Assortative mating and self-fertilization differ in their contributions to reinforcement, cascade speciation, and diversification.

Abstract

Cascade speciation and reinforcement can evolve rapidly when traits are pleiotropic and act as both signal/cue in non-random mating. Reproductive isolation due to these traits can be rapid, facilitating cascade speciation by limiting gene flow among sympatric and allopatric populations of the same species. Here we examine the evolution of two key traits-- assortative mating and self-fertilization--and their possible contribution to reinforcement and (by extension) cascade speciation. First, we use a population genetic model of reinforcement to address conditions that favor the evolution of assortative mating and/or self-fertilization in the face of deleterious gene flow from a heterospecific population. Our results indicate that, although both traits can contribute to reinforcement independently, when they are allowed to evolve simultaneously the evolution of increased self-fertilization prohibits the evolution of assortative mating, at least when inbreeding depression is low or moderate. Thus we predict that, under conditions of low inbreeding depression, mating system transitions are more likely to be involved in reinforcement and consequently cascade speciation, than assortative mating. Given that transitions to self-fertilization might be an important driver of reinforcement and cascade speciation we analyzed comparative data from two different groups to address the effects of self-fertilization on clade-wide diversification. We find mixed evidence for an association between self-fertilization and diversification: self-fertilization is associated with reduced diversification rates in the plant genus Mimulus (as found in several other plant groups) however we see a trend towards uniparental reproductive modes being associated with increased diversification rate in the Nematode phylum. Reinforcement driving speciation via transitions to self-fertilization might be short lived and/or unsustainable across longer time scales in plants, but not in animals