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The theory of evolution is one of the great intellectual
revolutions of human history, drastically changing
our perception of the world and of our place in it.
Charles Darwin put forth a coherent theory of evolution
and amassed a great body of evidence in support of
this theory. In Darwin's time, most scientists fully
believed that each organism and each adaptation was
the work of the creator.
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The basis of evolution is
the genes that are passed on from generation to generation; these
produce an organism's inherited traits. These traits vary within
populations, with organisms showing heritable differences (variation)
in their traits. Evolution itself is the product of two opposing
forces: processes that constantly introduce variation, and processes
that make variants either become more common or rare. New variation
arises in two main ways: either from mutations in genes, or from
the transfer of genes between populations and between species. In
species that reproduce sexually, new combinations of genes are also
produced by genetic recombination, which can increase variation
between organisms.
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