Divergent ecological adaptation in allopatry leads to behavioral isolation through female resistance

Natural selection can be a potent contributor to speciation, but how exactly this occurs remains unclear. Studying behavioral isolation and the divergence of mating behaviors among multiple isolated populations can help uncover the importance of divergent adaptation in the evolution of reproductive isolation. Here, we utilize common-garden reared virgin Bahamas mosquitofish (Gambusia hubbsi) derived from wild populations adapted to either high- or low-predation risk environments. While we know that parallel evolution of female mate choice has contributed to behavioral isolation between contrasting predation regimes in this radiation, we here discovered that male mating behaviors expressed during one-on-one encounters have also consistently diverged between predation regimes, but have not consequently increased behavioral isolation. On the other hand, we found that female resistance toward a potential mate may represent an underappreciated component of behavioral isolation. Female-to-male aggression during one-on-one interactions was stronger when interacting with foreign males compared to native males, especially males originating from the different predation regime that also elicited female aggression more quickly. Female aggression tended to lead to lower fertilization success, with the lowest fertilization success occurring for mating pairs from different predation regimes. Males displayed their genitalia more toward native females, but performed more copulation attempts toward foreign females—yet, copulation attempts were least likely to lead to genital contact or fertilization when attempting to mate with females from a different predation regime. Overall, behavioral isolation between divergent ecological environments in Bahamas mosquitofish appears to have arisen largely from the actions of one sex. Multiple female mating behaviors contribute to behavioral isolation, while males primarily showed behaviors that seem to attempt to enhance their mating success in the presence of resistant females. Thus, in coercive mating scenarios with indiscriminate males, ecological divergence in traits linked to female mate discrimination can still lead to parallel speciation.