Biodiversity depends on the interplay of maintenance and dissolution of reproductive isolating barriers between species. However, the completeness of reproductive barriers may not be uniform across a species' geographic range, resulting in a mosaic of hybridization and genetic exchange between species. A mosaic pattern of hybridization appears within the overlapping geographic ranges of Rainbow Darters (Etheostoma caeruleum) and Orangethroat darters (E. spectabile) in the southeastern United States. This project will investigate the formation of hybrids and the selection pressures that act against them, using experimental crosses within and between these darter species, and will determine geographic patterns in the strength and symmetry of post-mating barriers.
Hybridization can be a serious threat to endangered species, but the relationship between human-mediated disturbance and breakdown of reproductive barriers is not understood. Nearly one third of the approximately 230 species of darters are considered vulnerable, threatened, or endangered, and this research will inform conservation and management of these and other species. This project also provides training in population genetics, reproductive biology, museum techniques, and field-based research to undergraduate students and contributes specimen and data to museum collections and public data bases.
One of the great mysteries of biology is how new species originate. The process of species formation is called speciation. Ever since the mid-19th Century biologists have focused on the ability of distinct species to form viable and reproductively fit hybrids as a tool to examine how species originate and maintain their reproductive cohesiveness. Christen Bossu’s Ph.D. dissertation work examined the role of hybridization in the formation and maintenance of species in darters, a lineage of North American freshwater fishes that contains approximately 250 species. This work had demonstrated a remarkable degree of exchange of genetic material via hybridization between closely and somewhat distantly related dater species. This surprisingly high degree of hybridization has resulted in complicated genetic histories that make complicate the use of genetic material to infer evolutionary relationships among these species. The males of many darter species exhibit elaborate nuptial coloration that is associated with mate choice and/or aggressive interactions among males. The evolution of distinct and species specific color patterns is thought to facilitate species recognition and help in preventing hybridization. However, being colorful seems to impose ecological limits, as the darter lineages that are colorful are also the least variable ecologically. This result implies that bright colors of darters impose a cost and result in apparent strong ecological constraints.
