Charles Darwin (18091882) published in 1859 a vastly important work: On The Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life. Darwin in 1858 had coauthored (with Alfred Russel Wallace) the theory of natural selection, which says that "favourable (biological) variations tend to be preserved." In the struggle for existence, the fit are not those who survive but those who reproduce. Natural selection also leads to diversification as different organisms adapt to particular ecological circumstances. Darwin said all biological similarities and differences are caused by "descent with modification." He concluded that all organisms are descended from at most four or five ancestors, and perhaps only one. Evolution is the name for this biological process that (according to modern molecular biology) goes back to one common ancestor 31/2 billion years ago.
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