintain that coconut pearls exist and continue to
perpetuate this assumption in the literature. They say: "Just because
there is no proof of their existence, does this mean that coconut pearls
do not exist?" A scientist would say: "With the complete lack of
proof for the existence of coconut pearls, the probability of finding
one inside a coconut is extremely unlikely, and their existence appears
to be based on faith rather than objective facts." There are websites
where you can actually purchase "coconut pearls." One site claims that
the authenticity of their "coconut pearls" is based on psychic
verification by a trained shaman. They also state that they cannot
guarantee the authenticity of their "coconut pearls" with 100 percent
certainty, but this "does not mean the pearls and stones are fake." I
suppose it isn't too surprising to see "coconut pearls" for sale on the
Internet since there are also websites offering extraterrestrial real
estate for sale on the moon.
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The "Maharajah coconut pearl." It was discovered on Celebes Island in
the Java Sea and presented to Dr. David Fairchild in 1940. This alleged
"pearl" given to Dr. Fairchild was not in its original coconut, so
there is substantial doubt as to its authenticity.
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31. Propositions For The Origin Of Life On Earth
- Religious
debates over the origin of life are often false dichotomous arguments.
If the scientific explanation is not adequate then the alternative
argument for creation must be true. The problem here is that this
debate is not limited to two alternative arguments. There are several
scientific hypotheses for the origin of life and numerous explanations
for supernatural creation throughout the world. In false dichotomous
arguments, if one side is wrong this does not mean that the other side
is correct. In true dichotomous arguments there are only two choices.
If one side is false then the other side must be true. For example, in a
dichotomous key to the duckweed family, one statement says that roots
are present, while a second statement says that roots are absent. Only
one of these statements is true regarding the duckweed species you are
trying to identify.
- There
are literally thousands of non-scientific theories proposed for the
origin of life, including intelligent design. Supernatural arguments
for the origin of life are not scientific theories because they have no
empirical evidence and cannot be tested or proven. They are based on
faith in a creator (God). Advocates of intelligent design are very
careful not to mention God as the designer. If the "designer" is not
God, then is it an intelligent being from another galaxy? Advocates of
intelligent design also argue that their "theory" has been tested and
the results published in reputable, peer-reviewed scientific journals,
but this is simply not the case. In 2004, an Intelligent Design article
was accepted by the editor of Proceedings of the Biological Society of Washington
without the approval of the Society's governing council. The 250
indignant members of this Society vehemently protested its publication.
See "Creationism's Holy Grail: The Intelligent Design of a
Peer-Reviewed Paper" by Robert Weitzel in (Skeptic Volume 11 (Number 4) 2005, pages 66-69. Although Charles Darwin mentioned a Creator in some editions of his Origin of Species,
there is considerable speculation regarding his religious convictions.
In a letter to Mr. J. Fordyce (1879) Darwin wrote: "... In my most
extreme fluctuations I have never been an Atheist in the sense of
denying the existence of God. I think that generally (and more and more
as I grow older), but not always, that an Agnostic would be the most
correct description of my state of mind." For more information about
Darwin's religious beliefs, please refer to Finding Darwin's God by Kenneth R. Miller, HarperCollins, 1999.
- The following quotations come from the last two paragraphs of Chapter 24 in Darwin's Origin of Species (1859). They do not appear to be the words of an atheist:
"Authors of the highest eminence
seem to be fully satisfied with the view that each species has been
independently created. To my mind it accords better with what we know
of the laws impressed on matter by the Creator, that the production and
extinction of the past and present inhabitants of the world should have
been due to secondary causes, like those determining the birth and death
of the individual. When I view all beings not as special creations,
but as the lineal descendents of some few beings which lived long before
the first bed of the Silurian system was deposited, they seem to me to
become ennobled. ..."
... There is grandeur in this view of
life, with its several powers, having been originally breathed [by the
Creator] into a few frms or into one; and that, whilst this planet has
gone cycling on according to the fixed law of gravity, from so simple a
beginning endless forms most beautiful and most wonderful have been, and
are being, evolved."
"By the creator" was inserted into this last paragraph of Origin of Species in the 2nd edition (1860).
- As
I stated above, Darwin expressed his agnostic state of mind in a letter
to Mr. J. Fordyce in 1879 (K.R. Miller, 1999). However, in his
autobiography he expressed a theistic point of view. In his old age
Darwin wrote down his recollections for his own amusement and the
interest of his children and their descendents. He completed the main
narrative of 121 pages in 1876, but added 67 pages of addenda during the
last six years of his life. The following quotation comes from page 60
of The Autobiography of Charles Darwin 1809-1882 published by his grandaughter Nora Barlow in 1958:
... ----Believing as I do that
man in the distant future will be a far more perfect creature than he
now is, it is an intolerable thought that he and all other sentient
beings are doomed to complete annihilation after such long-continued
slow progress. To those who fully admit the immortality of the human
soul, the destruction of our world will not appear as dreadful. Another
source of conviction in the existence of God, connected with the reason
and not with the feelings, impresses me as having much more weight.
This follows from the extreme difficulty or rather impossibility of
conceiving this immense and wonderful universe, including man with his
capacity of looking far backwards and far into futurity, as the result
of blind chance or necessity. When thus reflecting I feel compelled to
look to a First Cause having an intelligent mind in some degree
analogous to that of man; and I deserve to be called a Theist."
- Many
evolutionists are not atheists in any sense of the word. Some prefer
to be called naturalists. Dr. Greg Graffin, lead singer of the punk
band Bad Religion and coauthor of the book Anarchy Evolution (2010) has problems with the term "atheist."
"It defines what someone is not
rather than what someone is. It would be like calling me an
a-instramentalist for Bad Reliogion rather than the band's singer.
Defining yourself as against something says very little about what you
are for. ...There's another problem with defining yourself in opposition
to a particular worldview. Because atheism is defined through
negation, it's never clear which meaning of "God" one opposes. Some
believers revere an interventionist God who regularly influences
physical events. Others believe that God rarely if ever exerts any
influence over human affairs. Some people believe that God is evident
in nature, while others believe that the existence of God can be
revealed only through supernatural revelation. Many people believe in
more than one god or even in a vaguely defined "spirituality" that does
not require the existence of a specific god or gods."
- The
development of life from self-replicating organic molecules in the
original "primordial soup" is a scientific hypothesis called biopoiesis.
Scientists have created the molecular building blocks of life from
gasses in a primitive earth atmosphere. They have also created what
appear to be the precursors of cells; however, the precise mechanism and
biochemical pathway for biopoiesis remain hypothetical. If one
hypothesis does not adequately explain the origin of life, this does not
mean that an alternative hypothesis is necessarily true. In fact,
there are several hypotheses for the origin of life on earth, including
an extraterrestrial origin. One of these is called the panspermia
hypothesis which states that the earth was "seeded" by extraterrestrial
prokaryotic cells similar to archaebacteria that were carried to earth
by meteors. Under a strict definition, a hypothesis must be testable
and verifiable before it becomes a scientific theory. Will a scientific
theory be developed to explain the origin of life? Only time will
tell.
- In
the classic spark discharge experiments by Stanley Miller and Harold
Urey (1953 and 1959), amino acids were formed in a primitive, non-oxygen
earth atmosphere containing water, methane, ammonia and hydrogen;
however, the reducing atmospheric conditions used in this experiment are
not consistent with the earth's primitive atmosphere (Kasting and
Catling, 2003). Early earth probably had an atmosphere dominated by
carbon dioxide like our neighboring planets Venus and Mars.
Furthermore, it is is difficult to synthesize prebiotic compounds in a
non-reducing atmosphere containing oxygen (R. Stribling and S.L. Miller,
1987). Another electric discharge mixture using hydrogen sulfide
instead of hydrogen was tested by Miller in 1958 but never reported.
Eric Parker and his colleagues analyzed this mixture in 2011 using state
of the art high performance liquid chromatography and time-of-flight
mass spectroscopy. Their test samples contained a large assortment of
amino acids and amines, including numerous sulfur amino acids.
According to Parker et al., the hydrogen sulfide gas mixture may not
have been ubiquitous throughout the primitive atmosphere; however, it
may have been prominent on a regional scale near volcanoes. Their
results suggest that a mixture of oxidized and reduced gases, including
hydrogen sulfide, may have aided the synthesis of amino acids and amines
on the primitive earth.
32. Alkaline Vents On The Ocean Bottom: Ideal Hatcheries For Life
- Biochemist Nick Lane discusses the origin of life in chapter one of his book Life Ascending: The Ten Great Inventions of Evolution
(2009). According to Dr. Lane, the problem with the "primordial soup"
described by other authors is that it is thermodynamically flat:
"Nothing in the soup particularly wants to react, at least not in the
way that hydrogen and oxygen want to react. There is no disequilibrium,
no driving force to push life up, up, up the very steep energetic hill
to the formation of truly complex polymers, such as proteins, lipids,
polysaccharides, and most especially RNA and DNA. The idea that
replicators like RNA were the first figments of life, predating any
thermodynamic driving force, is, in Mike Russell's words, 'like removing
the engine from an automobile and expecting the regulating computer to
do the driving.' But if not from a soup, where did the engine come
from?"
- Lane
describes in detail a place where life on earth could have evolved:
The mineral rich hydrothermal deep sea vents along oceanic rifts where
all the precursors for biological molecules occur naturally, and where a
plausible energy source exits to synthesize the polymers of life. Some
of these vents have been called "black smokers" because of broiling
metal sulphides welling up from the magma furnace below, reaching
temperatures of 400 degrees Celsius before precipitating in the cold
ocean waters.
- A
second type of hydrothermal vent called "alkaline vent" has
temperatures much more conducive to life. It was originally proposed by
Mike Russel (see Russel & Martin, 2004). Unlike the acidic black
smokers, this second type of vent arises from serpentine bedrock and is
alkaline. It has a steady supply of hydrogen gas that reacts with
carbon dioxide to form organic molecules. Microscopic examination
reveals that these vent chimneys are composed of a labryinth of
interconnecting compartments which retain and concentrate any organic
molecules formed, making the assembly of polymers, like RNA, far more
likely. Without the microscopic confines of micropores, newly formed
monomers might disperse into the oceans and not polymerize into the
macromolecules of life. In the early days of life, 4 billion years ago,
the oceans were loaded with dissolved iron. According to Mark Lane
(2009), "the microcompartments would have had catalytic walls, composed
of iron-sulphur minerals, like the fossil vents at Tynagh, in Ireland.
They would have worked, in fact, like natural flow reactors, with
thermal and electrochemical gradients circulating reactive fluids
through catalytic compartments."
"A rocky labyrinth of mineral cells, lined with catalytic walls composed
of iron, sulpur and nickel, and energized by natural proton gradients.
The first life was a porous rock that generated complex molecules and
energy, right up to the formation of proteins and DNA itself."
- Mark
Lane refers to electrochemical gradients many times in his book. This
may be defined as ions (usually protons) moving across membranes. A
simplified explanation is the production of energy from anaerobic
methane bacteria (Archaea): The electrons and H+ ions (protons) from
hydrogen gas are used to reduce carbon dioxide (CO2) to methane (CH4). In the reaction, the H+ ions combine with the oxygen from CO2 to form water (H2O).
During this process, the electrons are shuttled through an anaerobic
electron transport system within the bacterial membrane which results in
the phosphorylation of ADP (adenosine diphosphate) to form ATP
(adenosine triphosphate). ATP is the vital energy molecule of all
living systems which is absolutely necessary for key biochemical
reactions within the cells. In fact, varnish bacteria living on
sun-baked boulders make their ATP in a similar fashion, only the
electrons are coming from the aerobic oxidation of iron and manganese.
During the oxidation process, the electrons are shuttled through an
iron-containing cytochrome enzyme system on the inner bacterial
membrane. The actual synthesis of ATP from the coupling of ADP
(adenosine diphosphate) with phosphate (PO4)
is a lot more complicated and involves a mechanism called chemiosmosis.
The electron flow generates a higher concentration (charge) of
positively-charged hydrogen (H+) ions (or protons) on one side of the
membrane. When one side of the membrane is sufficiently "charged,"
these protons recross the membrane through special channels (pores)
containing the enzyme ATP synthetase, as molecules of ATP are produced.
In eukaryotic cells, including the cells of your body, ATP is produced
by a similar process within special membrane-bound organelles called
mitochondria. In fact, some biologists believe that mitochondria (and
chloroplasts) within eukaryotic animal and plant cells may have
originated from ancient symbiotic bacteria that were once captured by
other cells in the distant geologic past. This fascinating idea called
the "Endosymbiont Hypothesis was originally proposed by Lynn Margulis.
See Acquiring Genomes: A Theory of the Origins of Species by L. Margulis and D. Sagan (2002).
- Alkaline
vents have the raw materials and necessary electrochemical gradients
(proton gradients) for most of the above biochemical reactions of life.
The electrochemical gradient between the alkaline vent fluid and the
acidic seawater leads to the spontaneous formation of acetyl phosphate
and pyrophosphate, which act just like adenosine triphosphate (ATP), the
chemical that powers all living cells. These molecules drove the
formation of amino acids--the building blocks of proteins, and
nucleotides--the building blocks of DNA and RNA. According to Nick Lane
and other researchers, life is inevitable on planets with oceans of
acidic water and alkaline vents during millions of years of natural
selection.
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When phosphates are transferred to other
molecules, a process known as phosphorylation, the phosphorylated
molecules have a much lower activation energy and react at lower
temperatures. This is how all of the remarkable biochemical reactions
can occur within cells of your body. For example, the relatively stable
glucose molecule becomes glucose-phosphate during cellular respiration.
It becomes more reactive and can be enzymatically broken down into
carbon dioxide and water within seconds, thus releasing its electrons
into the cytochrome enzyme system (electron transport) within
mitochondria. Phosphate donors like acetyl phosphate and pyrophosphate
must have occured early in the origin of life in order to produce
complex macromolecules.
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Serpentine (technically called serpentinite) is a greenish, shiny rock
that is exposed throughout the Coast Ranges of central and northern
California, and the Sierra Nevada foothills. The name is derived from
its resemblance to a snake skin. According to the California Geological
Survey, serpentinite has been designated California's official state
rock. Serpentinite is a magnesium silicate rock with a waxy luster and a
shiny, marblelike appearance. It varies from cream white through all
shades of green to black. Higher grade, deeply-colored serpentinites
are used for animal carvings, particularly in Africa. Some polished
serpentinites resemble jade in color and are used in pendants and rings.
Serpentinite is formed on the sea floor at tectonic plate boundaries by a
process called serpentinization: A hydration and metamorphic
transformation of untramafic (igneous) rock from the Earth's mantle.
This is a geological low-temperature metamorphic process involving heat
and water in which low-silica mafic and untramafic rocks are oxidized
(anaerobic oxidation of Fe 2+ by the protons of water leading
to the formation of hydrogen gas) and hydrolyzed with water into
serpentinite. It is only seen on land in subduction zones where oceanic
rocks are preserved. Serpentinite is low in plant nutrients and high
in toxic metals. Serpentinite outcrops in California often contain many
species of rare endemic plants adapted to this rock type, including
several species of cypress (Cupressus = Hesperocyparis).
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- Nick
Lane has published several scholarly articles on the origin of life in
alkaline vents, including "Bioenergetics and the Probability of Life" (Journal of Cosmology
Volume 10, 2010). A detailed discussion of his paper is beyond the
scope of this page on evolution. The following summary is quoted from
his article with Michael Le Page in New Scientist (14 October 2009):
How Life Evolved: 10 Steps to the First Cells
by Nick Lane and Michael Le Page (14 October 2009)
- "Water percolated down into newly formed rock under the seafloor,
where it reacted with minerals such as olivine, producing a warm
alkaline fluid rich in hydrogen gas, sulphides and other chemicals - a
process called serpentinisation. This hot fluid welled up at alkaline
hydrothermal vents like those at the Lost City, a vent system discovered
near the Mid-Atlantic Ridge in 2000."
- "Unlike today's seas, the early ocean was acidic and rich
in dissolved iron and carbon duioxide. When upwelling hydrothermal
fluids reacted with this primordial seawater, they produced carbonate
rocks riddled with tiny pores and a "foam" of iron-sulphur bubbles."
- "Inside the iron-sulphur bubbles, hydrogen reacted with
carbon dioxide, forming simple organic molecules such as methane,
formate and acetate. Some of these reactions were catalysed by the
iron-sulphur minerals. Similar iron-sulphur catalysts are still found at
the heart of many proteins today."
- "The electrochemical gradient between the alkaline vent
fluid and the acidic seawater leads to the spontaneous formation of
acetyl phosphate and pyrophospate, which act just like adenosine
triphosphate or ATP, the chemical that powers living cells. These
molecules drove the formation of amino acids--the building blocks of
proteins--and nucleotides, the building blocks for RNA and DNA."
- "Thermal currents and diffusion within the vent pores
concentrated larger molecules like nucleotides, driving the formation of
RNA and DNA--and providing an ideal setting for their evolution into
the world of DNA and proteins. Evolution got under way, with sets of
molecules capable of producing more of themselves starting to dominate."
- "Fatty molecules coated the iron-sulphur froth and
spontaneously formed cell-like bubbles. Some of these bubbles would have
enclosed self-replicating sets of molecules--the first organic cells.
The earliest protocells may have been elusive entities, though, often
dissolving and reforming as they circulated within the vents."
- "The evolution of an enzyme called pyrophosphatase, which
catalyses the production of pyrophosphate, allowed the protocells to
extract more energy from the gradient between the alkaline vent fluid
and the acidic ocean. This ancient enzyme is still found in many
bacteria and archaea, the first two branches on the tree of life."
- "Some protocells started using ATP as well as acetyl
phosphate and pyrophosphate. The production of ATP using energy from the
electrochemical gradient is perfected with the evolution of the enzyme
ATP synthase, found within all life today."
- "Protocells further from the main vent axis, where the
natural electrochemical gradient is weaker, started to generate their
own gradient by pumping protons across their membranes, using the energy
released when carbon dioxide reacts with hydrogen. This reaction
yields only a small amount of energy, not enough to make ATP. By
repeating the reaction and storing the energy in the form of an
electrochemical gradient, however, protocells "saved up" enough energy
for ATP production."
- "Once protocells could generate their own
electrochemical gradient, they were no longer tied to the vents. Cells
left the vents on two separate occasions, with one exodus giving rise to
bacteria and the other to archaea."
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Building Blocks Of DNA From Outer Space?
- One
of the most interesting pieces of evidence for an extraterrestrial
origin of complex biological molecules is the discovery of DNA bases
(nucleobases) in carbon-rich meteorites (carbonaceous chondrites) from
Antarctica. According to Dr. Michael Callahan of NASA's Goddard Space
Flight Center (2011): "For the first time, we have three lines of
evidence that together give us confidence these DNA building blocks
actually were created in space." Callahan is lead author of a paper on
the discovery appearing in Proceedings of the National Academy of Sciences,
2011. Extracts from carbon-rich meteorites were analyzed by liquid
chromatography and mass spectrometry to determinine the chemical
compounds and their stuctural formulas. Adenine and guanine, two of the
four DNA bases of life, were isolated plus a variety of nucleobase
analogs not found in living organisms. These nucleobase analogs are
important because they would not be expected from terrestrial
contamination. In fact, none of them were found in the nearby ice. The
well-studied Murchison meteorite in Australia also contained nucleobase
analogs and over 100 amino acids (Zita Martins, et al. 2008). These
nucleobases were synthesized in non-biological reactions in the
laboratory from chemicals present in asteroids, including hydrogen
cyanide, ammonia and water. According to N. Lane, J. Allen and W.
Martin (2010), the presence of complex biological molecules in
meteorites does not mean the life must have arisen in outer space.
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Meteorite fragment from Arizona and illustration of 4 DNA nucleobases: Adenine (C5H5N5), thymine (C5H6N2O2), cytosine (C4H5N3O) and guanine (C5H5N5O).
For those counting atoms in the above molecules, each one is missing
one green hydrogen because it is on the reverse side and not visible.
Two purine bases, adenine and guanine, were found in carbon-rich
Antarctic meteorites plus a variety of nucleobase analogs. If complex
biological molecules were formed in space, were the building blocks for
life on earth seeded from meteorites? It is intriguing to ponder DNA
nucleobases (the rungs of DNA ladders) traveling through space inside
meteorites!
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Simplified model of
a small section of ladder-shaped DNA. The "rungs" are composed of
nucleobase pairs. Adenine (A) always pairs with thymine (T) and guanine
(G) always pairs with cytosine (C). The "rails" are composed of
alternating phosphates (P) and deoxyribose sugars (D). Although some
people doubt whether these chemicals could have formed in a primitive
earth atmosphere, nucleobases have been discovered in carbon-rich
meteorites that reached the earth.
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33. The Danger Of Imposing Non-Science Dogma In Science Courses
- Advocates
of intelligent design would like to see their dogma taught alongside
evolution in science courses. As I have stated in this report,
intelligent design is not a scientific theory. It is not based on
empirical evidence and cannot be tested or proven. It is dangerous to
impose non-scientific dogma in science courses. One case in point is
the teaching of acquired characteristics in the Soviet Union between
1948 and 1964. This hypothesis was proposed by the French biologist
Jean-Baptiste de Lamarck in the early 1800s, and was actually proposed
thousands of years earlier by Greek scholars. According to acquired
characteristics, the environment can bring about inherited change. One
classic example given by Lamarck is that the long neck of the giraffe
developed over time because animals stretched their necks to browse high
in trees and then passed on the propensity for a longer neck to their
offspring. Although the environment is a factor in evolution, the
mechanism of acquired characteristics has been thoroughly disproved
during the last century: Phenotypic changes acquired during an
organism's lifetime do not result in genetic changes that can be passed
to subsequent generations. Potential gene-bearing gametes are set aside
in the form of mother cells (oocytes and spermatocytes) early in an
animal's embryonic development. The long neck of the giraffe is
explained by genetic variabilty and selection for longer-necked
offspring over thousands of generations. By the mid 1800s both Charles
Darwin and Alfred Russel Wallace independently came up with their
hypotheses of "survival of the fittest" to explain the origin of species
by natural selection; but it was the Austrian Monk Gregor Mendel who in
1865 first described a mechanism to explain variability and the
transmission of traits from parents to offspring. The story behind the
teaching of acquired characteristics in the Soviet Union is a good
example of why politics and religion should not interfere with
scientific research.
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Evolution of the giraffe's long neck is
still debated by authorities. The giraffe doesn't have special genes
for a long neck. Like the mouse, genes controlling neck growth during
embryonic development may be switched on for a longer time, so the
giraffe ends up with a longer neck. As Richard Dawkins points out in
his book Climbing Mount Improbable (1996), the long neck
of a giraffe doesn't require a macromutation. There are two main
hypotheses for the origin of long necks. 1. Giraffes have a
competitive advantage in reaching higher foliage that is out of reach
for other browsing herbivores; however, research indicates that giraffes
also feed on lower branches (at or below shoulder height) during the
drought season when browsing competition should be the highest. 2. The
long neck evolved as a result of sexual selection (R.E. Simmons and L.
Scheepers, 1996). Long-necked males have an advantage in "necking" when
they strike a male opponent with their head. The winner of these duels
has greater access to a female in estrus. Longer necks and heavier
heads of males may be an advantage in "necking," but it doesn't explain
the long necks of female giraffes (G. Mitchell, S.J. van Sittert and
J.D. Skinner, 2009).
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- In 1948, a decree of the Presidium of the Soviet Academy of Sciences appeared in Pravda.
It stated that communists must teach and say that acquired
characteristics are inherited. All research in agriculture and biology
was controlled by Trofim Lysenko, the "dictator of genetic research."
Refusal to follow acquired characteristics resulted in the dismissal,
exile and execution of a number of Russian geneticists. During the late
1940s and 1950s, the Soviet Union fell behind the rest of the world in
genetics research and there was a gradual failure in Soviet agriculture.
Shortly after the downfall of Nikita Kruschev in 1964, Lysenko was
dismissed from his administrative position. Soviet research in acquired
characteristics was exposed as a fraud and the translation of western
textbooks containing Mendelian genetics was ordered. The imposition of
non-scientific dogma in the Soviet Union for a period of almost two
decades resulted in a missing generation of trained geneticists.
Politics and religion should not interfere with scientific research or
the teaching of science in our schools.
- As I have stated above in this essay, the K-12 general biology supplement Of Pandas and People
contains some misleading information. It also makes some very
simplistic comparisons that are inappropriate for an academic science
course designed to prepare young minds for the future of science and
technology. For example, if we see the words "John Loves Mary" written
in the sand, we know that this message was written by an intelligent
designer called Homo sapiens sapiens. Then why can't a message
written in the DNA of our genes also be made by an intelligent designer?
This is another example of an oversimplified comparison. We know from
experience that people write messages in the sand. Probably everyone
has done this at one time in their life. But we can only hypothesize
about the origin of coded messages in DNA, or for that matter, the
origin of DNA. We can say that DNA is so marvelous and complex that is
must have come from an intelligent designer, but now we are basing our
conclusion on faith, not science. That is why intelligent design does
not belong in a science class. Although proponents of intelligent
design are very careful not to specify who their "designer" is, the
logical conclusion is that it must be God. We have freedom of religion
in the United States guaranteed by the First Amendment of the
Constitution. It is not wrong to believe that our God is the
intelligent designer; however, explanations of the natural world based
on faith just don't belong in a science class.
- During
the famous Dover School Board Trial of fall 2005, proponents of
intelligent design argued that their "theory" was not based on biblical
creationism. They even claimed that their recommended supplemental
biology textbook Of Pandas and People (2nd. Edition, 2004)
did not center around creationism. The latter testimony was repudiated
by Dr. Barbara Forest after she discovered early drafts of the textbook
with the words "creation" instead of intelligent design. In one draft
(pp. 3-41), the word "creationists" was incompletely replaced by the
words "intelligent design." It is clear that intelligent design is
synonymous with creationism.
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Biology and Creation (1986): Evolutionists think the former is correct, creationists accept the latter view."
Biology and Origins (1987): Evolutionists think the former is correct, creationists accept the latter view."
Of Pandas and People (1987): Evolutionists think the former is correct, creationists accept the latter view."
Of Pandas and People (1987): Evolutionists think the former is correct, cdesign proponentsists accept the latter view."
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- The
Biophysical Society is an international society of scientists
established to encourage development and dissemination of knowledge in
biophysics. The following comes from their concluding paragraph
regarding the teaching of alternatives to evolution in K-12 science
classrooms (November 5, 2005): "Attempts to suppress or compromise the
teaching of evolutionary science in the United States are misguided
actions that will deprive our youth of a clear understanding of the
scientific process, and of the scientific skills that they need to
compete in a global economy: one that is increasingly driven by science
and technology. Moreover, current efforts to disguise theology as
science do a severe disservice to the scientific profession and to the
people of the United States."
- Since
intelligent design is not science, it does not belong in the science
curricula of the nation's primary and secondary schools. This is the
position statement of the following prestigious scientific
organizations: National Academy of Sciences, American Association for
the Advancement of Science, National Science Teachers' Association,
American Geophysical Union, American Chemical Society, American
Association of Physics Teachers, and the American Astronomical Society.
All of these organizations emphasize the importance of scientific
methodology as well as articulating well-established scientific
theories.
34. Evolution Of Macromolecules & Cells by Random Probability
- Some authors have used the extrapolation of Darwin's Origin of a Species by Means of Natural Selection
to include the first macromolecules and living cells, although this has
never been proven through rigorous scientific analysis. They have even
speculated that macromolecules and living cells evolved purely by
random chance. A functional protein may contain more than 500 amino
acids arranged in a specific orderly sequence with a unique
3-dimensional structure. The probability for the 2-dimensional
arrangement of 500 amino acids is astronomical. It is equivalent to 1
in 20500 or roughly 1 in 10650. Some models of the visible universe list 1080
for the total number of subatomic particles (or electrons), depending
on the reference. William A. Dembski, a staunch proponent of
intelligent design has come up with his universal probability bound, a
numerical value that gauges the likelihood that a given event could have
occurred by chance in nature, or whether it occurred by intelligent
design (i.e. by a natural or supernatural intelligence). Dembski's
probability bound is a cutoff point between probability and a "creator."
It is based on a very improbable number (1 in 10150),
derived from the inverse of the product of several astronomical
numbers, including the number of subatomic particles in the universe.
Whether this number is the cutoff point between random probability and a
creator is impossible to prove. For an intelligent review of Dembski's
mathematical explanation for intelligent design, please read "The Dream
World of William Dembski's Creationism" by Mark Perakh in Skeptic Volume 11 (Number 4) 2005, pages 54-65.
- Given
the plausible conditions and energy forces of a primitive earth
atmosphere, the origin of life was probably a lot more likely than
simple random probability. Some of these "natural" conditions and
forces may have included biological compounds in a "primordial
soup," possibly seeded by carbon-rich meteorites; shallow pools lined
with clay particles bearing electrostatic charges; heat and electrical
discharges (lightning); and a time window of countless million of years.
Whether a supernatural force was also involved in this process is
perhaps one of the most controversial topics ever discussed by educated
people.
35. Did All Life On Earth Evolve From A Common Ancestor?
- All
life on earth is determined by the same genetic code consisting of the
four bases Adenine, Guanine, Cytosine and Thymine. The fact that we
also share some of the same DNA sequences (genes) as eukaryotic cells is
strong evidence that we are all related. The probability that
identical genes evolved independently in diverse organisms is
mathematically too unlikely. The following explanation is summarized
from Eric Roston's fascinating book entitled The Carbon Age. The thermophilic bacterium Aquifex aeolicus
lives in hot springs at Yellowstone National Park in water that is
nearly boiling at 95° C (203° F). This rod-shaped bacterium 5
micrometers in length has a genome containing 1,551,335 base pairs.
Since each base pair has four possible arrangements (see paragraph #15
above), the total number of different DNA combinations for this minute
organism is 41,551,335. Thats four multiplied by itself 1,551,335 times. Compare this number with 1080 or 4133,
the total number of electrons in some models of the visible universe.
The DNA in a human nucleus contains at least three billion base pairs or
4 3,000,000,000 different
sequences. "These numbers are so preposterously large that the
likelihood for random overlap is all but mathematically zero. So if
human beings and A. aeolicus have any genes--just one--in common, they are not random, and humans and A. aeolicus share a genetic code descended from a single ancestor." In fact, we actually share several dozen genes with A. aeolicus.
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A boiling hot spring in Yellowstone National Park containing Aquifex aeolicus, photosynthetic cyanobacteria, and Thermus aquaticus,
a heterotrophic bacterium that survives on minute amounts of organic
matter in the water. The latter species is the original source of TAQ
polymerase used in the amplification of DNA using the polymerase chain
reaction (PCR). Scientists from throughout the world are studying the
amazing bacteria flora at Yellowstone National Park. This is one of the
best places on earth to study these organisms in their natural
protected habitats. In other parts of the world, similar hot springs
have been destroyed for the production of geothermal energy. Life as we
know it may have first arisen more than three billion years ago in a
high temperature environment of boiling water. Thermophilic bacteria in
hot springs of Yellowstone National Park may be relict populations of
the first life on earth. In fact, these thermophilic bacteria may be
the ancestors of all other life forms, including humans.
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36. Addendum: Origin Of The Magnificent Grand Canyon
- Standing
on the rim of this enormous canyon and gazing out at the colorful
strata representing over a billion years of geologic time, you realize
the brief life span of a mortal human. Was this magnificent canyon
created in its present form by God, or was it formed during millions of
years of sedimentation, uplifting and erosion? If it was underwater
during the worldwide biblical flood, how did all the animals become
stratified into layers dating back to the Cambrian period over 500
million years ago? Were all these creatures treading water at the same
time during the great flood, only to die and settle out in layers?
- Fossil
evidence indicates that numerous species of animals lived during
different periods of time. As layers of sediments were deposited,
animals from each time period were recorded in the strata as fossils.
About 40-70 million years ago, the Grand Canyon region was uplifted into
a high plateau during a tremendous mountain building era known as the
Laramide Orogeny. This massive uplifting gave rise to the Rocky
Mountains and the origin of the Colorado River drainage. During the
past six million years, the Colorado River has cut through this high
plateau region, as it winds its way to the low Colorado Desert and
eventually to the Gulf of California. The river's steep gradient of 8
feet per mile (1.5 m per km) and high sediment load contributed to its
cutting power. Rockfalls, landslides, flash floods from torrential
rains, and ongoing erosion in tributary canyons, have worked in tandem
with the Colorado River to widen the canyon and form the intricate cliff
and slope pattern seen today.
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Trilobites such as these Elrathia kingi from Utah lived in
shallow Cambrian seas. Trilobite fossils are also present in the lower
sedimentary strata of the Grand Canyon. They are often placed in the
class Trilobita within the phylum Arthropoda. They flourished during
the Cambrian period over 500 million years ago. The last of the
trilobites disappeared in the mass extinction at the end of the Permian,
about 250 million years ago. They are a famous and well-known fossil
group, possibly second only to the dinosaurs.
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- There
is overwhelming evidence from paleontology and geology that the Grand
Canyon's strata and all of its ancient life evolved during the past 550
million years. To suggest that it was created in its present form, or
that all of this occurred during the last 6,000 years as stated in the
Bible, is based purely on faith. Some people with a much broader
interpretation of the Bible believe their faith does not conflict with
the objective scientific evidence.
37. True or False Summary Statements
- A scientific theory is an explanation for the cause or causes of
complex natural phenomena based on observable facts and rigorous tests
that have been repeatedly verified by scientists.
- Scientific theories are generally more complex and dynamic
than scientific laws, and they may be changed as the body of
experimental data and analysis develops.
- Scientific laws are strictly empirical and explain a
single action or set of actions; they can sometimes be expressed in
terms of a single mathematical equation.
- Hypotheses are tentative explanations or propositions
about the causes of natural phenomena; they are not elevated to the
status of scientific theories and scientific laws without rigorous
testing and review by scientists.
- Scientific theories and scientific laws both start out as
hypotheses that have been repeatedly tested by scientists and have
survived.
- A scientific theory is a set of statements, including
scientific laws, that has been tested repeatedly on new data; it is not
subordinate to, or lesser than, a scientific law.
- A scientific theory is not "just a theory," it is as good as it gets when it comes to explaining complex natural phenomena.
- Dynamic scientific theories do not necessarily become
scientific laws; along with scientific laws they are the foundations of
scientific knowledge and together explain our complex natural world.
- True scientific theories have been developed through the
rigorous scientific method; they are much different from "common
theories" that are used by laypersons and creationists.
- Most common theories are only educated guesses; at best they are only tentative hypotheses
- The "endosymbiont theory" should be called the "endosymbiont hypothesis."
- Symbiogenesis is technically a scientific hypothesis rather than a scientific theory.
- Some scientists incorrectly use the term "theory" to
explain phenomena that has not been repeatedly tested and verified by
the scientific method.
- One scientist cannot create a scientific theory.
- A scientific theory should not be called a fact.
- It is still Einstein's "theory of relativity," even though
it is probably as close to a fact as anyone can get in science. Even
though it has survived the test of time and has passed all tests so far,
it is still subject to challenge as the body of experimental data and
analysis develops.
- A common or layperson's theory is equivalent to an educated guess or hunch.
- All facts in science are provisional and subject to challenge.
- The scientific theory of evolution assumes the existence of life and is directed to an explanation of how life evolved.
- The scientific theory of evolution does not deal with the
origin of life, and it does not presuppose the absence of a creator or
God.
- Scientific explanations for the origin of life are more properly referred to as hypotheses rather than scientific theories.
- Intelligent design is a non-scientific (non-testable) argument or assertion that life owes its origin to a master intellect.
- According to creationists, all life originated abruptly in its present form.
- Faith is the belief in the existence of something without proof or verifiable empirical evidence.
- Politics and religion should not interfere with scientific research or the teaching of science in our schools.
- The existence of true coconut pearls is based on faith rather than empirical evidence.
- Intelligent design as the only alternative to evolution is a false dichotomous argument.
- The argument whether evolution is a theory or a fact is an invalid comparison.
- The evidence showing that life evolved on this planet is
overwhelming; however, the exact mechanism for the origin of life is
hypothetical.
- Biopoiesis is a scientific hypothesis for the origin of
life from self-replicating organic molecules in the original "primordial
soup."
- Panspermia is a hypothesis which states that the earth was
"seeded" by extraterrestrial prokaryotic cells similar to
archaebacteria that were carried to earth by meteors.
- Natural selection is a well-tested and verifiable
mechanism to explain the origin of species according to the scientific
theory of evolution.
- Genetic variability is the raw material for evolution.
- The evolution of many species in a new habitat from an ancestral species is called adaptive radiation.
- The remarkable "Silver Sword Alliance" and the unusual
lobelioids on the Hawaiian Islands is a good example of adaptive
radiation.
- The word "theory" is commonly used by scientists and lay
people for tentative explanations that have not been universally tested
and accepted by the scientific community. In this case, the term
hypothesis is more appropriate.
- The word "theory" is commonly used by scientists and lay
people for explanations that have been repeatedly verified and
universally accepted by the scientific community. In this case, the
term scientific theory is more appropriate.
- The African euphorbias and North American cacti are classic examples of convergent evolution.
- Parallel evolution is similar to convergent evolution,
except the organisms being compared may not have an overall resemble to
each other.
- The noun "theory" associated with time-tested explanations
such as evolution, relativity and plate tectonics, should be modified
by the adjective "scientific" in order to distinguish it from a "common"
or "layman" theory that is essentially a tentative explanation or
untested hypothesis.
- Using the term homoplasy avoids the confusing distinction between parallel and convergent evolution.
- The origin of the eye is different phyla of animals is probably not a good example of homoplasy.
- Humans once had 24 pairs of chromosomes like present-day great apes (orangutans, gorillas and chimpanzees).
- Contrary to anti-evolution propaganda, there are many
excellent examples of "missing links" in the fossil record,
evolutionary transitions between distantly related animal groups.
- Fossils known as "missing links" represent major
phylogenetic branches (clades) giving rise to successive levels of life
on earth.
- Evolution is best explained as a well-established scientific theory based on numerous facts.
- Evolution can also be referred to as a fact according the definition of fact in the Merriam-Webster Unabridged Dictionary of the English Language.
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