Geographic Overlap of Potentially Hybridizing Species in the Fish Family Poeciliidae in Their Native Ranges

Understanding the factors that influence a species' geographic range and the overlap between the ranges of distinct species remains a central question in both ecology and evolution. Here we investigate the influences of hybridization and the breakdown of reproductive isolation and a key innovation, matrotrophy (the provision of energy for embryonic development after fertilization), on species ranges and range overlaps in the fish family Poeciliidae. We assessed whether pairs of species that hybridize have different amounts of range overlap than pairs of species with similar divergence times that do not hybridize. We also assessed whether matrotrophic species have larger ranges than lecithotrophic species (lecithotrophic species provide all energy for embryonic development before fertilization). To investigate our hypotheses, we reviewed the literature on poeciliid fishes and plotted the ranges of all extant species in ArcGIS. The data we worked on was compiled by M. Tobler and Z. Culumber in 2019 and consisted of museum collection records for individuals of distinct species. We quantified the ranges for 407 different fish species. We then we added data on the estimated time of divergence between sister taxa, based on the phylogenetic tree (Furness et al. 2019). We found that the areas of overlap between sister taxa depended upon time since divergence. However, the slopes of range overlap and divergence time were of opposite sign between species pairs that hybridize and species pairs of similar divergence times that do not hybridization and range overlap (km2). Based on our results, we believe that the species that are closely related but not hybridizing have smaller areas of overlap because they are still in the stage of primary differentiation, are experiencing secondary contact after prior geographic separation that no longer exists or are in a stable hybrid zone. When comparing the ranges of closely related placentals and non-placentals, we found that multiple closely related species that shared the same reproductive strategy did not have any range overlap. This lends support to the theory that placentation evolved separately in different geographic areas. A future question that researchers could answer using these data would be what is causing the geographic overlap between species. Another future biogeographic study may be an evaluation of whether water features are leading to a decline in species diversity based on habitat preferences or their normal range.