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Motoo Kimura (1924-1994) was excited by the living
world, especially plants, from an early age. After beginning
his biological training in secondary school, he studied botany
at Kyoto Imperial University. Upon graduating, he was hired
by the renowned geneticist Hitoshi Kihara, a professor in
the Universities school of Agriculture. While most other students
and workers in the lab spent their idle hours reading up on
a particular species, Kimura dove into the works on J.B.S.
Haldane, Theodosius Dobzhansky, and Sewall Wright. He taught
himself whatever math he needed to know along the way. One
of his earliest theoretical accomplishments was the development
of the "stepping stone" model of migration, a more
realistic version of Wright's island model.
In 1949, he was hired as a research assistant at the newly
established National Institute of Genetics in Mishima, where
he was to be employed there for the rest of his life. Four
years later, in 1953, Kimura left for United States to study
on a Fullbright Fellowship. After nine unsatisfying months
at Iowa State, he joined James Crow's laboratory at the University
of Wisconsin, from which he received his Ph.D. in 1956. During
this two-year period, he wrote several important, highly mathematical
papers on random genetic drift that impressed the few population
geneticists who were able to understand them (most notably,
Wright). In one paper, he extended Fisher's theory of natural
selection to take into account factors such as dominance,
epistasis and fluctuations in the natural environment. After
a five year period in Japan, Kimura returned to Crow's lab
at Wisconsin in 1961, where spent the next to years working
out such important problems as the fixation probability of
a newly occurring mutation and developing the "infinite
alleles model" along with Crow.
In 1963, he returned to Japan once again, and set out to
develop ways to use the new data pouring in from molecular
biology to solve problems of population genetics. Using data
on the variation among hemoglobins and cytochromes-c in a
wide range of species, he calculated the evolutionary rates
of these proteins. After extrapolating these rates to the
entire genome, he concluded that there simply could not be
strong enough selection pressures to drive such rapid evolution.
He therefore decided that most evolution at the molecular
level was the result of random processes like mutation and
drift. Kimura spent the rest of his life advancing this idea,
which came to be known as the "neutral theory of molecular
evolution."
While Kimura did a great deal of important theoretical and
experimental work in the 1970s and 1980s (much of it in collaboration
with Tomoko Ohta), he is most remembered for his tireless
and dogmatic championing of the neutral theory. Some have
argued that proving his many detractors wrong became an obsession.
Nevertheless, he still found time to make profound contributions
to the field of population genetics. Not only was he able
to work out the time it takes for a neutral allele to become
fixed in a population (4Ne), he also calculated the number
of heterozygous nucleotide sites in a finite population in
which new mutations are constantly occurring.
Throughout his career, Kimura authored several hundred papers.
He also wrote or co-wrote 6 books, including An Introduction
to Population Genetics Theory (1970; with James Crow) and
The Neutral Theory of Molecular Evolution (1983). His most
widely cited papers are collected in the 1994 volume Population
Genetics, Molecular Evolution, and the Neutral Theory: Selected
Papers. He received innumerable awards during his long career,
including: The Genetics Society of Japan Prize (1959); The
Weldon Memorial Prize (1965); The Japanese Order of Culture
(Emperor's Prize [1976]), the Chevalier de L'Ordre National
du Merite (1986), and the Darwin Medal (Royal Society [1992]).
He was elected Foreign Member of the National Academy of Sciences
(USA) in 1973, and of the Royal Society in 1993. He died on
his 70th birthday, November 13, 1994, after a fall caused
by Amyotrophic Lateral Sclerosis.
Selected Bibliography:
- M. Kimura, "The theory of the chromosome substitution
between two different species," Cytologia (1950),
15: 281-94.
- M. Kimura, "'Stepping-stone model of population,"
Annual Report of the National Institute of Genetics
(1953), 3: 62-63.
- M. Kimura, "Process leading to quasi-fixation of
genes in natural populations due to random fluctuation of
selection intensities," Genetics (1954), 39:280-295.
- M. Kimura, "Solution of a process of random genetic
drift with a continuous model," PNAS (1955),
41: 144-150.
- M. Kimura, "Stochastic Processes and distribution
of gene frequencies under natural selection," Cold
Spring Harbor Symp. Quant. Biol. (1955), 20: 33-53.
- M. Kimura, "On the probability of fixation of mutant
genes in a population," Genetics (1962), 47:
713-19.
- M. Kimura and J.F. Crow, "The measurement of effective
population number," Evolution (1963), 17: 279-288.
- M. Kimura and J. F. Crow, "The number of alleles
that can be maintained in a finite population," Genetics
(1964), 49: 725-38.
- M. Kimura, Diffusion Models in Population Genetics
(London: Methun, 1964)
- M. Kimura, "A stochastic model concerning the maintenance
of genetic variability in quantitative characters,"
PNAS (1965), 54: 731-736.
- M.
Kimura, "Evolutionary Rate at the Molecular Level,"
Nature (1968), 217: 624-26.
- M. Kimura, "The number of heterozygous nucleotide
sites maintained in a finite population due to steady flux
of mutations," Genetics (1969), 61: 893-903.
- M. Kimura and T. Ohta, "The average number of generations
until fixation of a mutant gene in a finite population,"
Genetics (1969), 61: 763-71.
- M. Kimura, "Theoretical foundation of population
genetics at the molecular level," Theoretical Population
Biology (1971), 2: 174-208.
- T. Ohta and M. Kimura, "A model of mutation appropriate
to estimate the number of electrophoretically detectable
alleles in a finite population," Genet. Res.
(1973), 22: 201-204.
- M. Kimura, The Neutral Theory of Molecular Evolution
(Cambridge: Cambridge University Press, 1983).
- M. Kimura, "The neutral theory of molecular evolution:
a review of recent evidence," Japanese Journal of
Genetics (1991), 66: 367-86.
Secondary Sources:
- James Crow, "Motoo Kimura: An Appreciation,"
in T. Ohta and K. Aoki (eds.), Population Genetics and
Molecular Evolution (Tokyo: Japan Scientific Societies
Press, 1985) p. 1.
- James Crow, "Twenty-Five Years Ago in Genetics: Motoo
Kimura and Molecular Evolution," Genetics (1987),
119:??.
- James Crow, "Twenty-Five Years Ago in Genetics: The
Infinite Alleles Model," Genetics (1989), 121:
631-4.
- James Crow, "Motoo Kimura, 1924-1994," Genetics
(1995), 140: 1-5.
- James Crow, "Memories of Moto," Theoretical
Population Genetics (1996), 49: 122-127.
- Motoo Kimura, "Genes, Populations, and Molecules:
A Memoir," in T. Ohta and K. Aoki (eds.) Population
Genetics and Molecular Evolution (Tokyo: Japan Scientific
Societies Press, 1985), pp. 459-481.
- Motoo Kimura, "Thirty Years of Population Genetics
with Dr. Crow," Japanese Journal of Genetics
(1988), 63: 1-10.
- Wen-Hsiung Li, "Kimura's Contributions to Molecular
Evolution," Theoretical Population Biology (1996),
49: 146-153.
- G. A. Watterson, "Motoo Kimura's Use of Diffusion
Theory in Population Genetics," Theoretical Population
Biology (1996), 49: 154-188.
- Tomoko Ohta, "Motoo Kimura," Annu. Rev. Genet.
(1996), 30: 1-5.
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