Relationships between reproductive character displacement in genital morphology and the population-level cost of interspecific mating: implications for the Templeton effect

Natural selection against maladaptive interspecific reproductive interactions may cause greater divergence in mating traits between sympatric populations than between allopatric populations in a pair of species, known as reproductive character displacement (RCD) which is evidence for the lock-and-key hypothesis of genital evolution. However, the relative importance of various processes contributing to RCD in genital morphology (e.g. reinforcement, reproductive interference, and population filtering or the Templeton effect) is not clear. Here, we examined hypotheses for RCD in genital morphology, with a special focus on the Templeton effect (which predicts that only highly differentiated populations can exist in sympatry). We examined population-level fitness costs in interspecific mating between Carabus maiyasanus and Carabus iwawakianus with RCD in genital morphology. A mating experiment using populations with various degrees of RCD in genital morphology showed no evidence for consistently lower interspecific mating costs in C. maiyasanus populations in contact with displacement in genital morphology than in remote populations, contrary to the predictions of the Templeton effect. Alternatively, interspecific mating costs varied among populations. Observed relationships between the sizes of genital parts concerning isolation and interspecific mating costs across populations suggested that population-level fitness costs do not necessarily decrease during the process leading to RCD. Our results provide insight into ecological and evolutionary processes during secondary contact in closely related species.

Automated summary

This study examined hypotheses for reproductive character displacement (RCD) in genital morphology, with a focus on the Templeton effect. The results showed no evidence for the predictions of the Templeton effect, as the interspecific mating costs varied among populations and the population-level fitness costs did not necessarily decrease during the process leading to RCD.