Motoo Kimura (1924-1994) OR decades the field of mathematical population prize-winners. When our textbook (CROW and KIMURA F genetics and evolutionary theory was dominated 1970) was published, he used his royalties to build a by the three pioneers,J. B. S. HALDANE,R. A. FISHER, tiny greenhouse attached to his home. Every Sunday and SEM'A1.L. WRIGHT.M'ith WRIGI-IT'Sdeath (CROW was orchid day. He used his artistic talent to paint pic- 1988), and for some time before, the leadingsuccessor tures of his favorite flowers, usually on chinaware. to this great heritage was MOTOO KIMURA.Although From age 17 to 19 KIMURA was in high school, where best known for his daring neutral theory of molecular a friendly and scientifically literate teacher encouraged evolution, a concept of great interest andequally great his study of chromosome morphology, and hebecame controversy, he is admired by populationgeneticists a plant cytogeneticist. At that time, cytogenetics was even more forhis deep contributions to the mathemati- very popular in Japan,and he joined thearmy of chro- cal theory. mosome watchers. During this period he was also fasci- MOTOO KIMURA was born November 13,1924 in Oka- nated by a physics course. HIDEKIYUKAWA, later to win zaki, Japan. He diedNovember 13, 1994, on theseventi- the Nobel Prize for predicting the meson, became his eth anniversary of his birth. For some timehe had been scientific hero, and KIMURA began to take an interest a victim of amyotrophic lateral sclerosis and was pro- in mathematics as the language of science. gressively weakening. Nevertheless, his death was acci- Japan was then in themidst of World War 11, and the dental. He fell, hitting his head, and never regained normal high school period was shortened from three consciousness. Death can be merciful; he had nothing to two and a half years. In 1944 he was admitted to to look forward to but further deterioration. Kyoto Imperial University. HITOSHIKIHARA wasJapan's KIMURA'S father was a businessman who loved flowers foremost geneticist (CROW1994), a world leader in the and raised ornamentals in the home. Young MOTOO cytogenetics of wheat, and KIMURA might have been was fascinated by their beauty and curious as to their expected to study with him. Nevertheless, KIHARA ad- development.When his fatherbought him a micro- vised him to enroll in botany. There was a reason. At scope, he spent hours with it. In school he developed that time students in botany were exempt from military an interestin plants and decided to become a systematic service until graduation. Curiously, students of agricul- botanist. At the same time he en-joyed mathematics, ture-KIHARA'S area-did not enjoy this privilege. especially EUCLID.He showed definite promise, for his In this way KIMURA escaped military service, but life teacher advised him to become a mathematician, which was far from easy. There was the irritation of regular advice he ignored. military drill, and there was never enough good food. &MUM never lost his love of plants and retained an The bomb onHiroshima came beforehis first university interest in botany throughout his life. He later became year was finished, but conditions immediately after the an avid orchid breeder, andseveral of his creations were war were even worse. Food was even harder to obtain 2 J. F. Crow and KIMURA made regular Sunday visits to a cousin for his papers. That KIMURA persisted in working alone in a good meal. The cousin was a quantum physicist, so an indifferent, if not hostile, environment was charac- these Sundays were also occasions for scientific talk. teristic. Then, as later, he was confident of his own Although still a student of cytology, KIMURA became abilities and knew what he wanted to do. increasingly interested in mathematical questions. He KIMURA’S chance to study abroad came through DUN- was first attracted to mapping functions and through CAN MCDONALD,who was workingwith the Atomic this learned ofJ. B. S. HAL.DANE. Reading DOBZHANSKY‘S Bomb Casualty Commissionin Hiroshima. MCDONALD, Genetics and the origzn of Species led him to the work of coming from Dartmouth College, knew that theCollege SEWALLWRIGHT. By this time, the war was over and he had a fund for support of genetics and was generous had moved to KIHARA’S department. KIHARAwas doing with it. This, plus a Fulbright travel fellowship, allowed backcrosses in wheat to introduce parts of a genome KIMURA to come to theUnited States.MCDONALD into a different cytoplasm, and KIMURA helped him by thought he should study with WRIGHT,but WRIGHTwas deriving the frequency distribution of introduced chro- considering retirement and recommended that he go mosomes in successive backcrossed generations. This to Iowa State University with J. L. LUSH. led to hisfirst published scientific paper (KIMURA My acquaintance with KIMURA began as he was just 1950). starting his graduate work at Iowa State. We met by KIHARAassigned no specific duties, and KIMURA bus- accident in the University of Wisconsin Union where ied himself reading WRIGHT’Spapers. There was only a the Genetics Society was meeting. I had heard of KI- single copy and of course no duplicating facilities, so MURA through my student, NEWTONMORTON, who had he copied the papers by hand. (I recall, years later, been MCDONALD’Scolleague in Hiroshima. I must have seeing and marveling at KIMURA’S neatly copied version been almost unique in the United States in recognizing of WRIGHT’S 63-page 1931paper, complete with occa- the name KIMURA. Thus began a friendship that lasted sional notes and derivations of his own.) for the rest of his life. He brought with him a paper In 1949 KIMURA joined the research staff of the Na- that he had written on the ship between Japan and tional Institute of Genetics in Mishima, a position he Seattle. The paper dealt with fluctuating selection coef- retained for the rest of his life. The laboratory was a ficients. I was greatly impressed, for he had found a crude wooden building that had been an airplane fac- transformation that converted a cumbersome partial tory during the war. Mishima was small and provincial, differential equation into the familiar expression for a striking contrast to the intellectual and cultural at- heat conduction. The paper was reviewed by WRIGHT, tractions of Kyoto, but on clear days it did provide a who praised it lavishly-unusual for WRIGHT-and it magnificent view of Mount Fuji. Actually,KIMURA spent was published in GENETICS(KIMURA 1954). much of his time in KIHARA’S laboratory in Kyoto where KIMURA soon became dissatisfied with the direction there was a much better library. Studying probability of research at Iowa State, with its emphasis on subdivi- textbooks, he discovered that the FOKKFLR-PLANCKequa- sion of epistatic variance. Finding little interest there tion that WRIGHThad used was only one of two KOLMO- in stochastic models, he wrote asking to work with me GOROV equations, the forward one. Later, KIMURA was at the University of Wisconsin.I gladly accepted and he to be especially creativein his use of the backward equa- came to Wisconsin early in the summer of 1954. Before tion. the summer was over, he had worked out the complete By this time KIMURA was devoting his time entirely solution of neutral random drift in a finite population to mathematical genetics. As was his lifetime habit, he (KIMURA 1955). A few months later, WRIGHTmoved to learned the subject for himself. His formal training in Madison, and KIMURA finally had his dream-a chance mathematics was quite limited; he simply learned what to study with WRIGHT. Actually,although there was mu- he had to learn to solve the problem at hand. He was tual admiration, they never worked together. Their ap- helped, of course, by being exceptionally gifted. During proaches were too different. this time he proposed the “stepping stone” migration KIMURA was invited to the Cold Spring Harbor Sym- model (KIMURA 1953).WRIGHT’S island model, the stan- posium of 1955, where he met the leading population dard of the time, assumed that immigrants come at geneticists. The attendees by this time had heard of the random from a larger population. KIMURA introduced Japanese phenomenon, although most could under- the more realistic model that immigrants come from a stand neither his mathematics nor his English. Another nearneighbor. By adding long-range migrants, he meeting at StonyBrook allowed him to meet H. J. could include the island model as a special case. MULLER,whom he greatly admired, and they became This was a lonely period for KIMuRA, for none of his regular correspondents. KIMURA always insisted that associates understood his work, or thought it to be of MULLERshould occupy a place among the great pio- any interest. The exception was TAKUKOMAI, also in neers in evolutionary thinking. Kyoto. KOMAI had studied with T. H. MORGANin the His two years in Wisconsin as a graduate studentwere United States and, although he didn’tunderstand remarkably productive. He wrote a number of papers WRIGHT’Smathematics, he encouraged KIMURA to study extending the drift model to multiple alleles, mutation, Motoo Kimura (1924-1994) 3 migration, and selection. It was during this period that approximation to truncation selection is almost as effec- he first used the KOLMOGOROVbackward diffusion tive as strict truncation. This made the theory much equation to find the probability of ultimate fixation of more realistic for natural populations. a mutant with arbitrary dominance, a finding laterto be KIMURA’S early Wisconsin workon the probability of of great use in molecular evolution. During this period, fixation of a new mutant was followed by studies of KIMURA worked out the conditions fora stable equilib- the average time until fixation, the time until loss, the rium in a multi-allelic locus and for the evolution of number of individuals carrying the mutant gene and closer linkage by selection with epistasis.