Drift and dispersal hinder the evolution of facultative asexual reproduction.

Facultative parthenogenesis is a flexible reproductive strategy in which females can reproduce asexually if males are unavailable. When males are present, females can incorporate potentially beneficial genes from males into their offspring genomes; when males are absent, females can still produce offspring and benefit from the twofold advantage of parthenogenesis. Given these advantages, it is puzzling that this reproductive strategy is not more widespread. While a number of selection-based explanations have been proposed, fundamental questions remain about the role of population ecology in the evolution of facultative parthenogenesis. Here, we consider the roles of dispersal and genetic drift in the evolution of facultative parthenogenesis within a sexually reproducing species. We develop and analyze a simple mathematical model with two parameters: the dispersal intensity and the population size. We find that a combination of drift and dispersal forms a barrier to the invasion of obligately sexual populations by facultatively parthenogenetic mutants. Although these factors are unlikely to be the only forces that limit facultative parthenogenesis, they represent a parsimonious null model for the observed relative rarity of this reproductive strategy in some taxonomic groups.