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THEORY OF EVOLUTION
Since Charles Darwin gets most of the credit for the theory of evolution
we will concentrate largely on the development of his ideas. However, Darwin was not the first to consider the evolution of life. Many
others had pondered the subject long before him. The C.
Warren Irvin, Jr., Collection of Charles Darwin and Darwiniana has been
formed to reflect Darwin's writings and interests and to place them in the
context of the work of his immediate predecessors.
CHARLES DARWIN
I. Charles Darwin (1809-1882)
A. Englishman who spent two years in medical school at Edinburgh and then
trained as a clergyman at Cambridge for three years. He was mostly bored by
his studies but while at college he became friends with some of the most
respected scientists of the time
B. Through a college contact he accepted an unpaid post as naturalist
aboard the HMS Beagle which sailed around
the world for 5 years (1831-1836)
C. When Darwin left England he believed the church dogma of special
creation as told in Genesis. However, his observations over five years led him
to question it
D. Darwin took along Lyell's just published Principles of Geology which
challenged belief that the earth was created by divine plan only 6,000 years
ago. He was impressed by Lyell's emphasis on the great antiquity of the
earth's rocks and he gradually began to think that the characteristics of
organisms as well as the face of the earth could change over time
E. Darwin got off the ship wherever possible and he collected specimens and
took notes on virtually every fossil and living organism he encountered. These
observations sowed the seeds of his theory of evolution
F. Darwin was amazed by the variety of organisms he encountered during his
voyage. Everywhere he looked, he saw new and oddly shaped trees, exotically
colored flowers and birds, and beetles and other insects
G. Darwin quickly realized that the diversity of living organisms was only
part of the mystery of life. He found even greater numbers of fossil species.
In Argentina, he discovered fossil armadillos, giant ground sloths, peculiar
horses, and creatures that reminded him of the hippopotamus.
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Darwin soon became convinced that living organisms (extant) were vastly
outnumbered by extinct forms. He was convinced that extinction and the
appearance of new species were real phenomena that had to be explained
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Fossils of giant
armadillos and sloths
caused him to think that although they belonged to extinct forms, they
were constructed on the same basic plan (ground plan) as the small extant
species of the region
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This started him thinking of a fossil sequence of animal species through
the ages and the causes of extinction. He realized that living species
have ancestors
H. Darwin also noted that organisms differed markedly from continent to
continent and on opposite sides of natural barriers such as mountains,
deserts, and large rivers
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On traveling from north to south in South America he observed that one
species was replaced by similar, but slightly different species. E.g. he
noted the differences in ostriches from area to area. He couldn't imagine
that god would create several minor versions of species separately, one
for each locality.
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He concluded that not only did species change through time, but also
they changed with geographical distance.
I. Darwin wrote in his journal "it was evident that such facts could
only be explained on the supposition that species gradually became modified;
and the subject haunted me"
J. Darwin also witnessed the forces of geology in action:
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He saw the eruption of a volcano in Chile and later learned that another
volcano 500 miles away had blown its top the same night.
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He also experienced an earthquake that lifted beds of marine mussels
"still adhering to the rocks, ten feet above the high water
mark".
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He observed beds of fossil marine organisms thousands of feet above sea
level and he began to understand how the Andes Mountains were formed by
uplift. His belief in geological uniformitarianism was strengthened.
K. Throughout his journey, Darwin marveled at the "perfection of
structure" that made it possible for organisms to do whatever they needed
to do to stay alive and produce offspring.
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He called this perfection of structure fitness, by which he meant
the combination of all traits that help organisms survive and reproduce in
their environment.
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Darwin also began to question what process had "fit" these
organisms to their physical environments and to each other.
II. Darwin's visit to the Galapagos
Islands, a cluster
of rugged volcanic islands only a few miles apart, 600 miles west of Ecuador
had the most profound impact on his thinking.
A. Darwin noted that these islands were inhabited by a surprising number of
bizarre and often beautiful plant and animal species. He surmised correctly
that many of these species were endemic (found nowhere else)
B. Darwin wrote about the Galapagos Islands:
"Considering the small size of these islands we feel the more
astonished at the number of their aboriginal beings, and their confined
range. Seeing every height crowned with its crater, and the boundaries of
most lava-streams still distinct, we are led to believe that within a period
geologically recent the unbroken ocean was here spread out. Hence both in
space and time, we seem to be brought somewhat near to that great fact-that
mystery of mysteries-the first appearance of new beings on this earth"
C. Darwin noticed that although the tortoises were clearly different from
island to island they not only all resembled each other but they also
resembled those of the mainland. This was also true for a group of small birds
now called Darwin's Finches.
He reasoned that an ancestral stock had migrated to the islands where
they underwent profound changes under the different conditions of the
individual islands. Apparently a single ancestral group could give rise to
several different varieties or species.
III. Darwin develops his theory of evolution
A. When Darwin left England in 1831 he accepted the religious dogma of
special creation. On his return to England in 1836 he was convinced of the
idea of "descent with modification", i.e. all organisms, including
humans, are modified descendents of previously existing forms of life
B. Darwin thoughts developed in two stages:
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the realization that organisms are not fixed and unchangeable
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to provide an explanation of the process of evolutionary change
C. Darwin spent the next 20 years formulating his theory while also working
on a number of other scientific projects
D. Darwin became interested in the domestication of plants and animals and
how breeders artificially select different varieties of dogs, horses, fowl and
crop and ornamental plants from one species.
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The breeders had only vague notions as to the origin or inheritance of
the variable traits
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Darwin acknowledged the unlimited variability in organisms but was never
able to explain satisfactorily how a variant trait was inherited
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In 1837 Darwin wrote "how [artificial] selection could be applied
to organisms in a state of nature remained a mystery to me.
E. In 1838 Darwin read "An essay on the principle of population, as
it affects the future improvement of society" by a mathematician
named Malthus. Malthus maintained that human population increases
geometrically yet the means of sustenance increase arithmetically. He claimed
that sometime in the future human population growth would exceed resources and
there would be intense competition causing war, misery and famine.
1. From this Darwin concluded that competition exists among all living
things and a "struggle for existence" might be the means by which
well adapted individuals survive and the ill adjusted are eliminated
2. Darwin realized that perpetual selection existed in nature in the form
of natural selection. In contrast to artificial selection, the
breeder is replaced by conditions in the environment that prevent the
survival and reproduction of certain individuals. Natural selection occurs
without a conscious plan or purpose.
F. In 1858, while Darwin was working on his manuscript outlining his theory
of evolution, he received a manuscript for review by a young English
naturalist named Wallace. The title was "On the tendencies of
varieties to depart indefinitely from the original type". To Darwin's
surprise Wallace had independently come up with the same theory of natural
selection!
Wallace was a naturalist working mostly in the Malay Archipelago. He had
also read Essay on the principle of population by Malthus and it came into
his mind during a malarial fever. He wrote..."suddenly there
flashed upon me the idea of survival of the fittest".
G. Darwin wanted to let Wallace publish first but Lyell convinced him that
they should jointly present their work. Papers by both Darwin and Wallace were
read before the Linnaean Society of London on July 1, 1858.
H. Lyell urged Darwin to publish his full theory of evolution as soon as
possible. "On the origin of species by means of natural selection, or the
preservation of favoured races in the struggle for life" was published
and sold out on Nov. 24, 1859.
1. The complete text of Darwin's "Origin of Species" is available
on the web here.
I. Both Darwin and Wallace share credit for the theory of natural
selection, however there is much more to the theory of evolution than this.
Over more than 20 years Darwin amassed massive amounts of meticulously
documented evidence to support his theory of "descent with
modification" which can be summarized as follows:
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all species reproduce in excess of the numbers that can survive
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yet adult populations remain relatively constant
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therefore there must be a severe struggle for survival
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all species vary in many characteristics and some of the variants confer
an advantage or disadvantage in the struggle for life
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the result is a natural selection favoring survival and reproduction of
the more advantageous variants and elimination of the less advantageous
variants
J. Darwin's theory came under severe criticism from the church however it
gained almost immediate acceptance by most scientists of the day.
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