Darwin came to
understand that any population consists of individuals that are all slightly
different from one another. Those individuals having a variation that
gives them an advantage in staying alive long enough to successfully reproduce
are the ones that pass on their traits more frequently to the next generation.
Subsequently, their traits become more common and the population
evolves. Darwin called this "descent with
modification."
The Galápagos
finches provide an excellent example of this process. For instance,
among the birds that ended up in arid environments, the ones with beaks that
were better suited for eating cactus got more food. As a result, they
were in better condition to mate. In a very real sense, nature selected
the best adapted varieties to survive and to reproduce. This process has
come to be known as natural selection.
Darwin did not
believe that the environment was producing the variation within the finch
populations. He correctly thought that the variation already existed and
that nature just selected for the most suitable beak shape and against less
useful ones. Darwin and his supporters ultimately described this process
as the "survival of the fittest." This is very different from
Lamarck's incorrect idea that the environment altered the shape of individuals
and that these acquired changes were then inherited.
Nineteenth
century critics of Darwin thought that he had misinterpreted the Galápagos
finch data. They said that God had created the 13 different species as
they are and that no evolution in beak shape has ever occurred. It was
difficult to conclusively refute such counter arguments at that time.
However, 20th century field research has proven Darwin to be
correct.
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Thomas Malthus
(1766-1834) |
In
1798, Thomas Malthus, an
English clergyman and pioneer economist, published an extensive article
entitled Essay on the Principles of Population. In it he observed
that human populations will double every 25 years unless they are kept in
check by limits in food supply. By the late 1830's, Darwin came to
realize that all plant and animal populations have this same potential to
rapidly increase their numbers unless they are constantly checked by
limitations in food, water, and other resources that are essential for
survival. This fact was key to his understanding of the process of
natural selection. He realized that the most fit individuals in a
population are the ones that are least likely to die of starvation and,
therefore, are most likely to pass on their traits to the next
generation.
A good example of
natural selection was discovered among "peppered" moths living near English
industrial cities. These insects have varieties that vary in wing and
body coloration from light to dark. During the 19th century, sooty smoke
from coal burning furnaces killed the lichen on trees and darkened the
bark. When moths landed on these trees, the dark colored ones were
harder to spot by birds who ate them and, subsequently, they more often lived
long enough to reproduce. Over generations, the environment continued to
favor darker moths. As a result, they progressively became more
common. By 1900, 98% of the moths in the vicinity of English cities like
Manchester were mostly
black. Since the 1950's, air pollution controls have significantly
reduced the amount of heavy particulate air pollutants reaching the trees.
As a result, lichen has grown back, making trees lighter in
color. Now, natural selection favors lighter moth varieties so they have
become the most common. This trend has been well documented by field
studies undertaken between 1959 and 1995 by Sir Cyril Clarke from the
University of Liverpool. The same pattern of moth wing color
evolutionary change in response to increased and later decreased atmospheric
pollution has been carefully documented by other researchers for the
countryside around Detroit, Michigan.
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Dark moths on light colored
bark are easy targets for
hungry birds but are hidden on
pollution darkened
trees.
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Darwin did not
rush his ideas about natural selection into print. There was a Christian
evangelical fervor in England at the time. He could have been charged
with sedition and blasphemy for publishing his unpopular theory. After
returning from the voyage around the world on H.M.S. Beagle, he settled down
in England, married Emma
Wedgwood (his first cousin), raised a large family, and quietly continued his
research. In 1842 and 1844, he wrote relatively short summaries of his
theory, but they were not widely read outside of British scientific
circles. It was not until he was 50 years old, in
1859, that he finally published his theory of evolution in full for his fellow
scientists and for the public at large. He did so in a book entitled
On the Origin of Species. It was very popular and
controversial from the outset. The first edition came out on November
24, 1859 and sold out on that day. It went through six editions by
1872.
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Alfred
Wallace
1823-1913
|
What finally
convinced Darwin that he should publish his theory was the draft of an essay
that he received in the summer of 1858 from a younger British naturalist named
Alfred Wallace, who was
then hard at work collecting biological specimens in Southeast Asia for sale
to museums and private collectors. Darwin was surprised to read that
Wallace had come upon essentially the same explanation for evolution.
Being a fair man, Darwin later insisted that Wallace also get credit for the
theory during debates over its validity carried out by the British Royal
Society.
However, it is
Darwin's book rather than Wallace's essay that had the most impact on the
Victorian public. Darwin not only described the process of natural
selection in more detail, but he also gave numerous examples of it. It
was his On the Origin of Species that convinced most educated people
in the late 19th century that life forms do change through time. This
prepared the public for the acceptance of earlier human species and of a world
much older than 6000 years.
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Gregor
Mendel
1822-1884 |
Both Darwin and
Wallace failed to understand an important aspect of natural selection.
They realized that plant and animal populations are composed of individuals
that vary from each other in physical form. They also understood that
nature selects from the existing varieties those traits that are most suited
to their environment. If natural selection were the only process
occurring, each generation should have less variation until all members of a
population are essentially identical or clones of each other. That does
not happen. Each new generation has new variations. Darwin was
aware of this fact, but he did not understand what caused the variation.
The first person to begin to grasp why this happens was an obscure Central
European monk named Gregor Mendel.
Through plant breeding experiments carried out between 1856 and 1863, he
discovered that there is a recombination of parental traits in
offspring. Sadly, Darwin and most other 19th century biologists never
knew of Mendel and his research. It was not until the beginning of the
20th century that Mendel's pioneer research into genetic inheritance was
rediscovered. This was long after his death. He never received
the public acclaim that was eventually showered on Darwin.
Charles Darwin's
convincing evidence that evolution occurs was very threatening to many
Christians who believed that people were created specially by God and that
they have not changed biologically since that creation. The idea that
there could have been prehistoric humans who were anatomically different from
us was rejected for similar reasons. However, Charles Lyell's geological
evidence that the Earth must be much older than 6,000 years and the rapidly
accumulating fossil record of past evolution convinced educated lay people in
the 1860's to think what had been unthinkable earlier.
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Boucher de Perthes
(1788-1868)
|
Archaeological
confirmation of the existence of prehistoric Europeans had been accumulating
since the 1830's. However, until the late 1850's, it had been widely
rejected or misinterpreted. Much of this evidence had been collected by
Jacques Boucher Crèvecoeur de Perthes , a customs officer in northern France in the early
1800's. His hobby was collecting ancient stone tools from deep down in
the Somme River gravel deposits. Since he found these artifacts in
association with the bones of extinct animals, he concluded that they must
have been made at the time that those animals lived.
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Prehistoric artifact
incorrectly thought
to be a "lightning bolt remnant"
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Boucher de Perthes
tried to publish his findings in 1838. They were rejected by all
important scientists and journals. The prehistoric stone tools usually
were dismissed as being only "lightning stones" (i.e., the remnants of
lightning bolts). However, by 1858, his claims were beginning to be
accepted by some enlightened Western European scientists. Darwin's
publication of On the Origin of Species the following year convinced
even more educated people that Boucher de Perthes had been
right.
Darwin's popularizing
the idea of evolution also made it possible for scientists to begin to accept
that some of the makers of Boucher de Perthes' prehistoric tools had already
been discovered and that their bones were in museums. These bones had
been found in several Western European countries during the first half of the
19th century. However, they had all been dismissed as being from odd
looking modern people. During the 1860's, some were correctly determined
to be from an earlier species or variety of people who had lived during the
last ice age--i.e., long before recorded history. We now know that these
ancient people were mostly Neandertals, who lived about 130,000-30,000 years
ago.
