SEWALL WRIGHT AND EVOLUTIONARY BIOLOGY
William B. Provine. University of Chicago Press, Chicago, 1986. 561 pp., illus. $30.
William Provine's important and excellent book is more than a biography of a towering figure in population genetics; it is an examination of the development of the neo-Darwinian synthesis that is the core of modern evolutionary theory. Every student of evolution will profit by reading the book.
Wright, whose publications span the years 1912 to (most recently) 1984, made important contributions to animal breeding and to physiological genetics. But he is, properly, best known as the co founder, with R.A. Fisher and J.B.S. Haldane, of the mathematical theory of population genetics— the genetical theory of evolution.
His formulations of the theories of inbreeding, gene flow, isolation by distance, genetic drift, the effects of selection on systems of interacting genes, and many other topics are central to modern evolutionary thought.
His views profoundly influenced Theodosius Dobzhansky and through him much of the community of evolutionary biologists. His longstanding conflict with R.A. Fisher had an enormously stimulating effect on empirical population genetics. As Provine shows, the difference between Wright's and Fisher's views of evolution— the one affirming the importance of population structure, the interaction of drift with selection, and gene interactions, the other de voted to the sufficiency and power of mass selection—has been the central debate in the theory of evolutionary processes and persists today in arguments about genetic variation and speciation.
Some may be surprised to learn that Wright views himself as a committed selectionist and has little interest in neutral molecular evolution because he sees in it no evolutionary importance.
Although, as Provine shows, Wright's view of the adaptive versus the neutral nature of species differences has changed over the years, he has indeed sought unwaveringly since 1930 to describe the genetic conditions that favor a “continuous and essentially irreversible evolutionary process” of progressive adaptation, as he wrote in a 1931 paper.
Wright believes he described such favorable genetic conditions in his “shifting balance” theory, which he views as his central contribution and thinks has been ignored and misunderstood for de cades. This theory is designed to explain not local differentiation or differences among species but long term progress. It is a theory of mac-roevolution.
One of Wright's best known contributions is the concept of the adaptive landscape, which represents the mean fitness of a population as a function of allele frequencies at one or more loci. (Provine's introductory description of the now standard concept is marred by one of his very few errors of scientific interpretation.)
Even in a constant environment, the landscape can have numerous peaks of different heights, owing to different combinations of interacting genes. The theory of shifting balance describes movement to progressively higher peaks of adaptation.
Wright developed this theory, which in its emphasis on population structure and gene interaction differs profoundly from Fisher's simpler theory of mass selection, by analogy with the most effective procedures used by breeders in artificial selection.
Although it has not really been ignored—at least partial descriptions of it are presented in almost every major book on evolution since 1937—the theory is not widely understood, and it has certainly never been tested. As Provine notes, it is a verbal theory that does not necessarily follow from the individual elements that Wright developed mathematically with such elegance.
Provine notes some of the difficulties with the shifting balance theory but nevertheless—mistakenly, I think—calls it one of “the few really robust theories of evolutionary change.”
From Provine's analysis, it appears that the impact of the shifting balance theory was lost partly because Wright was an ineffective self-publicist and had few students in population genetics and partly because he came to be viewed, erroneously, as a champion of drift over selection.
As the synthesis became more selectionist (in part because selection was found to operate in the very cases that had been thought to exemplify nonadaptive variation), Wright's identification with drift led to a decline in the stature of his theories.
His stature has recently grown greatly, and he may well be, as Provine calls him, “perhaps the single most influential evolutionary theorist of this century,” but his central theory, of shifting balance, cannot be evaluated without further theoretical and empirical study.
Provine's history is rich in detail and insight. Anyone familiar with population genetics will be fascinated by the significance of the conflict over the evolution of dominance, by Wright's interaction with Dobzhansky, Mayr, Kimura, and others, by the surprisingly significant story of Linanthus parryae, and by innumerable other events that bear on today's controversies.
For example, elements of Wright's theory are remarkably similar to Mayr's theory of speciation by peak shifts, which is central to the punctuated equilibrium debate; I cannot help but think that there is more background to the friction between Wright and Mayr than Provine describes.
But this is only one episode in a long, productive life, through the prism of which Provine displays the evolution of much of today's evolutionary theory. Provine understands the theory well; the result is a book rich not only in history but in scientific substance.
Futuyma is a professor in the Department of Ecology and Evolution, State University of NewYork, StonyBrook, 11794.