Motoo Kimura achieved international recognition for his numerous contributions to the fields of evolution and population genetics. He is considered the founder of the neutral theory of molecular evolution. According to this theory, evolutionary change and most of the variability within a species are caused at the molecular level by the random drift of mutant genes. By comparison, English naturalist Charles Darwin's theory of natural selection was based on the concept that evolution occurs at the species level, with those individuals best adapted to the environment most fit to survive. Kimura focused on the molecular changes that occur in the nucleotides of DNA, and concluded that the resulting mutant genes are neutral and subject to random drift, or changes, in gene frequencies due to pure chance. Kimura developed his theory quantitatively and thus, according to James F. Crow, writing in Population Genetics and Molecular Evolution, "laid a very strong foundation for a mathematical theory of evolution." At first these ideas were met with considerable skepticism by many other geneticists. With the accumulation of more evidence, however, they gained more acceptance
Kimura was born in Okazaki, Japan, to Issaku Kimura and Kana Kaneiwa. After receiving his M.Sc. degree from Kyoto University in 1947, he served as an assistant at the university for the next two years. In 1949, Kimura was appointed as a research member of the National Institute of Genetics in Mishima. He then came to the United States in 1953, where he was a graduate student at Iowa State College. In 1956, he received his Ph.D. from the University of Wisconsin, where he worked in the laboratory of James F. Crow. Shortly afterward he returned to the National Institute of Genetics and became the laboratory head, a position he was to hold until 1964. In that year, Kimura was appointed head of the department of population genetics, serving until 1988, when he became professor emeritus. Kimura married Hiroko Mino in 1957, and the couple had one son, Akio.
During his career, Kimura established a mathematical approach to the field of population genetics. This branch of science deals with the distribution of genes in a population, where all individuals are considered to share the same gene pool. As the individuals interbreed, genes are exchanged, resulting in many recombinations and consequent variations among members. The significance of gene frequency in evolution of species was first recognized in 1908 by Godfrey Harold Hardy and Wilhelm Weinberg when they independently arrived at the same conclusion. Their findings, now summarized in the Hardy-Weinberg law, state that the gene pool of a population remains constant from generation to generation under the following conditions: the population is large and characterized by random matings, and there are no new factors such as mutations or migration. When these theoretical conditions are not present, gene frequencies change, leading to the emergence of new species.
Kimura used computer technology to calculate the genetic composition of populations and the gene frequencies to be expected under various conditions over hundreds of generations. He prepared mathematical equations to depict a variety of possible influences such as inbreeding, mutations, crossbreeding, selection, chromosomal aberrations, natural selection, and random drift. In so doing he was able for the first time to establish a mathematical basis for the entire process of change in the gene frequency of populations.
Over the years Kimura published more than one hundred research papers. He was elected to foreign membership in the U.S. National Academy of Sciences, L'Académie des Sciences of Toulouse, the Genetical Society of Great Britain, and the Royal Society of London. He served as visiting professor at the Universities of Wisconsin, Pavia (Italy), Princeton, and Stanford. Kimura was honored with a number of Japanese awards, including a D.Sc. from Osaka University, the Genetics Society of Japan Prize, the Japan Academy Prize, the Japan Society of Human Genetics Prize, the Order of Culture National Medal from the Emperor, the Honorary Citizen of Okazaki award, and the Asaki Prize. Abroad, he received additional awards from Oxford University, the French government, the U.S. Academy of Sciences, as well as the International Prize for Biology, honorary degrees from the universities of Chicago and Wisconsin, and the Darwin Medal of the Royal Society. In his spare time Kimura raised and hybridized various orchids. Kimura died in Tokyo at age 70.
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