Mechanical reproductive isolation facilitates parallel speciation in western North American scincid lizards

Mechanical reproductive barriers have been dismissed as a major driver of animal speciation, yet the extent to which such barriers cause reproductive isolation in most animal groups is largely unknown and rarely tested. In this study, we used hierarchical Bayesian modeling of mate compatibility experiments to show that body size divergence in lizards of the Plestiodon skiltonianus complex contributes to reproductive isolation in at least three ways: males preferably court females that are more similar in size, females reject males that are highly divergent in size, and the size difference of a male and female in copula constrains the ability to align the genitalia for intromission. We used a predictive model to estimate the contributions of behavioral and mechanical barriers to reproductive isolation between populations with differing degrees of size divergence. This model shows that the mechanical barrier is more important than behavioral barriers at small and intermediate degrees of size divergence, suggesting that it acts earlier during speciation when body morphology is more similar between diverging lineages. Given that correlated divergence in size and ecology is common in animals, similar constraints imposed by the geometry of the mating posture may apply to a variety of major animal lineages and merit further attention in speciation research.