Hybridization offers unique insight into the evolutionary process of speciation and the forces that maintain barriers to reproduction. In northeast Florida, a hybrid zone between two species of killifish, Fundulus heteroclitus and F. grandis, has been identified near Flagler Beach, FL, although the exact boundaries of the hybrid zone remain unknown (Gonzalez et al. 2009). This study aims to determine the fine-scale spatial genetic patterns of the hybrid zone and test the hypothesis that species ranges are influenced by changes in dominant vegetation. Results from an ADMIXTURE analysis and FST data indicated the boundary of the area of overlap between the two parental species to exist at Marineland, FL in the north and Tomoka Basin, FL in the south, while the boundaries of hybridization (i.e. the hybrid zone) existed in a smaller region between Bings Landing, FL and Tomoka Basin, FL. The area of overlap showed a highly mosaic pattern of allele frequencies, suggesting the spatial genetic structure of the hybrid zone is influenced by exogenous selection. Reproductive barriers were found to be the strongest in sites where F. heteroclitus was the more abundant species, but weaker in areas with predominantly F. grandis. No single environmental factor correlated significantly with the genetic distribution throughout the zone of overlap, and it is likely a combination of multiple factors that influence genetic variation, suggesting that habitat differences were likely important in the diversification of these two species
