Does sympatry predict life history and morphological diversification in the Mexican livebearing fish Poeciliopsis baenschi?

Understanding why some species coexist and others do not remains one of the fundamental challenges of ecology.Although there is evidence to suggest that closely-related species are unlikely to occupy the same habitat because of competitive exclusion, there are many cases where closely-related species do co-occur.Research comparing sympatric and allopatric populations of co-occurring species provides a framework for understanding the role of phenotypic diversification in species coexistence.In the present study, we compare phenotypic divergence between sympatric and allopatric populations of the livebearing fish, Poeciliopsis baenschi.We focus on life-history traits and body shape, comprising two sets of integrated traits likely to diverge in response to varying selective pressures.Given that males and females can express different phenotypic traits, we also test for patterns of divergence among sexes by comparing size at maturity and sexual dimorphism in body shape between males and females in each population type.We take advantage of a natural experiment in western Mexico where, in some locations, P. baenschi co-occur with a closely-related species, Poeciliopsis turneri (sympatric populations) and, in other locations, they occur in isolation (allopatric populations).The results obtained in the present study show that sympatric populations of P. baenschi differed significantly in life-history traits and in body shape compared to their allopatric counterparts.Additionally, males and females showed different responses for size at maturity in sympatric conditions versus allopatric conditions.However, the amount of sexual dimorphism did not differ between sympatric and allopatric populations of P. baenschi.Hence, we conclude that not all traits show similar levels of phenotypic divergence in response to sympatric conditions.