One of this century's leading evolutionary biologists, Motoo Kimura revolutionized the field with his random drift theory of molecular evolution—the neutral theory—and his groundbreaking theoretical work in population genetics. This volume collects 57 of Kimura's most important papers and covers forty years of his diverse and original contributions to our understanding of how genetic variation affects evolutionary change. Kimura's neutral theory, first presented in 1968, challenged the notion that natural selection was the sole directive force in evolution. Arguing that mutations and random drift account for variations at the level of DNA and amino acids, Kimura advanced a theory of evolutionary change that was strongly challenged at first and that eventually earned the respect and interest of evolutionary biologists throughout the world. This volume includes the seminal papers on the neutral theory, as well as many others that cover such topics as population structure, variable selection intensity, the genetics of quantitative characters, inbreeding systems, and reversibility of changes by random drift. Background essays by Naoyuki Takahata examine Kimura's work in relation to its effects and recent developments in each area.