Male success against con- and heterospecific competitors indicates a positive but modest role for sexual selection as driver of speciation

For sexual selection within species to drive the evolution of reproductive isolation between lineages, sexually selected and reproductive isolating traits must both share underlying mechanisms and operate in the same direction. While some work has been done to evaluate mechanistic overlap, fewer studies have evaluated whether intraspecific sexually-selected variation is associated with elevated reproductive isolation between species. Here we evaluate this association by assessing the relationship between male reproductive success against conspecifics versus heterospecific males at each of two different mating stages. We find that male precopulatory performance (remating success following a conspecific versus a heterospecific first mating) was not associated between conspecific and heterospecific contexts, but postcopulatory success (sperm competition against conspecific versus heterospecific males) was modestly positively correlated. We discuss two lines of evidence that suggest this modest association is due to incomplete mechanistic overlap between postcopulatory competition in conspecific and heterospecific mating contexts. This study provides an explicit test of a necessary condition for sexual selection to drive speciation, and finds that while sexual selection is not individually sufficient to explain the magnitude of reproductive isolation in this system, it could nonetheless facilitate the evolution of isolation via postcopulatory sperm competition.