Understanding the mechanisms that generate biodiversity is a major goal of evolutionary biology. While hybridization is increasingly recognized as common, its ultimate role in the evolutionary process remains unclear. While hybrid zones have long been of interest to evolutionary biologists, the ecology of hybrid zones is not well studied. Specifically, the role of ecological gradients in regulating the structure and dynamics of hybrid zones is not well understood. Many well studied hybrid zone systems exist over large areas, and are often unreplicated. The Fundulus notatus species complex is unique in that species in the group have broadly overlapping ranges, and species typically segregate over well defined linear stream gradients. Fundulus olivaceus usually occurs in the headwaters, while F. notatus is found downstream in larger backwaters. Hybrid zones between these species are typically found near confluences of smaller creeks and larger rivers. A fundamental question that can be addressed in this system is whether replicate hybrid zones between the same parental species display similar rates of hybridization and genomic introgression. Similarly, are rates of hybridization between parental species the same in common garden settings where the role of underlying ecological gradients have been removed?
Results/Conclusions
We present data from field sampling of multiple natural hybrid zones, and experimental hybrid zone trials in meocosms using the same populations sampled in the field. We use a variety of population genomic analyses of large SNP libraries (Genotyping by Sequencing) to assess the rate of hybridization and introgression in the field. These data are compared to mesocosm trials where we collected data on hybridization rate (the proportion of deposited eggs that were parental or hybrid), survival, and recruitment of pure and hybrid offspring. Overall, hybridization rates in the field were dramatically different among replicate hybrid zones. Results ranged from a hybrid swarm dominated by F1 hybrids, to little sign of any hybridization. Mesocosm trials generally coincided with field results indicating a limited role of local ecological conditions in reproductive isolation among these species.