Wright, Sewall

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(b. Melrose, Massachusetts, 21 December 1889;

d. Madison, Wisconsin, 3 March 1988), population genetics, evolutionary theory.

Wright, along with John Burdon Sanderson Haldane and Ronald Aylmer Fisher, founded modern evolutionary theory, that is, mathematical population genetics. Wright is probably best known for his general evolutionary theory, the shifting balance theory. But he is also responsible for the mathematical theory of inbreeding and population structure. In mathematics, Wright invented the method of path analysis. For his groundbreaking work in population genetics, he was the recipient of the Elliott and Kimball awards from the National Academy of Sciences, the Lewis Prize from the American Philosophical Society, the Weldon Medal from Oxford University, the Darwin Medal from the Royal Society of London, the National Medal of Science from the United States, and the Balzan Prize from Italy. Wright’s life’s work is represented by his four-volume Evolution and the Genetics of Populations (1968–1978). The authoritative biography of Wright is William Provine’s Sewall Wright and Evolutionary Biology (1986).

Early Years. Sewall Wright was born in Melrose, Massachusetts, on 21 December 1889, to Elizabeth Quincy Sewall and Philip Green Wright. Wright had two brothers, Quincy and Theodore. Each was well known in his respective field, namely, law and aeronautical engineering. Wright spent his childhood and early adulthood in Galesburg, Illinois, where his father held a teaching post at Lombard College. Wright enrolled at Lombard College, taking his BSc degree in 1911. Wright studied mathematics mainly, but by his last year Wright’s interests in biology took center stage because of the mentoring of Wilhelmine Entemann Key (who was one of the first women to earn a PhD at the University of Chicago). Indeed, in the summer of 1911 Key sent Wright to Cold Spring Harbor Laboratory on Long Island, New York, where Wright learned from world-class biologists, including Key’s former mentor Charles Benedict Davenport (1866–1944). Wright found his experience at Cold Spring Harbor rewarding, and he returned there during the summer of 1912. In the fall of 1911, Wright entered the graduate program in biology at the University of Illinois at Urbana-Champaign.

By the spring of 1912, Wright completed his master’s thesis on the anatomy of the trematode Microphallus opacus. Wright completed the thesis in short order on account of a chance meeting after a lecture by the geneticist William Ernest Castle (1867–1962) of Harvard University’s Bussey Institute, another student of Davenport. Castle lectured on his selection experiments on hooded rats and on mammalian genetics. Wright was fascinated and approached Castle about working with him. Castle was sufficiently impressed by Wright, and they decided that Wright would quickly finish his master’s thesis and enroll at Harvard University in the fall of 1912.

At Harvard, Wright engaged in original experimental research in physiological genetics. Wright’s research was directed by Castle, but he was also considerably influenced by the geneticist Edward Murray East. While at Harvard and the Bussey Institute, Wright worked closely with Castle on his hooded rat selection experiments and on the genetics of small mammals. By 1915 Wright completed his doctoral thesis on coat color inheritance in guinea pigs. Wright’s research demonstrated the existence of multiple loci and alleles affecting coat color in the animal; it further set out the hypothesis that enzyme pathways and pigment precursors provided the physiological basis for observed patterns of inheritance of coat coloration. Around the time Wright was finishing his thesis, he accepted a position as senior animal husbandman at the U.S. Department of Agriculture (USDA) in Beltsville, Maryland. Wright’s work as a physiological geneticist permeated his work in evolutionary theory.

At the USDA. Wright was with the USDA for ten years, from 1915 to 1925. During this period, Wright published widely on physiological genetics, inbreeding and crossbreeding, and statistics, where he created the method of path analysis. In 1921 he married Louise Williams, a member of the faculty of biology at Smith College. The Wrights had three children, Elizabeth Rose, Richard, and Robert.

In 1917–1918, Wright published a ten-paper series on coat color inheritance in mice, rats, rabbits, guinea pigs, cattle, horses, swine, dogs, cats, and humans (1917; 1918a). Wright’s work was cutting-edge at the time: He found similarities and purported homologies among genes with similar effects in all these species, some of which were later confirmed; he also interpreted coat colors in terms of contemporaneous knowledge of enzymes and pigment chemistry. In addition, Wright analyzed the inheritance of size factors into components based on correlation of various body parts (1918b). He partitioned the variance in size into components of general size, limb-specific factors, forelimb and hind-limb factors, upper-limb and lower-limb factors, and special factors for each part.

Two publications of Wright’s from 1921 stand out. The first, “Correlation and Causation,” described Wright’s invention of the method of path analysis, that is, the method for estimating the magnitude and significance of hypothesized causal connections between sets of variables (1921a). Wright used partial regression analysis, a standard statistical tool, but his approach was unique. He hypothesized what the causal paths were and then aimed to determine the relative magnitudes of different paths. Wright’s procedure was to diagram a series of paths. A path of causal influence was indicated by an arrow, while unanalyzed correlations were indicated by double arrows. Each step in a pathway was associated with a “path coefficient,” a partial regression coefficient standardized by being measured in standard deviation units. A path

coefficient is a measure of the relative contribution of this step in the pathway. Wright devised rules by which the relevant equations may be written from the path diagram. The equations can then be solved for the unknown variables. Wright’s method of path analysis was used widely by animal breeders early on, but later was superceded by other methods. But it is common to see path analysis used by social scientists.

The other “stand out” work in 1921 was a series of papers published under the heading “Systems of Mating” (1921b). While a graduate student at Harvard, Wright devised a method for measuring the proportion by which the heterozygosity of an individual is reduced by inbreeding. The method results in what is familiarly known as Wright’s “inbreeding coefficient.” Wright subsequently analyzed the history of inbreeding in American shorthorn cattle using his method. This work formed the basis of Wright’s well-known F-statistics, where F (or f) is the measure of inbreeding, and later helped Wright in elaborating his isolation-by-distance model in the 1940s (1943b; 1951b).

Wright’s work on physiological genetics and inbreeding during his tenure at the USDA profoundly affected the way he understood the genetic basis of evolutionary change and population structure. Indeed, much of his thought had crystallized by 1925, but his results were not published until 1931.

The Shifting Balance Theory. In 1926 Wright joined the faculty of biology at the University of Chicago. There Wright’s contribution as an architect of the synthesis of Darwinism and Mendelism was completed. Wright’s most famous paper, “Evolution in Mendelian Populations,” was published in 1931. Wright demonstrated the mathematical unification of Darwinian natural selection and the principles of Mendelian heredity, and he communicated this synthesis in the form of his famous shifting balance theory of evolution. To be sure, Wright’s long 1931 paper is a masterpiece of mathematical population genetics. But the central ideas of Wright’s shifting balance theory are inextricably tied to his communication of them through his “adaptive landscape” diagram in 1932.

Wright’s aim in the 1931–1932 papers was to determine the ideal conditions for evolution to occur given specific assumptions about the relationship between Mendelian heredity and the adaptive value of gene complexes (1931, p. 158; 1932, p. 363). Wright’s view was that his “shifting-balance” process of evolution described those conditions. Its driving assumptions were part and parcel of Wright’s understanding of genetic interaction, mating, and population structures. According to Wright (1932, pp. 361–363), accurately representing the population genetics of the evolutionary process requires thousands of dimensions. This is because the field of possible gene combinations in the field of gene frequencies of a population is vast (approximately 101000). Indeed, Wright begins the 1932 paper by asking about the nature of this field of possible gene combinations. Figure 1 is Wright’s first illustration, in which he depicts the combinations of two to five allelomorphs (see Figure 1). Here, Wright illustrates how quickly the dimensionality of the field expands as the number of combinations expands: for the case of thirty-two combinations, five dimensions are required, plus a sixth to represent adaptive value. In the case of a species, with 101000 combinations, the required dimensions number at nine thousand.

Wright used the two dimensional graphical depiction of an adaptive landscape in Figure 2 as a way of intuitively conveying what he thought can be realistically represented only in thousands of dimensions. The surface of the landscape is typically understood as representing the joint gene frequencies of all genes in a population graded for adaptive value. The surface of the landscape is very “hilly,” says Wright, because of epistatic relations between genes, the consequences of which (for Wright) are that genes adaptive in one combination are likely to be maladaptive in another. Given Wright’s view of epistasis and the vastness of the field of gene combinations in a field of gene frequencies, Wright estimates the number of adaptive “peaks” separated by adaptive “valleys” at 10800. Peaks are represented by “+”; valleys are represented by “–”.

The adaptive landscape diagram sets up Wright’s signature problem, namely, the problem of peak shifts. That is, given that the adaptive landscape is hilly, the ideal conditions for evolution to occur must allow a population to shift from peak to peak to find the highest peak. In his 1931 paper Wright demonstrated mathematically the statistical distributions of genes under alternative assumptions of population size, mutation rate, migration rate, selection intensity, and so forth. In the 1932 paper the graphs displaying the results appear, and he uses them in

combination with the landscape diagram to argue for his three-phase shifting-balance model of the evolutionary process (window F in Figure 4) as the solution to his problem of peak shifts via assessments of alternative models of the process (windows A–E in Figure 4). Evolution on the shifting-balance process occurs in three phases: Phase I— random genetic drift causes subpopulations semi-isolated within the global population to lose fitness; Phase II— selection on complex genetic interaction systems raises the fitness of those subpopulations; Phase III—interdemic selection then raises the fitness of the large or global population. In his 1932 paper Wright used the adaptive landscape diagram to demonstrate why he thought such an apparently complicated process was required for the ideal conditions for evolution to occur. The central assumptions of Wright’s shifting balance theory have been challenged since he first published them. In fact, in the early twenty-first century the shifting balance theory is probably less well received than it was during Wright’s career.

Controversies and Collaborations. Wright was a key figure in one of the great controversies of evolutionary genetics, that between himself and another architect of population genetics, Fisher. Wright also participated in one of the great collaborations of evolutionary genetics, that between himself and Theodosius Dobzhansky (1900–1975). The controversy between Wright and Fisher was central, fundamental, and very influential. And the collaboration with Dobzhansky popularized Wright’s shifting balance theory as it helped refine it.

From 1929 until 1962, Wright and Fisher debated the very fundamentals of adaptive evolution: its ecological context, genetic basis, major processes, and modes of speciation. The debate has persisted long after the involvement of its principals, expanding on old problems and raising news ones. Illustrative of the disagreements between Fisher and Wright is their debate over the importance of genetic drift in evolution.

In 1947 Fisher, with the ecological geneticist Edmund Brisco Ford(1901–1988), published an experimental paper aimed at discrediting Wright’s shifting balance theory and substantiating Fisher’s panselectionism. Fisher and Ford’s paper describes and analyzes data from what was at the time a fairly novel field experimental technique, the capture and release protocol, targeting wing coloration in populations of the moth, Panaxia dominula in Oxfordshire, England. Fisher and Ford argued that even in small(ish) populations (between 103 and 104) Wright’s assumed norm, genetic drift—Wright’s most important evolutionary factor, according to Fisher and Ford—was evolutionarily inefficacious. Indeed, Fisher and Ford argued further that natural selection, even in smallish populations, is the driving factor of evolution.

Fisher and Ford claimed that their data showed that fluctuations in the frequencies of the heterozygous form of the moth (called medionigra) were too large from year to year to be due to genetic drift. Their specific argument was that even though population size was sufficiently small for genetic drift to be effective, drift nevertheless was not a factor (because the gene frequency changes were too high to be due merely to chance). For the years Fisher and Ford studied the population, the average population size was in the range of thirty-two hundred to four thousand moths, with approximately 11 percent overall being the medionigra form and a total gene frequency change of approximately 6 percent (1947, pp. 150, 164). Fisher and Ford ultimately inferred that because changes in gene frequencies in the moth populations were not due to genetic drift, they must be due to natural selection.

In 1948 Wright published a critique of Fisher and Ford’s study. Wright objected (1) that Fisher and Ford had misinterpreted the role Wright had assumed for random genetic drift—in their reading of Wright’s work, they attributed more of a role to drift than Wright himself did—and (2) that their inference that selection must be the cause of the changes in gene frequencies in the populations of the moths was not justified experimentally. Fisher and Ford did not provide any direct evidence that selection is the cause; they only infer it after rejecting drift. Wright’s paper drew an acerbic attack from Fisher and Ford published in 1950. Wright again responded in 1951 (1951a). The substancef the disagreement after Wright’s 1948 paper is the problem of interpreting Wright’s view of the role of genetic drift in evolution.

The field work on P. dominula continued long after the 1950s and has come to form one of the longest-running field experiments in ecological genetics. Ironically,

however, since the late 1990s it has become clear that the selectionist interpretation that has held sway from the beginning is on shaky ground. But genetic drift is not the right interpretation, either: temperature fluctuations affecting the expression of wing color in the moths explains much of the fluctuation of the medionigra form.

Probably more heat than light was generated by the controversy between Fisher and Wright. The opposite may be said about the collaboration between Wright and Dobzhansky. Wright clearly had a major influence on Dobzhansky’s important 1937 book, Genetics and the Origin of Species and also on the Genetics of Natural Populations series published between 1937 and 1975. However, Dobzhansky’s influence on Wright and in particular the promulgation of the shifting balance theory cannot be underestimated. Wright’s influence on evolutionary biology was powerfully communicated through Dobzhansky’s own work and through his collaboration with Wright through the 1950s. Genetics and the Origin of Species provided Wright with the empirical underpinning he knew his abstract theoretical work lacked. And naturalists who did not possess Wright’s mathematical expertise were finally able, with Dobzhansky’s book, to see Wright’s particular synthesis of Mendelian heredity and Darwinian natural selection. Moreover, Dobzhansky’s tireless work in the field yielded theoretical insights Wright may not have otherwise had, including his model of isolation by distance, published in 1943.

Population structure is central to the shifting balance theory. But it was not until Dobzhansky asked Wright to analyze data on the distribution of the desert flower Linanthus parryae that Wright was able to fully articulate the model. Indeed, in the 1930s he had only been able to produce a qualitative theory of isolation by distance. As it happens, Dobzhansky (with Carl Epling [1894–1968]) published “Microgeographic Races in Linanthus parryae” in 1942 as part of the Genetics of Natural Populations series before Wright was able to develop the quantitative model. Wright published the model in 1943 (1943b).

Wright used his theory of F-statistics to partition a series of related inbreeding coefficients that were individually relevant to a particular population structure. Wright invented inbreeding coefficients for an individual relative to its subpopulation, for an individual with respect to the total population, and for the correlation between randomly uniting gametes drawn from the same subpopulation. With these coefficients and derived equations for the gene frequencies of subgroups, their effective population sizes, migration index, and mean gene frequency of the population, Wright was able to develop models that captured the evolutionary effect of isolation by distance. The equations showed that when populations were small (<103), considerable random differentiation would be expected in population subdivisions. As population size increased (>104), values for the effect of isolation by distance were essentially what would be expected of a randomly interbreeding population. A companion piece followed Wright’s isolation-by-distance paper, in which he applied his models to Dobzhansky’s data (Wright, 1943a). Because of the centrality of isolation by distance to Wright’s shifting balance theory, the L. parryae fieldwork stood as a major illustration of the theory as a whole (Provine, 1986, p. 379).

The controversies and collaborations of which Wright was a part helped refine his evolutionary thought. Just as Fisher forced Wright to clarify his theoretical assumptions, Dobzhansky led him to new theoretical insights that shaped and reshaped his shifting balance theory. More generally, these controversies and collaborations would direct the field of evolutionary biology and have lasting influence.

Retirement to Madison. In 1955 Wright was subject to mandatory retirement from the University of Chicago. He became Leon J. Cole Professor of Genetics at the University of Wisconsin, Madison, where he remained for the rest of his long career. Wright was prolific throughout. And perhaps there is no better testament to that than the publication, between 1968 and 1978, of his four-volume magnum opus, Evolution and the Genetics of Populations. These volumes were the culmination of his work in evolutionary theory. Each volume painstakingly reworks the evolutionary problems he originally attacked starting in the 1920s. In particular, he revisits all of the problems that were central to his debates with Fisher and his collaborations with Dobzhansky. Thus, he revisits the problem of the genetic basis of evolutionary change, population structure, and the ecological genetics of P. dominula and L. parryae, as well as other work not discussed here, including the evolution of the nematode Cepaea nemoralis, the statistical distribution of Drosophila pseudoobscura, and so on. Wright was ruthless with his examination, reviewing huge amounts of updated literature on these topics. The quantity and quality of the work is staggering.

But Wright’s career did not end with Evolution and the Genetics of Populations, in spite of the fact that the fourth and final volume was published when he was eighty-eight years old. Indeed, Wright continued reading and writing, publishing his last paper the year he died, in 1988. The paper was a mostly favorable reaction to his biography published by William B. Provine two years earlier. Wright died at the age of ninety-eight, on 3 March 1988, of complications from a broken pelvis after a slip on an icy sidewalk during one of his usual walks.

Wright is remembered as a towering figure among American evolutionary biologists. His adaptive landscape diagram permeates evolutionary thought, and his statistical theory of inbreeding is standard in evolutionary genetics. Wright was the recipient of numerous awards, including the Weldon Medal of the Royal Society of London in 1947, the National Medal of Science in 1966, and the Medal of the Royal Society of London in 1980. And many of those who worked with Wright, including James Crow, Motoo Kimura, Janice Spofford, and Michael Wade, staked out careers that are a testament to his stature. Wright the man was described by his friends and associates as shy, but warm, and unflinching when discussion turned to his interests.



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