Hybridization and introgression in two ecologically dissimilar Fundulus hybrid zones

Hybridization and introgression appear more common in rapidly evolving groups, suggesting an important role in the evolutionary process. Detailed studies of how extrinsic or intrinsic forces regulate hybridization and introgression have the potential for broadening our understanding of mechanisms generating diversity. Species in the Fundulus notatus species complex have broad overlapping ranges and occur in replicated hybrid zones along predictable stream gradients. Typical hybrid zone structure has F. olivaceus in headwaters, F. notatus downstream, and hybrid zones near confluences or abrupt shifts in habitat. Rarely, the typical upstream-downstream orientation is reversed raising questions as to how hybrid zones are formed and maintained. We used next generation sequencing data to study hybridization and introgression in hybrid zones in neighboring drainages that differ in orientation (typical and reversed). We predicted extrinsic forces linked to stream gradients would result in noti...