Genetic Variation and Phenotypic Evolution During Allopatric Speciation

Abstract

Quantitative genetic models of phenotypic evolution in small isolated populations are presented, from the initial founding event to continued random genetic drift and natural selection toward a new optimum phenotype. The basic features of complex morphologies included are polygenic inheritance with multiple allelism, pleiotropy, recombination and mutation. Gene flow, inbreeding depression, gene interaction, and genetic homeostasis are also discussed. Simpson's adaptive zones for phenotypes (analogous to Wright's adaptive topography for gene frequencies) are formulated probabilistically for small populations. It is concluded that Mayr's theory of allopatric speciation overemphasized both the genetic cohesion of widespread species and the founder effect on heterozygosity and quantitative genetic variation. However, data on the strength of natural selection and the spontaneous mutability of quantitative characters, in conjunction with the models, provide a feasible microevolutionary mechanism for substantial and geologically rapid phenotypic evolution in small isolated populations.