ABSTRACT
Sexual conflict is a strong driver of evolution. The evolutionary outcomes of sexual conflict can, in turn, influence ecological processes within populations, for example, demography. However, evidence for the latter hypothesis is scarce, especially in the wild. Here, we show that sexual conflict is associated with demographic processes determining population size in the ground beetle Carabus insulicola with elaborate male and female genitalia based on individual- and population-level analyses. We found that sexually antagonistic selection can operate on the genitalia: longer male genitalia can be beneficial in sperm competition but decrease female reproductive success with increased egg dumping, whereas longer female genitalia are resistant to this male harassment via decreased egg dumping and increased fertilization rate. As expected from sexually antagonistic coevolution due to sexual conflict, we detected coevolutionary divergence between male and female genital sizes among populations. In parallel with decrease in female reproductive success, more harmful males with longer genitalia and less resistant females with shorter genitalia were related to small effective population sizes. Thus, sexual conflict may promote coevolutionary diversification between sexual traits, and this was associated with a demographic process. Our findings provide an insight into sex-driven eco-evolutionary dynamics in the wild.