Genetic Analysis of Hybrid Zones

Abstract

When two distinct gene pools meet and produce fertile hybrids, the outcome varies from gene to gene. At some loci a universally favorable allele has been established on one side. Such alleles soon spread through the whole population and hence differences are rarely observed. At other loci different alleles may be favored in different environments or genetic backgrounds; selection maintains these differences in the face of random mixing. At other loci—perhaps at most of those we observe in molecular surveys—different alleles may have been established by chance and may have no appreciable effect on fitness. These differences gradually fade away, at a rate that depends on the strength of selection against introgression at the other loci with which they are associated.