Sewall Wright

Sewall Wright (1889 to 1988) was an American geneticist who, with Ronald Fisher and J. B. S. Haldane, laid the mathematical foundations of population genetics. He is known for the ideas of genetic drift, the inbreeding coefficient and the fitness landscape.

Wright began with the inheritance of coat color in guinea pigs while working for the US Department of Agriculture, and from years of breeding experiments he developed a theory of inbreeding. He introduced the inbreeding coefficient F, which measures the probability that two copies of a gene in an individual are identical by descent, and also invented path analysis, a statistical method for tracing causes that is still widely used.

Wright showed that in small populations the frequencies of alleles can change by chance, a process called genetic drift, and that the strength of drift depends on the effective population size. In 1932 he pictured the fitness of all combinations of alleles as a landscape of peaks and valleys. His shifting balance theory proposed that a species divided into small, semi-isolated groups could cross valleys by drift and then climb to higher peaks through selection.

Wright and Fisher disagreed for decades over the importance of drift compared with selection, a disagreement that shaped the modern synthesis and later the neutral theory. Modern genomics still uses his measures of population structure, the F-statistics. He worked at the University of Chicago and the University of Wisconsin, and remained active into his nineties.

• Genetic Drift — a force he formalized
• Fitness (Biology) — the quantity on his landscape
• Ronald Fisher — his major colleague and rival