Nei was chosen to receive the award in recognition of his research achievements, which are the foundation of the theoretical framework of the field of molecular evolutionary biology. According to a statement by the society, Nei has contributed enormously to the development not only of his own field, but to every branch of biology that is concerned with diversity and evolution. "By developing various statistical methods and applying them to molecular data, he pioneered new ways of studying the genetic diversity of populations, evolutionary relationships among organisms, the times of species divergence from common ancestors, the location of gene regions in which natural selection is operating, and related areas. The methods he introduced have made it possible to obtain quantitative estimates of various parameters of evolutionary importance that could not have been measured experimentally. Through these achievements, Dr. Nei has not only made the latest findings at the molecular level available to evolutionary biologists, but has contributed greatly to the birth of molecular evolutionary biology and its establishment as a positive science in which hypotheses can be verified quantitatively, rather than being discussed solely on a conceptual level. "
Among his many achievements, Nei has constructed a mathematical theory of the relationships between species using molecular data obtained from their genes. With one of his graduate students, he developed the neighbor-joining method of inferring these relationships, known as molecular phylogenies. This method has become the most widely used worldwide for constructing phylogenetic trees. "It was a particularly charming moment when Emperor Akihito of Japan, who has studied the taxonomy and evolution of gobioid fishes, mentioned in his congratulatory address during the award ceremony that he has used the neighbor-joining method to construct phylogenetic trees during his studies of these fishes," Nei says.
In a career spanning more than forty years, Nei has worked with many collaborators in his development of various statistical methods to determine the molecular mechanisms of biological diversity and of evolution. He also has expanded this underlying theory and has applied his methods to the analysis of data.
For example, in his research on mathematical modeling of DNA evolution, Nei devised several statistical methods--now used by a large number of investigators--for estimating the number of changes, or substitutions, that are likely to occur in the nucleotide building blocks of the DNA molecule during its evolution. Nei then applied these methods in his studies of the genes of the major histocompatibility complex (MHC), which is involved in immune responses such as the rejection of transplanted organs. Among his discoveries concerning the major histocompatibility complex is that the puzzling diversity of its genes in human populations is due to a special pattern of DNA evolution that occurs when genes protect their hosts from invaders such as viruses and bacteria.
One of the best-known statistical methods developed by Nei, which was published in 1972 but still is in frequent use by scientists worldwide, defines the degree of genetic difference between populations and estimates this genetic distance from molecular data gleaned from DNA and protein molecules. Known as Nei's genetic distance, this measure makes it possible to estimate from the molecular data the origins of populations and the times of their divergence from common ancestors. Nei applied this technique to human populations and obtained the first evidence pointing to the African origins of modern humans. This paper on genetic distance has been listed among the 1000 most-cited papers in all scientific fields.
Nei also is the author of many influential papers and four books, has served on numerous editorial and review boards in the United States and in Japan, and was the cofounder and coeditor of Molecular Biology and Evolution, the leading journal in the field.
Nei earned a bachelor's degree in genetics at the Miyazaki University of Japan in 1953, a master's degree in genetics at Koyoto University in Japan in 1955, and a doctoral degree in quantitative genetics at Koyoto University in Japan in 1959.
He was an assistant professor at Kyoto University in Japan from 1958 to 1962, a geneticist at the National Institute of Radiological Sciences in Japan from 1962 to 1969, and head of the Population Genetics Laboratory at the National Institute of Radiological Sciences in Japan from 1965 to 1969. He moved to the United States in 1969, where he held the positions of associate professor and professor at Brown University from 1969 to 1972, acting director of the Center for Demographic and Population Genetics at the University of Texas at Houston from 1979 to 1980 and from 1986 to 1987, and professor of population genetics at the University of Texas at Houston from 1972 to 1990. Nei joined the Penn State faculty in 1990 as distinguished professor of biology and founding director of the Institute of Molecular Evolutionary Genetics, and was named Evan Pugh Professor of Biology in 1994. He was a visiting professor of biology at Tokyo Institute of Technology in Japan for three months during 2001.
He was elected an honorary member of the Genetics Society of Japan in 1989, a Fellow of the American Academy of Arts and Sciences in 1990. He was elected a Fellow of the American Association for the Advancement of Science in 1993, an honorary member of the Japan Society of Human Genetics in 1996, a member of the U. S. National Academy of Sciences in 1997, an honorary member of the Japan Society for Histocompatibility and Immunogenetics in 2000, and an honorary member of the Japan Society of Animal Genetics and Breeding in 2001.
Among his numerous honors and awards, in addition to the 2002 International Prize for Biology and numerous invited lectureships, Nei has received the Japan Society of Human Genetics Award in 1977, the Kihara Prize of the Genetics Society of Japan in 1990, a certificate of award from the Institute for Scientific Information as a "Highly Cited Researcher" in 2000, and an honorary doctoral degree from Miyazaki University in Japan in 2002. In addition, the Society for Molecular Biology and Evolution established the annual Masatoshi Nei Annual Lecture in his honor in 2000.