Sewall Wright

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Sewall Wright
Sewall Wright.jpg
Wright in 1954
Born(1889-12-21)December 21, 1889
DiedMarch 3, 1988(1988-03-03) (aged 98)
Alma mater Lombard College
University of Illinois
Harvard University
(Sc.D., 1915) [1]
Known for Population genetics
Causal graph
Fixation index
Fitness landscape
Genetic rescue
Genetic drift
Inbreeding coefficient
Path analysis
Shifting balance theory
Threshold model
Awards Daniel Giraud Elliot Medal (1945)
Weldon Memorial Prize (1947)
National Medal of Science (1966)
Darwin Medal (1980)
Thomas Hunt Morgan Medal (1982)
Balzan Prize (1984)
Foreign Member of the Royal Society (1963) [2]
Scientific career
Fields Genetics
Institutions University of Chicago [3]
University of Wisconsin [3]
Doctoral advisor William Ernest Castle

Sewall Green Wright FRS(For) HFRSE (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. He was a founder of population genetics alongside Ronald Fisher and J. B. S. Haldane, which was a major step in the development of the modern synthesis combining genetics with evolution. He discovered the inbreeding coefficient and methods of computing it in pedigree animals. He extended this work to populations, computing the amount of inbreeding between members of populations as a result of random genetic drift, and along with Fisher he 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. Wright also made major contributions to mammalian and biochemical genetics. [4] [5] [6]



Sewall Wright was born in Melrose, Massachusetts to Philip Green Wright and Elizabeth Quincy Sewall Wright. His parents were first cousins, [7] an interesting fact in light of Wright's later research on inbreeding. The family moved three years later after Philip accepted a teaching job at Lombard College, a Universalist college in Galesburg, Illinois.

As a child, Wright helped his father and brother print and publish an early book of poems by his father's student Carl Sandburg.

He was the oldest 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 at the Bussey Institute 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 (1966), the Balzan Prize (1984), and the Darwin Medal of the Royal Society (1980). He was a member of the National Academy of Sciences and a Foreign Member of the Royal Society. [2] The American Mathematical Society selected him as the Josiah Willards Gibbs lecturer for 1941. [8] [9] For his work on genetics of evolutionary processes, Wright was awarded the Daniel Giraud Elliot Medal from the National Academy of Sciences in 1945. [10]

He died in Madison, Wisconsin on 3 March 1988.


Wright married Louise Lane Williams (1895–1975) in 1921. [11] [12] They had three children: Richard, Robert, and Elizabeth. [13] [14]

Sewall Wright worshipped as a Unitarian. [15] [16]

Scientific achievements and credits

Population genetics

Visualization of a fitness landscape. The X and Y axes represent continuous phenotypic traits, and the height at each point represents the corresponding organism's fitness. The arrows represent various mutational paths that the population could follow while evolving on the fitness landscape. Visualization of two dimensions of a NK fitness landscape.png
Visualization of a fitness landscape. The X and Y axes represent continuous phenotypic traits, and the height at each point represents the corresponding organism's fitness. The arrows represent various mutational paths that the population could follow while evolving on the fitness landscape.

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, [17] cumulative stochastic changes in gene frequencies that arise from random births, deaths, and Mendelian segregations in reproduction. In this work he also introduced the concept of effective population size. 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 evolutionary landscapes. On these landscapes mean population 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.

Evolutionary theory

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.

Path analysis

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.

Plant and animal breeding

Wright strongly influenced Jay Lush, who was the most influential figure in introducing quantitative genetics into animal and plant breeding. From 1915 to 1925 Wright was employed by the Animal Husbandry Division of the U.S. Bureau of Animal Husbandry. His main project was to investigate the inbreeding that had occurred in the artificial selection that resulted in the leading breeds of livestock used in American beef production. He also performed experiments with 80,000 guinea pigs in the study of physiological genetics. Further more he analyzed characters of some 40,000 guinea pigs in 23 strains of brother-sister matings against a random-bred stock. (Wright 1922a-c). The concentrated study of these two groups of mammals eventually led to the Shifting Balance Theory and the concept of "surfaces of selective value" in 1932. (Wright 1988 Pg 122 American Naturalist)

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. An anecdote about Wright, disclaimed by Wright himself, describes a lecture during which Wright tucked an unruly guinea pig under his armpit, where he usually held a chalkboard eraser: according to the anecdote, at the conclusion of the lecture, Wright absent-mindedly began to erase the blackboard using the guinea pig.[ citation needed ]

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 endorsed a form of panpsychism. He 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. [18]


Wright and Fisher, along with J.B.S. Haldane, were the key figures in the modern synthesis that brought genetics and evolution together. Their work was essential to the contributions of Dobzhansky, Mayr, Simpson, Julian Huxley, and Stebbins. The modern 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.

"The Book of Why" by Judea Pearl and Mackenzie (2018) describes the contribution of Wright's work on path analysis and delays in its acceptance by several technical disciplines (specifically statistics and formal causal analysis).

OpenMx has as its icon a representation of Wright's Piebald Guinea Pig.


Related Research Articles

Genetic drift The change in the frequency of an existing gene variant in a population

Genetic drift is the change in the frequency of an existing gene variant (allele) in a population due to random sampling of organisms. The alleles in the offspring are a sample of those in the parents, and chance has a role in determining whether a given individual survives and reproduces. A population's allele frequency is the fraction of the copies of one gene that share a particular form. Genetic drift may cause gene variants to disappear completely and thereby reduce genetic variation. It can also cause initially rare alleles to become much more frequent and even fixed.

Modern synthesis (20th century) Combination of Darwins theory of evolution with natural selection and Mendels findings on heredity

The modern synthesis was the early 20th-century synthesis reconciling Charles Darwin's theory of evolution and Gregor Mendel's ideas on heredity in a joint mathematical framework. Julian Huxley coined the term in his 1942 book, Evolution: The Modern Synthesis.

Population genetics Study of genetic differences within and between populations including the study of adaptation, speciation, and population structure

Population genetics is a subfield of genetics that deals with genetic differences within and between populations, and is a part of evolutionary biology. Studies in this branch of biology examine such phenomena as adaptation, speciation, and population structure.

Evolutionary biology Study of the processes that produced the diversity of life

Evolutionary biology is the subfield of biology that studies the evolutionary processes that produced the diversity of life on Earth. In the 1930s, the discipline of evolutionary biology emerged through what Julian Huxley called the modern synthesis of understanding, from previously unrelated fields of biological research, such as genetics and ecology, systematics and paleontology.

E. B. Ford British ecological geneticist

Edmund Brisco "Henry" Ford was a British ecological geneticist. He was a leader among those British biologists who investigated the role of natural selection in nature. As a schoolboy Ford became interested in lepidoptera, the group of insects which includes butterflies and moths. He went on to study the genetics of natural populations, and invented the field of ecological genetics. Ford was awarded the Royal Society's Darwin Medal in 1954.

Fisher's fundamental theorem of natural selection is an idea about genetic variance in population genetics developed by the statistician and evolutionary biologist Ronald Fisher. The proper way of applying the abstract mathematics of the theorem to actual biology has been a matter of some debate.

Index of evolutionary biology articles Wikimedia list article

This is a list of topics in evolutionary biology.

Motoo Kimura Japanese biologist

Motoo Kimura was a Japanese biologist best known for introducing the neutral theory of molecular evolution in 1968. He became one of the most influential theoretical population geneticists. He is remembered in genetics for his innovative use of diffusion equations to calculate the probability of fixation of beneficial, deleterious, or neutral alleles. Combining theoretical population genetics with molecular evolution data, he also developed the neutral theory of molecular evolution in which genetic drift is the main force changing allele frequencies. James F. Crow, himself a renowned population geneticist, considered Kimura to be one of the two greatest evolutionary geneticists, along with Gustave Malécot, after the great trio of the modern synthesis, Ronald Fisher, J. B. S. Haldane and Sewall Wright.

<i>Genetics and the Origin of Species</i> book by Theodosius Dobzhansky

Genetics and the Origin of Species is a 1937 book by the Ukrainian-American evolutionary biologist Theodosius Dobzhansky. It is regarded as one of the most important works of the modern synthesis, and was one of the earliest. The book popularized the work of population genetics to other biologists, and influenced their appreciation for the genetic basis of evolution. In his book, Dobzhansky applied the theoretical work of Sewall Wright (1889-1988) to the study of natural populations, allowing him to address evolutionary problems in a novel way during his time. Dobzhansky implements theories of mutation, natural selection, and speciation throughout his book to explain habits of populations and the resulting effects on their genetic behavior. The book explains evolution in depth as a process over time that accounts for the diversity of all life on Earth. The study of evolution was present, but greatly neglected at the time. Dobzhansky illustrates that evolution regarding the origin and nature of species during this time in history was deemed mysterious, but had expanding potential for progress to be made in its field.

Will Provine American historian

William Ball "Will" Provine was an American historian of science and of evolutionary biology and population genetics. He was the Andrew H. and James S. Tisch Distinguished University Professor at Cornell University and was a professor in the Departments of History, Science and Technology Studies, and Ecology and Evolutionary Biology.

Evolution in Mendelian Populations is a lengthy 1931 scientific paper on evolution by the American population geneticist Sewall Wright.

Tempo and Mode in Evolution (1944) was George Gaylord Simpson's seminal contribution to the evolutionary synthesis, which integrated the facts of paleontology with those of genetics and natural selection.


Mutationism is one of several alternatives to evolution by natural selection that have existed both before and after the publication of Charles Darwin's 1859 book, On the Origin of Species. In the theory, mutation was the source of novelty, creating new forms and new species, potentially instantaneously, in sudden jumps. This was envisaged as driving evolution, which was thought to be limited by the supply of mutations.

James F. Crow American geneticist

James Franklin Crow was Professor Emeritus of Genetics at the University of Wisconsin–Madison and a prominent population geneticist whose career spanned from the modern synthesis to the genomic era.

An evolutionary landscape is a metaphor, a construct used to think about and visualize the processes of evolution acting on a biological entity. This entity can be viewed as searching or moving through a search space. For example, the search space of a gene would be all possible nucleotide sequences. The search space is only part of an evolutionary landscape. The final component is the "y-axis," which is usually fitness. Each value along the search space can result in a high or low fitness for the entity. If small movements through search space cause changes in fitness that are relatively small, then the landscape is considered smooth. Smooth landscapes happen when most fixed mutations have little to no effect on fitness, which is what one would expect with the neutral theory of molecular evolution. In contrast, if small movements result in large changes in fitness, then the landscape is said to be rugged. In either case, movement tends to be toward areas of higher fitness, though usually not the global optima.

Shifting balance theory A theory suggesting that adaptive evolution may proceed most quickly when subpopulations have restricted gene flow

The shifting balance theory is a theory of evolution proposed in 1932 by Sewall Wright, suggesting that adaptive evolution may proceed most quickly when a population divides into subpopulations with restricted gene flow. The name of the theory is borrowed from Wright's metaphor of fitness landscapes, attempting to explain how a population may move across an adaptive valley to a higher adaptive peak. According to the theory, this movement occurs in three steps:

  1. Genetic drift allows a locally adapted subpopulation to move across an adaptive valley to the base of a higher adaptive peak.
  2. Natural selection will move the subpopulation up the higher peak.
  3. This new superiorly adapted subpopulation may then expand its range and outcompete or interbreed with other subpopulations, causing the spread of new adaptations and movement of the global population toward the new fitness peak.
The eclipse of Darwinism The period when evolution was widely accepted, but natural selection was not

Julian Huxley used the phrase “the eclipse of Darwinism” to describe the state of affairs prior to what he called the modern synthesis, when evolution was widely accepted in scientific circles but relatively few biologists believed that natural selection was its primary mechanism. Historians of science such as Peter J. Bowler have used the same phrase as a label for the period within the history of evolutionary thought from the 1880s to around 1920, when alternatives to natural selection were developed and explored—as many biologists considered natural selection to have been a wrong guess on Charles Darwin's part, or at least as of relatively minor importance. An alternative term, the interphase of Darwinism, has been proposed to avoid the largely incorrect implication that the putative eclipse was preceded by a period of vigorous Darwinian research.

Russell Scott Lande is an American evolutionary biologist and ecologist, and an International Chair Professor at Centre for Biodiversity Dynamics at the Norwegian University of Science and Technology (NTNU). He is a fellow of the Royal Society and a member of the United States National Academy of Sciences.

Michael J. Wade American biologist

Michael J. Wade is a professor of biology at Indiana University Bloomington. Since 2009 he has been the Associate Vice Provost for Faculty and Academic Affairs at Indiana University. He is also affiliated faculty in the following departments and centers at Indiana University: Center for the Integrative Study of Animal Behavior (CISAB), the Cognitive Science Program, and the Department of History and Philosophy of Science.

Beanbag genetics Genetic model

Beanbag genetics is a conceptual model of genetics which was used by early Mendelians, who used to keep coloured beans in bags as a way of tracking Mendelian ratios. The phrase was first coined by Ernst Mayr in describing the work of Ronald Fisher and J. B. S. Haldane who treated genes as independent entities to simplify their mathematical analysis of population genetics.


  1. "Sewall Wright - American geneticist". Retrieved 21 March 2018.
  2. 1 2 Hill, W. G. (1990). "Sewall Wright. 21 December 1889-3 March 1988". Biographical Memoirs of Fellows of the Royal Society . 36: 568–579. doi:10.1098/rsbm.1990.0044.
  3. 1 2 FOWLER, GLENN (1988-03-04). "Sewall Wright, 98, Who Formed Mathematical Basis for Evolution". The New York Times. New York Times. Retrieved 6 September 2015.
  4. Crow, J. F. (1988). "Sewall Wright (1889-1988)". Genetics. 119 (1): 1–4. Bibcode:1988Natur.332..492S. doi:10.1038/332492a0. PMC   1203328 . PMID   3294096.
  5. Crow, J. F.; Dove, W. F. (1987). "Sewall Wright and physiological genetics". Genetics. 115 (1): 1–2. PMC   1203043 . PMID   3549442.
  6. Hill, W. G. (1996). "Sewall Wright's "Systems of Mating"". Genetics. 143 (4): 1499–1506. PMC   1207415 . PMID   8844140.
  7. Allendorf, Fred W.; Luikart, Gordon H.; Aitken, Sally N. (2012). Conservation and the Genetics of Populations. John Wiley. p. 548. ISBN   978-1-118-40857-5.. So were Darwin and his wife Emma (Wedgwood).
  8. "American Mathematical Society". Retrieved 21 March 2018.
  9. Wright, Sewall (1942). "Statistical genetics and evolution". Bull. Amer. Math. Soc. 48 (4): 223–246. doi:10.1090/S0002-9904-1942-07641-5. MR   0006700.
  10. "Daniel Giraud Elliot Medal" . Retrieved 7 January 2018.
  11. "Ohio Marriages, 1800-1958," database, FamilySearch (  : 8 December 2014), Sewall Wright and Louise Lane Williams, 10 Sep 1921; citing Licking, Ohio, reference 508B; FHL microfilm 384,312.
  12. Provine, William B. (1989). Wright and Evolutionary Biology . University of Chicago Press. p.  106. ISBN   9780226684734 . Retrieved 7 January 2018. They were married in Granville on September 10, 1921... The Wrights had two boys, Richard and Robert, during the remaining four years in Washington.
  13. "United States Census, 1930," database with images, FamilySearch (  : accessed 7 January 2018), Sewall Wright, Chicago (Districts 0001-0250), Cook, Illinois, United States; citing enumeration district (ED) ED 208, sheet 11A, line 50, family 226, NARA microfilm publication T626 (Washington D.C.: National Archives and Records Administration, 2002), roll 423; FHL microfilm 2,340,158.
  14. "Sewall Wright Profile".
  15. Ruse, Michael (Jun 30, 2009). Monad to Man: The Concept of Progress in Evolutionary Biology. Harvard University Press. p. 376. ISBN   9780674042995. Archived from the original on January 7, 2018. Retrieved 7 January 2018. Wright worshipped as a Unitarian
  16. Provine, William B. (1989). Sewall Wright and Evolutionary Biology . University of Chicago Press. pp.  460, 497. ISBN   9780226684734 . Retrieved 7 January 2018. Unitarian.
  17. The Structure of Evolutionary Theory (2002) by Stephen Jay Gould, Chapter 7, sectionn "Synthesis as Hardening"
  18. Steffes, David M (2007). "Panpsychic Organicism: Sewall Wright's Philosophy for Understanding Complex Genetic Systems". Journal of the History of Biology . 40 (2): 327–361. doi:10.1007/s10739-006-9105-5. PMID   18175605.

Further reading