Describes the hypothesis that Darwin's "natural selection," reformulated by R.A. Fisher, J.B.S. Haldane, and S. Wright in the light of Mendelian genetics, is the exclusive mechanism for biological evolution. During the 1930s, alternatives such as Lamarchism, macromutations, and orthogenesis were rejected in favor of natural selection acting on small mutations, but there were disagreements about the role of random genetic drift in evolution. By the...