As author of the "shifting-balance" theory—concerned with how certain gene combinations spread throughout a population—Sewall Wright transformed scientific views concerning evolution. Much of his research was done on guinea pigs, not previously viewed as useful for scientific research; Wright was the first to make use of the creature whose name became virtually synonymous with "laboratory animal." Wright also contributed to genetics by encouraging the use of statistical analysis.
The oldest of three children, Wright was born on December 21, 1889, to Philip, a college professor, and Elizabeth Sewall Wright. A child prodigy who wrote a small book called "The Wonders of Nature" at age seven, Wright was an exceptional student. Intending to study languages, he began his higher education at Lombard College, where his father taught, but a teacher interested him in biology instead. In the summer after his graduation from Lombard in 1911, he worked at Columbia University's Cold Spring Harbor laboratory on Long Island, New York. Wright earned his M.S. in zoology at the University of Illinois in 1912, and his Sc.D. in that discipline from Harvard in 1915.
At Harvard's Bussey Institution, a research facility, Wright had begun working with guinea pigs, which he considered useful for research despite their relatively long reproductive cycles. He discovered gene sequences that produced various effects on coat and eye color in the guinea pigs, and wrote his dissertation on that subject.
Wright's studies of guinea pigs continued after he became senior animal husbandman at the U.S. Department of Agriculture (USDA) in Washington, D.C., in 1915. There he analyzed the results of a massive guinea pig inbreeding study that the USDA had begun in 1906, developing a mathematical theory of inbreeding that he published in 1921 as Correlation and Causation.
Back at Cold Spring Harbor in 1920, Wright met Louisa Williams, an instructor at Smith College in Massachusetts. The two married in 1921, and later had two sons, Richard and Robert. In 1925 Wright left the USDA to become a professor of zoology at the University of Chicago. The late 1920s saw Wright engaged in debate with geneticist Ronald A. Fisher (1890-1962), who maintained that the success of natural selection was a function of population size—i.e., the larger the sample, the faster and more effective the development of mutations.
By 1931 this debate had sparked Wright to present his own theory concerning evolution. This, the "shifting balance" theory, held that a random gene-frequency drift within subpopulations leads to an increase in the preferred combination of genes, and in turn brings about the dispersal of the preferred gene throughout the larger population. Later in the 1930s, Wright worked with geneticist Theodosius Dobzhansky (1900-1975), who adapted Wright's mathematical methods in his highly influential book Genetics and the Origin of Species (1937).
Wright continued to conduct research and publish his findings in the 1940s. In 1954 he left the University of Chicago for the University of Wisconsin, where he devoted most of his efforts to the four-volume Evolution and the Genetics of Populations (1968-78). He received the National Medal of Science (1967), the Darwin Medal (1980), nine honorary doctorates, and numerous other awards.
In 1975 Louisa died of pneumonia, and five years later Wright began to lose his vision. He remained physically active, however, and died in 1988 at the age of 99. His wrote and published his last paper in the year of his death. In 1991, three years after his death, Science published the results of an experiment by a pair of geneticists that confirmed Wright's shifting-balance theory.