Thoeries of Evolution


Evolution is the process by which living organisms originated on earth
and have changed their forms to adapt to the changing environment. The earliest
known fossil organisms are the single-celled forms resembling modern bacteria;
they date from about 3.4 billion years ago. Evolution has resulted in
successive radiations of new types of organisms, many of which have become
extinct, but some of which have developed into the present fauna and flora of
the world (Wilson 17).
Evolution has been studied for nearly two centuries. One of the
earliest evolutionists was Jean Baptiste de Lamarck, who argued that the
patterns of resemblance found in various creatures arose through evolutionary
modifications of a common lineage. Naturalists had already established that
different animals are adapted to different modes of life and environmental
conditions; Lamarck believed that environmental changes evoked in individual
animals direct adaptive responses that could be passed on to their offspring as
inheritable traits. This generalized hypothesis of evolution by acquired
characteristics was not tested scientifically during Lamarck\'s lifetime.
A successful explanation of evolutionary processes was proposed by
Charles Darwin. His most famous book, On the Origin of Species by Means of
Natural Selection (1859), is a landmark in human understanding of nature.
Pointing to variability within species, Darwin observed that while offspring
inherit a resemblance to their parents, they are not identical to them. He
further noted that some of the differences between offspring and parents were
not due soley to the environment but were themselves often inheritable. Animal
breeders were often able to change the characteristics of domestic animals by
selecting for reproduction those individuals with the most desirable qualities.
Darwin reasoned that, in nature, individuals with qualities that made them
better adjusted to their environments or gave them higher reproductive
capacities would tend to leave more offspring; such individuals were said to
have higher fitness. Because more individuals are born than survive to breed,
constant winnowing of the less fit-a natural selection-should occur, leading to
a population that is well adapted to the environment it inhabits. When
environmental conditions change, populations require new properties to maintain
their fitness. Either the survival of a sufficient number of individuals with
suitable traits leads to an eventual adaptation of the population as a whole, or
the population becomes extinct. Evolution proceeds by the natural selection of
well-adapted individuals over a span of many generations, according to Darwin\'s
theory(Microsoft 96).
The parts of Darwin\'s theory that were the hardest to test
scientifically were the interferences about the heritability of traits because
heredity was not understood at that time. The basic rules of inheritance became
known to science during the turn of the century, when the earlier genetic works
of Gregor Mendel came to light. Mendel had discovered that characteristics are
transmitted across generations in discrete units, known as genes that are
inherited in a statistically predictable fashion. The discovery was then made
that inheritable changes in genes could occur spontaneously and randomly without
regard to the environment. Since mutations were seen to be the only source of
genetic novelty, many geneticists believed that evolution was driven onward by
the random accumulationof favorable mutation changes. Natural selection was
reduced to a minor role by mutationist such as Vries. Morgan, and Bates.
While mutation was replacing Darwinism, the leading evolutionary theory,
the science of population genetics was being founded by Sewall Wright, J.B.S.
Haldine, and several other geneticists, all working independantly. They
developed arguments to show that even when a mutation that is immediately
favored appears, its subsequent spread within a population depends on such
variables as the following:

the size of the population
the length of generations
the degree to which the mutation is favorable
the rate at which the same mutation reappears in descendants

Furthermore, a given gene is favorable only under certain environmental
conditions. If conditions change in space, then the gene may be favored only
in a localized part of the population; if conditions change over time, the gene
may become generally unfavorable. Because different individuals usually have
different assortments of genes, the total number of genes available for
inheritance by the next generation can be large, forming a vast store of genetic
variability. This is called the gene pool. Sexual reproduction ensures that
the genes are rearranged in each generation, a process called recombonation.
Mutations provide the gene pool with a continuous supply of new genes; through
the process of natural selection the gene frequencies change so that
advantageous genes occur in greater proportions(Ardrey 24).
As the new evolutionary theory became enriched from such diverse sources,
it became known as the synthetic theory. Three American scientists made
controbutions that were especially important. The German-born Ernst Mayr, a
zoologist,