Abstract
We investigated structuring in a widespread livebearing fish, the Sailfin molly (Poecilia latipinna) combining genetic, life-history, and environmental data. Widespread species often experience significant environmental clines over the area the naturally occupy. This provides potential for genetic structuring, local adaptation and speciation. Sailfin mollies can be found in coastal freshwater habitats from roughly Tampico, Veracruz in Mexico to Wilmington, North Carolina, in the USA. In addition, they are found inland on the Florida peninsula. Using microsatellite DNA, we genotyped 168 individuals from 18 populations covering almost all of the natural range of the Sailfin molly. We further determined standard life history parameters for both males and females for these populations. Finally, we measured biotic and abiotic parameters as we collected. We find six distinct genetic clusters based on microsatellite data. Furthermore, there is very strong isolation by distance. We also found significant numbers of migrants between populations. The genetic clusters and the migration patterns do not match paleodrainages. Life histories vary between populations but are idiosyncratic. We suggest a role of humans in migration in the sailfin molly, for example in the form of a ship channel that connects southern Texas with Louisiana and might be a conduit for fish migration. Despite genetic structuring we did not find evidence of cryptic speciation.