December 21, 1889 (a Saturday)

Sewall Wright

Sewall Wright

On this date, the American mathematician and biologist Sewall Green Wright (he later dropped the middle name) was born. He was one of the founders, along with R. A. Fisher and J. B. S. Haldane, of modern theoretical population genetics. He researched the effects of inbreeding and crossbreeding with guinea pigs and, later on, the effects of gene action on inherited characteristics. The synthetic theory of evolution as described by Sewall Wright synthesizes (combines) the principles of natural selection outlined by Charles Darwin with the principles of genetics. Wright explained evolution in terms of changes in gene frequencies.

The classic example which supports this theory is that of the peppered moth in England. The moth can be either dark or light colored. Scientists have determined that body color in the peppered moth is controlled by a single gene with two alleles: the allele for dark body color is dominant and the allele for light body color is recessive. Prior to the industrialization of central England, the light-colored allele was most prevalent. The light-colored moths would hide on the white-barked trees and avoid bird predation. But the pollution generated by the new industries stained the light-colored trees dark. Gradually the light-colored moth was attacked and that allele became much less prevalent. In its place, the dark-colored allele became the most predominant allele because moths that carried that allele could camouflage themselves on the stained trees and avoid being eaten by their bird predators. Clearly the population had evolved to a better adaptive condition.

Wright is perhaps best known for his concept of genetic drift, formerly known as the “Sewall Wright effect.” Genetic drift results when small populations of a species are isolated and due to pure chance, the few individuals who carry certain relatively rare genes may fail to transmit them. The genes may therefore disappear and their loss may lead to the emergence of new species, although natural selection has played no part in the process. Genetic drift can be summarized as “bad luck, not bad genes.”

Wright had long been concerned with cases in which genes interacted in ways not predictable from their individual effects. He believed that evolutionary creativity often depended on putting together favorable combinations of genes that were individually deleterious. But natural selection will not ordinarily incorporate such genes in a large, sexually reproducing population. Wright’s answer was his “shifting balance theory“, which holds that the best opportunity for adaptive evolution lies in the population structure.

Wright thought that many, if not most, species were subdivided into small populations that exchanged only a few migrants with each other and thus were not completely isolated. Because of the small size of each of these populations, genetic drift would have a significant effect on the genetic composition of each, thus allowing the populations to differentiate genetically by an appreciable amount. In this way, each of the populations would act as a small experiment in evolution.

Wright’s shifting balance theory consists of three distinct phases:

  • Phase 1, the exploratory phase, is characterized by the action of genetic drift in a local population. One or more may drift into an advantageous gene combination.
  • In phase 2, a new advantageous combination of genes is naturally selected in one or more populations.
  • Finally, in phase 3, those populations will then increase or, more likely, send out migrants to adjacent local populations, introducing the advantageous gene combination of the immigrants. As a result of this process, eventually all of the populations attain the favorable gene combination.

Although Wright’s theory remains controversial, it has been very popular and influential in the biological community. It is one of the things that biologists argue over. They do not argue over whether or not evolution occurs; that evolution occurs is a biological fact.

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