Motoo Kimura

Motoo Kimura was a highly influential Japanese biologist and theoretical population geneticist. He is most famous for introducing the neutral theory of molecular evolution in 1968. This theory fundamentally changed how scientists view the way life changes over time at the molecular level. Before Kimura, many believed that natural selection was the primary driver of all genetic changes. Kimura proposed that most changes in DNA are actually neutral. This means these changes do not significantly help or harm an organism's ability to survive. Instead, he argued that these genetic shifts occur largely through a process called genetic drift.

To explain how these changes work, Kimura used complex mathematical tools. He applied diffusion equations to study the movement of genes within a population. Specifically, he used the Kolmogorov backward equation to calculate the probability of fixation. Fixation occurs when a specific version of a gene, known as an allele, becomes common to the entire population. Kimura's math allowed him to predict whether an allele would be beneficial, deleterious, or neutral. By using these equations, he could track how allele frequencies change due to chance. This approach turned biology into a precise, quantitative science.

Kimura developed several different models to describe how mutations occur. He introduced the "infinite alleles" and "infinite sites" models to help explain genetic diversity. He also created the "stepwise" mutation model, which is often called a "ladder model." This specific model is very useful for electrophoresis studies. In these studies, scientists look at homologous proteins that differ by whole units of charge. These different models allowed researchers to better understand the growing data from peptide and genetic sequences. Each model provided a different way to map the history of molecular change.

Kimura's journey began in Okazaki, Aichi Prefecture, where he was born on November 13, 1924. He showed an early interest in botany and a talent for mathematics. While recovering from food poisoning, he even taught himself geometry. He attended Kyoto Imperial University, where he studied plant morphology and cytology. During World War II, he entered university early to avoid military duty. After the war, he joined the National Institute of Genetics in Mishima. He later traveled to the United States to study at Iowa State College and the University of Wisconsin.

While studying in the United States, Kimura joined a powerful intellectual community. He worked alongside the renowned geneticist Sewall Wright. In 1955, he presented a paper at the Cold Spring Harbor Symposium. Although the math was very complex, the presentation earned him praise from Wright and J.B.S. Haldane. Kimura earned his PhD in 1956 before returning to Japan. He spent the rest of his career at the National Institute of Genetics. There, he worked on many problems in theoretical population genetics, often collaborating with Takeo Maruyama.

His theory of neutral evolution was met with significant debate when it was first released. Many evolutionary biologists disagreed with him because they focused on natural selection. However, many molecular biologists supported his ideas. As molecular biology expanded, Kimura continued to defend and refine his theory. He wrote a major monograph on the subject in 1983 titled "The Neutral Theory of Molecular Evolution." He also wrote "My Views on Evolution," which became a best-seller in Japan. Today, his work is a core part of how we study molecular evolution.

Kimura received many prestigious honors throughout his life for his scientific contributions. In 1968, he won the Japan Academy Prize. He was later named a Foreign Member of the National Academy of Sciences of the USA in 1973. In 1992, he was awarded the Darwin Medal from the Royal Society. He also became a Foreign Member of the Royal Society in 1993. These awards recognized his role as one of the greatest evolutionary geneticists. His work remains a bridge between the study of individual organisms and the study of molecules.

In his later years, Kimura faced health challenges due to amyotrophic lateral sclerosis. He passed away on November 13, 1994, following a cerebral hemorrhage caused by an accidental fall. He left behind a significant legacy in the field of genetics. He was married to Hiroko Kimura, and they had a son named Akio. His work continues to guide scientists as they explore the deep history of life on Earth. Through his math and his theories, he helped us see the invisible patterns of evolution.