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THE BASICS OF
BIOLOGICAL EVOLUTION
By Robert George Sprackland, Ph.D.
The fact of
biological evolution is one of the most
well-supported and thoroughly tested theories in science. In fact, it is
very possibly the only theory that has some form of support from virtually
every other field and discipline of science. It is the very
backbone of biology, and essential to the progress of almost every aspect
of the study of life. The classification system used for cataloguing life
is based on grouping evolutionarily related species more closely together
than they are to other groups.
In simple terms, biologists define evolution as a change in allele
frequencies in populations over time that lead to descent with
modification. Sufficient modification may lead to the formation of new
species, termed speciation. The process may be slow and be the
result of a very long-term (tens or hundreds of millions of years)
accumulation of genetic novelties (that is, new genes or alleles not found
in the parent population) termed gradualism, or may be the result
of a rather rapid change (say over a few million of years) in one or
a few key genes, termed punctuated equilibrium. The evolution of many
vertebrates seems to follow the former model, while snails, clams, orchids
and grasses seem to follow the latter. The net result of this allelic
changing is summed up in another way: biological evolution is descent with
inheritable (meaning genetic) modification.
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This is the famous exhibit
depicting the evolution of horses from a fossil ancestor known as Hyracotherium
(once known as Eohippus). Over time, horses evolved to loose
toes, a demonstration that evolution does not necessarily involved
addition or complication of features. Photo by Dr. R. G.
Sprackland. |
What, then, are alleles? Alleles are the specific variations of
genes, and genes are sequences of DNA that code for the proteins that make
up all life.
As an example, consider human eye color; "eye color" would
equate with the gene, while "brown" and "blue" would
be the alleles. If a predominantly blue-eyed population were to become
equally blue and brown eyed over time, then by definition evolution would
have occurred. Note, however, that evolution only takes place in
populations, not individuals. No matter how much you (or your genes) alter
in your lifetime, you have not evolved.
Biologists have discussed two levels of evolution, using
arbitrary terms as shorthand descriptions of the amount of modification
experienced by a population. The first, microevolution is a simple
shift in gene frequencies or phenotypes. The shift is measurable, but the
resultant population still is recognizable as the parent species. For
example, diet, hygiene and medicine have greatly altered the relative
adult size of contemporary humans over our ancestors even one hundred
years ago. Nevertheless, great-great grandmother was still quite human.
The other term is macroevolution, representing a
condition in a population that so greatly varies from the parental stock
that recognition of a distinct new species is warranted. Macroevolution,
then, is the result of a speciation event, and, as mentioned above, may
occur in a single generation or take many thousands or millions of
generations.
One question that is often asked regarding
macroevolution concerns the fate of ancestors. Typically, a biologist will
hear "if humans descended from monkeys, why are there still
monkeys?" The commonest (and correct) answer is that humans did not
evolve from monkeys. Rather, both species shared an even more remote
common ancestor. In time, one route developed into the monkeys, and the
other became the apes and, eventually, us. However, this answer still
confuses the point: what happens to the ancestors? Even if humans did
evolve from monkeys, there is no reason why monkeys should disappear.
After all, our parents and grandparents do not evaporate after our births.
Similarly (and commonly) the ancestral parents and the derived offspring
may live side by side (for quite some time if they learn to exploit
slightly different resources).
The science of evolutionary biology is divided into two
dynamic branches. One is experimental evolution, in which population
studies and genetic analyses allow specific and repeatable testing. Much
of modern medicine's arsenal of drug therapy is based on the results of
such evolutionary studies. Certainly the resistance to many drugs that
alarms so many contemporary physicians is a result of micro-organismal
evolution. The other branch is historical evolution, and involves
non-repeatable aspects of objective observation. Paleontology is a prime
area of historical science. Many people forget that all of science has
this second, historical/descriptive component when they speak of the
"scientific method." In fact, the main points of the method
apply to experimental sciences, though historical science is also subject
to testing, revision, and extensive modification in light of new data.
It is also worth adding here that evolutionary biology
is a distinct discipline from origin of life studies (though I can't
imagine an evolutionary biologist not at least somewhat interested in the
origins of life). Evolutionary studies focus on how life changes. Origins
are the domain of biochemistry and molecular biology. Evolution is not
concerned with how life got started; it is concerned with what it did
after it got here.
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DARWINIAN EVOLUTION
There is no name more closely allied
with biological evolution than that of Charles Robert Darwin.
Darwin, however, neither invented the concept of evolution, nor did
he use the term "evolution" but once (actually, as
"evolved," the very last word in his book) in his landmark
book, On the Origin of Species.
In fact, he did not come to the conclusion that evolution was real
during a moment of inspiration while at the Galapagos Islands.
What, then, did Darwin contribute to
evolutionary thought? After graduating from Cambridge, he served as
the captain's companion on HMS Beagle during a scheduled
3-year mapping trip to South America. He used this opportunity to
study the natural history of the places he would visit, and was thus
the ship's naturalist (though there was no such official position).
In an incident reminiscent of Gilligan's Island, the voyage
lasted much longer than planned--almost 5 years. Along the way,
young Darwin collected an immense number of biological and
geological specimens. He would spend the rest of his life studying a
part of that collection.
Darwin spent nearly two decades
thinking through his ideas, fearful to publish because of the
ramifications of a non-Christian explanation for the diversity of
life. In 1858, though, another biologist, Alfred Russell Wallace,
sent Darwin a short paper in which he presented what was a virtual
abstract of Darwin's theory. Darwin's and Wallace's papers were read
at the Linnean Society, and Darwin proceeded to write On The
Origin of Species by means of Natural Selection. It was
published in late 1859, and sold out by noon the first day. The book
was quickly reissued, and has been continuously in print ever since.
Darwin proposed a theory (in
scientific terms) of how life modified over time to produce new
species. Unlike earlier authors and philosophers, Darwin provided a
mechanism, which he termed natural selection, and offered
copious evidence to support each of his theory's foundations.
In brief, Darwin made four
observations that lead to his theory: 1) living things produce far
more offspring than can possibly survive to adulthood, 2) each
offspring has some variation that makes it an imperfect replica of
the parent(s), 3) some variations will confer greater survival
advantage than others, and 4) those individuals with the better
variations will generally tend to survive and produce more
successful offspring. The sorting of "good" from "not
good" individuals by their environment is what Darwin termed
"natural selection."
Ironically, Darwin's biggest gap was
in explaining how the process of inheritance worked (by the 1800s,
no one doubted inheritance was real--only the "how" of its
working was still a mystery). But within five years of the
publication of his book, an Austrian monk, Gregor Mendel, figured
out the principles of inheritance. His publications were sent to
Darwin but, because of their mathematical basis, were either unread
or ignored by the Englishman. Mendel's work would be rediscovered,
acknowledged and vindicated in 1901, many years after both he and
Darwin had passed away.
Today, though biologists argue about
the details of the causes and mechanisms of evolution in specific
cases, they do adhere to the theory of evolution by natural
selection. To scientists, it is a theory in the way science defines
"theory"-- a set of hypotheses that explain particular
phenomena and stand up to all tests to date. |
Charles
Darwin is known as an intellectual giant today, but his career was
anything but assured. He was at best a mediocre student through
college; he barely graduated from Cambridge University, intending to
become a country parson; and got his "break" to be the
naturalist on a round-the-world voyage because the captain of the
ship liked his personality. Darwin never held a job nor academic
post. He lived off the inheritance of his father and the dowry of
his wife, and conducted almost all his experimental work within a
few miles of his home in Down. As he grew older he attended fewer
and fewer public functions, but frequently held an academic court at
his country home. This statue of Charles Darwin,
arranged for by his colleague Thomas Huxley. Huxley, known today as
"Darwin's Bulldog" for his aggressive defense of his
colleague's then-radical ideas, set up a subscription to pay for
this posthumous memorial. It is displayed at
London's Natural History Museum and was dedicated by Huxley and
Darwin's intellectual nemesis Sir Richard Owen. Photo by Dr. R. G. Sprackland.
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This is the assembled skeleton of the
giant sloth which Charles Darwin collected on the southeastern coast
of South America. When Darwin questioned the existence and
subsequent disappearance of giant creatures so similar to living
species, he began to comprehend the concepts of extinction and
biological succession. The sloth is on display today at London's Natural History
Museum. Photo by Dr. R. G. Sprackland.
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In 1999
the Kansas State Board of Education dropped the teaching of
evolution and ancient-earth science from the required curriculum,
prompting the cartoon above. Evolution is not
"just a theory" as usually expressed, but one of the most
solidly-supported scientific theories ever devised. In February
2001, a newly elected Kansas school board reinstated the teaching of
science.
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| Further reading: Click
on a book to order a copy.
Butlin, Roger and Tom Tregenza. 1997. Is
speciation no accident? Nature 387: 551-552.
Davis, Jerrold. 1996. Phylogenetics,
molecular variation, and species concepts. BioScience 46(7):502-513.
Erwin, Douglas. 1998. After the end:
recovery from extinction. Science 279: 1324-1325.
Freeman, S. and J. Herron. 1998. Evolutionary
Analysis. Prentice Hall. ISBN: 0-13-568023-9.
Futuyma, Douglas. 1995. The uses of
evolutionary biology. Science 267: 41-42.
Gee,
Henry. 1999. In Search of Deep Time: Beyond the Fossil Record to a New
History of Life. The Free Press, NY. ISBN: 0-684-85421-X.
Gibbons, Ann. 1996. On the many origins
of species. Science 273: 1496-1499.
Goldschmidt, T. 1996. Darwin's
Dreampond: Drama in Lake Victoria. The MIT Press. ISBN: 0-262-07178-9.
Gould, Stephen. 1997. Nonoverlapping
magisteria. Natural History, 106(2): 16-62.
Grant,
Peter. 1999. Ecology and Evolution of Darwin's Finches. Princeton
University Press, NJ. ISBN: 0-691-04866-5.
Harvey, Paul and Robert May. 1997. Case
studies of extinction. Nature 385: 776-777.
Holland, Heinrich. 1997. Evidence for
life on earth more than 3850 million years ago. Science 275:
38-39.
Huelsenbeck, John and Bruce Rannala.
1997. Phylogenetic methods come of age: hypotheses in an evolutionary
context. Science 276: 227-231.
International Commission on Zoological
Nomenclature. 1999. The International Code of Zoological Nomenclature,
4th Edition. ISBN: 0-85301-006-4.
See the review in our Museum Library!
Jones, Steve. 2000.
Darwin's Ghost. Random House. ISBN: 0-375-50103-7.
Kerr, Richard. 1997. Does evolutionary
history take million-year breaks?
Leakey,
Richard. 1979. The Illustrated Origin of Species by Charles Darwin.
Hill and Wang. ISBN: 0-8090-5735-2.
Lewin,
Roger. 1999. Patterns in Evolution: The New Molecular View.
Scientific American Library. ISBN: 0-7167-6036-3.
Losos, J., T. Jackman, A. Larson, K. de
Queiroz and L. Rodríguez-Schettino. 1998. Contingency and determinism in
replicated adaptive radiations of island lizards. Science 279:
2115-2118.
Malhotra, Anita and Roger Thorpe. 1991.
Experimental detection of rapid evolutionary response in natural lizard
populations. Nature 353:
Mayr, E. and P. Ashlock. 1991. Principles
of Systematic Zoology. McGraw Hill. ISBN: 0-07-112701-1.
Milner, R. 1990. The Encyclopedia of
Evolution. Facts on File. ISBN: none.
Morell, Virginia. 1997. Predator-free
guppies take an evolutionary leap forward. Science 275:
1880.
Morell, Virginia. 1996. Starting species
with third parties and sex wars. Science 273: 1499-1502.
Nee, Sean and Robert May. 1997.
Extinction and the loss of evolutionary history. Science 278:
692-694.
Patterson, Colin. 1999. Evolution.
2nd Edition. Comstock Publishing. ISBN: 0-8014-8594-0.
Ridley,
Mark. 1996. Evolution. Second edition. Blackwell Science,
Cambridge, Mass. ISBN: 0-632-04384-9.
Roush, Wade. 1997. Hybrids consummate
species invasion. Science 277: 316-317.
Rundle, H., L. Nagel, J. Boughman and D.
Schluter. 2000. Natural selection and parallel speciation in sympatric
sticklebacks. Science 287(5451): 306-308.
Shreeve, James. 1999. Secrets of the
gene. National Geographic 196(4): 42-75.
Sprackland, Robert. 1999. It's a bird,
it's a plane! It's... a turtle?? Reptile & Amphibian Hobbyist 4(12):
18-19.
Vogel, Gretchen. 1998. For island
lizards, history repeats itself. Science 279: 2043.
Weiner, J. 1995. Evolution made visible.
Science 267: 30-33.
Weiner, J. 1994. The Beak of the
Finch. Vintage Press. ISBN: 0-09-946871-9.
Wray, G., J. Levinton and L. Shapiro.
1996. Molecular evidence for deep Precambrian divergences among metazoan
phyla. Science 274: 568-573.
Wuethrich, Bernice. 1997. How reptiles
took wing. Science 275: 1419.
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