The Role of Natural and Sexual Selection in the Origin and Maintenance of Divergence Within and Between the Mexican Sailfin Mollies, \u3cem\u3ePoecilia velifera\u3c/em\u3e and \u3cem\u3eP. petenensis\u3c/em\u3e

Understanding speciation remains a holy grail of evolutionary biology. One useful approach is studying the evolutionary mechanisms important in population divergence to infer the mechanisms important in speciation. This method is especially useful when closely related species can be compared to determine whether intraspecific differences parallel interspecific differences. I studied population divergence in two species of Mexican sailfin mollies, Poecilia velifera and P. petenensis. These closely related species are particularly useful for this type of study, as they live in habitats that may differ in the importance of natural selection. In addition, these species may differ in the importance and strength of sexual selection, as males exhibit secondary sexual morphological and behavioral traits. To understand population divergence, I compared morphology among populations in both species. In addition, I observed male mating behaviors to understand the pattern of behavioral differences among populations. Finally, I used microsatellite loci to determine neutral genetic differentiation both within and between the two species. Morphologically, I found that populations in both species were differentiated, and while some morphological trait differences were shared among populations in both species, important differences were also present. For example, caudal peduncle differences among populations in P. petenensis, but not P. velifera, suggest that habitat differences may shape some morphological differences. Males of P. velifera showed evidence of an alternative male mating strategy, with small males performing only gonopodial thrusts, while large males performed both courtship displays and gonopodial thrusts. Males of all sizes of P. petenensis performed both mating behaviors, regardless of body length. In addition, little variation existed between populations of P. velifera, however, males of P. petenensis showed more population specific rates of mating behaviors. Finally, microsatellite analysis revealed that while most populations were genetically distinct, patterns of genetic variation were not concordant with patterns of phenotypic variation, suggesting that selection, and not genetic drift, is likely promoting population divergence in P. velifera and P. petenensis. These results, taken together, suggest that differences in population divergence between these species are the results of both natural and sexual selection, which have been important evolutionary mechanisms in sailfin molly speciation