Establishment Process of a Magic Trait Allele subject to Both Divergent Selection and Assortative Mating

ABSTRACT Sexual selection and divergent selection are among the major driving forces of reproductive isolation, which could eventually result in speciation. A magic trait is defined such that a single trait is subject to both divergent selection and sexual selection through phenotype-based assortative mating. We are here interested in the evolutionary behavior of alleles in a genetic locus responsible for a magic trait. We assume that, in a pair of homogeneous subpopulations, a mutant allele arises at the magic trait locus, and theoretically obtain the probability that the new allele establishes in the population. We also consider the trajectory of allele frequency along the establishment. Divergent selection simply favors the new allele to fix where it is beneficial, whereas assortative mating works against rare alleles. It is theoretically demonstrated that the fate of the new allele is determined by the relative contributions of the two selective forces, divergent selection and assortative mating, when the allele is rare so that the two selective forces counteract. We also show that random genetic drift also plays an important role. The theoretical results would contribute to improve our understanding of how natural selection initiates speciation.