Motoo Kimura Research Paper

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Motoo Kimura was Japan’s foremost population and evolutionary geneticist. He is best known for his neutral theory of molecular evolution, which asserts that the great majority of evolutionary changes at the DNA level are driven by mutation and random survival of mutant genes, rather than by natural selection. He was also a major contributor to the mathematical theory of evolution and population genetics.

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1. Life

Motoo Kimura was born on November 13, 1924, in Okazaki, Japan. Early in his life he was fascinated by plants and had a deep interest in their structure. He also displayed an early talent for mathematics. He was admitted to the National High School in Nagoya in 1942. Here, thanks to a knowledgeable and sympathetic teacher, he studied the chromosomes of lilies. This institution also provided his first contact with biometry, and he realized for the first time that he could use his mathematical skills to study biological problems. In 1944 he was admitted to the Kyoto Imperial University.

Normally Kimura would have enrolled in the Faculty of Agriculture to study with the internationally respected geneticist, Hitoshi Kihara. But agriculture students were being called into military service, while botany students were not. So, on Kihara’s advice, Kimura enrolled in the Faculty of Science. By the time of his graduation the war was over, and he joined Kihara who encouraged his study of mathematical problems. During this time, Kimura had no specific duties and took the opportunity on his own to read systematically in mathematical genetics. His mathematics, formidable as it sometimes seems, was mainly self-taught. Life in wartime Japan was miserable, and remained so for some time afterward. Kimura was able to supplement a sparse physical and intellectual diet by visiting his cousin, a quantum physicist who lived in the suburbs and could supply both food and scientific talk. Kimura hoped for a theory of population genetics comparable to that of physics.

In 1949, he joined the staff of the National Institute of Genetics in Mishima, and retained this position for the rest of his life. Thanks to help from some American geneticists studying the effects of the atomic bomb in Hiroshima, he was able to obtain support for graduate study in the USA. After a year at Iowa State College Kimura transferred to the University of Wisconsin in 1954 as the author’s student. This also provided an opportunity for daily association with America’s most eminent population geneticist, Sewall Wright, who had recently moved to the University of Wisconsin.

Except for his graduate years and several short periods abroad, Kimura spent the remainder of his life in the National Institute of Genetics in Mishima, Japan. Late in his life, he developed amyotrophic lateral sclerosis and died on his 70th birthday, November 13, 1994.

2. Major Contributions

Kimura’s scientific life can be divided into two periods. The first began while he was still a student and continued until 1968. During this time he made major contributions to the mathematical theory of population genetics. In 1968, he introduced his neutral theory of molecular evolution. Although he never stopped making theoretical contributions, his major efforts from this time on were devoted to further developments and defense of the neutral theory.

The mathematical theory of evolution had its heyday in the period roughly from 1920 to 1950. It was almost completely dominated by three men, R. A. Fisher and J. B. S. Haldane in England, UK and Sewall Wright in the US. Kimura became the logical successor. Much of Kimura’s work utilized the stochastic equations of the Russian mathematician, A. Kolmogrov. Kimura showed a remarkable inventiveness in solving these difficult equations and applying them creatively to significant evolutionary problems.

Among the problems that Kimura solved are: the probability that a new mutant gene will ultimately spread through the population; the number of generations required for this process; if the mutation is lost, the number of generations it persists before loss; the number of individuals that carry a mutation during the time until it is fixed or lost; and the average age of a mutant gene segregating in a population.

At the same time he contributed to many other areas of population genetics theory. These included: a comparison of inbreeding systems, showing that systems minimizing the immediate increase in homozygosity are not the best in the long run; a further development of Fisher’s ‘fundamental theorem of natural selection’ with more explicit treatment of gene interactions; a new and influential model of population structure, the ‘stepping stone’ model, in which migrants are restricted to neighboring colonies; the efficiency of rank-order selection in changing gene frequencies and removing deleterious mutations from the population.

By 1968 the subject of molecular evolution was in a period of rapid development, which still continues. This was the year of Kimura’s bombshell. He argued that the great majority of amino acid and nucleic acid changes are selectively neutral. Instead of natural selection as the main directive force, these changes occur by mutation and whether they persist or are lost is a matter of chance. Remarkably, Kimura’s early work on stochastic processes turned out to be preadapted for the study of molecular evolution. He had a set of ready-made tools available, and he proceeded to exploit them with great vigor.

The neutral theory made a number of qualitative and quantitative predictions. Among these was that molecular changes occur at a rate independent of structural and physiological changes; Kimura noted that sharks, whose external form has hardly changed in an enormous time period, have as much molecular variation as rapidly evolving animal species, such as mammals. There were a number of similar predictions. One of the simplest and most remarkable is that the rate of neutral gene substitution in the population is equal to the rate of mutation of this gene per individual. This has permitted predictions of evolutionary rates; departures from these predictions provide evidence for natural selection. Rates that are slower than the neutral expectation can be attributed to ‘selective constraint;’ that is, the amino acids in this region are so specifically fitted to their function that any replacement doesn’t work as well. Thus, essentially all mutations are harmful and are removed by natural selection (sometimes called purifying selection). Several proteins, for example some histones, are in this category. Any change in the molecular structure is harmful. In contrast, regions that evolve faster than the neutral rate are attributed to positive selection. An appreciable number of the mutations are beneficial. One of the best understood examples is the DNA region governing tissue transplants.

The neutral theory has provided a rational basis for the idea of a ‘molecular clock’ guiding the rate of evolution. The rough constancy of the mutation rates for comparable genes in different organisms—in particular that they are much more stable than the forces of selection—has led to the clock assumption becoming the basis for construction of phylogenetic trees, a field of great activity as molecular data are increasingly used.

At first, Kimura’s theory was rejected out of hand by most evolutionists. But gradually, over the years, it has come to be widely accepted. It is now realized that a great deal of the DNA of higher organisms has no known function and that the actual genes constitute a small part of the DNA. At present it is accepted that most nucleic acid substitutions follow the neutral paradigm. As for amino acid changes, the jury is still out. Clearly some changes follow neutral kinetics, others clearly are selected, and the proportions are yet to be sorted out.

3. An Assessment

Let it be said at the outset that Kimura’s work has had its greatest impact on evolution theory rather than on the social and behavioral sciences. His work is very general and is applicable to the whole living world, of which that of greatest interest to social scientists, human evolution, and human behavioural evolution in particular, are only a part. Molecular evolution has yet to make the impact on human behavioral evolution that it is certain to make one day.

Kimura’s greatest contribution has been to impose a stochastic viewpoint on the study of evolution. Beneath surface regularities (and irregularities) of form and function are a multitude of molecular changes, governed by the laws of chance. At present, research in behavioral genetics and evolution is based mainly on deterministic theory. Problems such as separating genetic from environmental effects are usually studied by such means as observing adopted children or identical twins reared apart, and the techniques are usually those derived from analysis of variance. But as evolutionary and population genetics theory becomes further developed and richer, and as the molecular basis of behavioral traits (including those in humans) are better understood, stochastic aspects will play an increasingly important role.

The study of natural selection and adaptation has long had difficulty in formulating clearly testable and falsifiable hypotheses. This has been particularly evident in the abundance of ‘just so’ stories; because we can think of a plausible reason for a structure or function doesn’t necessarily mean that it arose for this reason (although it often has). One attribute of the neutral theory is that it can provide a null hypothesis for testing causes of evolutionary changes. For example, as mentioned above, the neutral theory makes a simple evolutionary rate prediction, against which departures can be measured. Thus far, this kind of reasoning has been used mainly for genes with physiological functions, but as the molecular basis of behavioral changes becomes better understood this kind of analysis will be increasingly applicable.

During his lifetime, Motoo Kimura received virtually every honor for which an evolutionary geneticist is eligible. In addition to numerous honors both in Japan and elsewhere, he was a foreign member of the United States National Academy of Sciences, the Royal Society of London, and the Chevalier de L’Ordre National du Merite in France. He received honorary doctorates from the Universities of Chicago and Wisconsin.

Kimura’s lasting impact is twofold. The first, best appreciated by population geneticists, is his enrichment of population genetic and evolutionary theory, especially as these are influenced by stochastic processes. The second, appreciated by a much larger audience, is his discovery of the enormous role that random processes play in molecular evolution. Kimura never denied the importance of Darwinian natural selection as the main evolutionary force shaping form and function, but beneath this, he emphasized, is an enormous play of random changes at the molecular level. In addition to Darwin’s ‘survival of the fittest,’ he added the slogan, ‘survival of the luckiest.’


  1. Kimura M 1983 The Neutral Theory of Molecular Evolution. Cambridge University Press, Cambridge, UK
  2. Kimura M 1994 Population Genetics, Molecular Evolution, and the Neutral Theory. University of Chicago Press, Chicago
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