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Sewall Wright

Sewall Green Wright (December 21, 1889 – March 3, 1988) was an American geneticist known for his influential work on evolutionary theory and also for his work on path analysis. With R. A. Fisher and J.B.S. Haldane, he was a founder of theoretical population genetics. He is the discoverer of the inbreeding coefficient and of methods of computing it in pedigrees. He extended this work to populations, computing the amount of inbreeding of members of populations as a result of random genetic drift, and he and Fisher pioneered methods for computing the distribution of gene frequencies among populations as a result of the interaction of natural selection, mutation, migration and genetic drift. The work of Fisher, Wright, and Haldane on theoretical population genetics was a major step in the development of the modern evolutionary synthesis of genetics with evolution. Wright also made major contributions to mammalian genetics and biochemical genetics.



Sewall Wright was born in Melrose, Massachusetts to Philip Green Wright and Elizabeth Quincy Sewall Wright. The family moved three years later after Philip accepted a teaching job at Lombard College, a Universalist college in Galesburg, Illinois.

He was the youngest of three gifted brothers – the others being the aeronautical engineer Theodore Paul Wright and the political scientist Quincy Wright. From an early age Wright had a love and talent for mathematics and biology. Wright attended Galesburg High School and graduated in 1906. He then enrolled in Lombard College where his father taught, to study mathematics. He was influenced greatly by Professor Wilhelmine Entemann Key, one of the first women to receive a Ph.D. in biology. Wright received his Ph.D. from Harvard University, where he worked with the pioneering mammalian geneticist William Ernest Castle investigating the inheritance of coat colors in mammals. He worked for the U.S. Department of Agriculture until 1925, when he joined the Department of Zoology at the University of Chicago. He remained there until his retirement in 1955, when he moved to the University of Wisconsin-Madison. He received many honors in his long career, including the National Medal of Science, the Balzan Prize, and the Darwin Medal of the Royal Society. He was a member of the National Academy of Science and a Foreign Member of the Royal Society.

Wright married Louise Lane Williams (1895-1975) in 1921. They had three children: Richard, Robert, and Elizabeth.

Scientific achievements and credits

His papers on inbreeding, mating systems, and genetic drift make him a principal founder of theoretical population genetics, along with R. A. Fisher and J. B. S. Haldane. Their theoretical work is the origin of the modern evolutionary synthesis or neodarwinian synthesis. Wright was the inventor/discoverer of the inbreeding coefficient and F-statistics, standard tools in population genetics. He was the chief developer of the mathematical theory of genetic drift, which is sometimes known as the Sewall Wright effect, cumulative stochastic changes in gene frequencies that arise from random births, deaths, and Mendelian segregations in reproduction. Wright was convinced that the interaction of genetic drift and the other evolutionary forces was important in the process of adaptation. He described the relationship between genotype or phenotype and fitness as fitness surfaces or fitness landscapes. On these landscapes fitness was the height, plotted against horizontal axes representing the allele frequencies or the average phenotypes of the population. Natural selection would lead to a population climbing the nearest peak, while genetic drift would cause random wandering.

Wright's explanation for stasis was that organisms come to occupy adaptive peaks. In order to evolve to another, higher peak, the species would first have to pass through a valley of maladaptive intermediate stages. This could happen by genetic drift if the population is small enough. If a species was divided into small populations, some could find higher peaks. If there was some gene flow between the populations, these adaptations could spread to the rest of the species. This was Wright's shifting balance theory of evolution. There has been much skepticism among evolutionary biologists as to whether these rather delicate conditions hold often in natural populations. Wright had a long standing and bitter debate about this with R. A. Fisher, who felt that most populations in nature were too large for these effects of genetic drift to be important.

Wright strongly influenced Jay Lush, who was the most influential figure in introducing quantitative genetics into animal and plant breeding. Wright's statistical method of path analysis, which he invented in 1921 and which was one of the first methods using a graphical model, is still widely used in social science. He was a hugely influential reviewer of manuscripts, as one of the most frequent reviewers for Genetics. Such was his reputation that he was often credited with reviews that he did not write.

He did major work on the genetics of guinea pigs, and many of his students became influential in the development of mammalian genetics. He appreciated as early as 1917 that genes acted by controlling enzymes.

Wright and philosophy

Wright was one of the few geneticists of his time to venture into philosophy. He found a union of concept in Charles Hartshorne, who became a lifelong friend and philosophical collaborator. Wright believed that the birth of the consciousness was not due to a mysterious property of increasing complexity, but rather an inherent property, therefore implying these properties were in the most elementary particles.


Wright and Fisher were the key figures in the neodarwinian synthesis that brought genetics and evolution together. Their work was essential to the contributions of Dobzhansky, Mayr, Simpson, Julian Huxley, and Stebbins. The neodarwinian synthesis was the most important development in evolutionary biology after Darwin. Wright also had a major effect on the development of mammalian genetics and biochemical genetics.


Since Wright developed the methods used for assessing the degree of inbreeding and its effects, it is notable that his own parents were first cousins. As a child he helped his father and brother print and publish an early book of poems by his father's student Carl Sandburg.


  • Crow, James F. (1988) "Sewall Wright (1889-1988)" Genetics 119 (1): 1-4.
  • Crow, James F. and W. F. Dove. (1987) "Sewall Wright and physiological genetics" Genetics 115 (1): 1-2.
  • Ghiselin, Michael T. (1997) Metaphysics and the Origin of Species. NY: SUNY Press.
  • Hill, William G. (1996) "Sewall Wright's 'Systems of Mating'" Genetics 143 (4): 1499-506.
  • Provine, William (1986). Sewall Wright and Evolutionary Biology. University of Chicago Press. ISBN 0-226-68473-3. 
  • Wright, Sewall (1932) "The roles of mutation, inbreeding, crossbreeding and selection in evolution" Proc. 6th Int. Cong. Genet. 1: 356–366.
  • Wright, Sewall (1986). Evolution: Selected papers. University of Chicago Press. ISBN 0-226-91053-9. 
This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Sewall_Wright". A list of authors is available in Wikipedia.
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