2 Sewall Wright

In the 1931 paper Evolution in Mendelian Populations, Wright analysed the gene frequencies in different types of populations under environmental and genetic pressures, such as recombination, mutation, and drift. It was noted that the mean gene frequency of a large population will reach a stable equilibrium. When that population is sub divided into partly isolated groups, each group will have similar mean values, if under similar conditions. However, when the groups do not have similar means, those which are more successful will grow in size and the others will decline, thus causing the mean gene frequencies of the entire population to change. Wright concludes:

``Finally in a large population, divided and subdivided into partially isolated local races of small size, there is a continually shifting differentiation among the latter (intensified by local differences in selection but occurring under uniform and static conditions) which inevitably brings about an indefinitely continuing, irreversible, adaptive, and much more rapid evolution of the species.''

Wright follows this with The Roles of Mutation, Inbreeding, Crossbreeding and Selection in Evolution, a 1932 paper that, with the 1931 work, Provine (1986) calls ``...Wright's two most seminal early papers on evolutionary theory". The adaptive landscape was introduced in this work as possible gene combinations, where peaks represent high adaptiveness. The main obstacle for a species would be to effectively explore the different peaks of an adaptive landscape that would be constantly moving in response to environment changes. To solve this dilemma, Wright stated:

``In order that this may occur, there must be some trial and error mechanism on a grand scale by which the species may explore the region surrounding the small portion of the field which it occupies. To evolve, the species must not be under strict control of natural selection. Is there such a trail and error mechanism?''

After showing the different ways a species may move around in the adaptive landscape, Wright described how a large species divided into small local ``races" would be able to spread across the adaptive landscape. The local races would rapidly explore their area of the landscape and when one finds a higher peak than the currently known, it will move the entire species toward that peak. Wright concluded:

``The conclusions is that subdivision of a species into local races provides the most effective mechanism for trail and error in the field of gene combinations.''