Biodiversity is generated by the process of speciation and maintained by factors preventing interbreeding of distinct species (i.e. reproductive isolation). Reproductive isolation is most readily studied in situations like that involving the minnow species, Luxilus cornutus and Luxilus chrysocephalus, which interbreed extensively in the eastern United States. Dr. Thomas Dowling of Arizona State University has been studying genetic interactions between these two hybridizing species, and he has identified geographic variation in the effectiveness of factors maintaining reproductive discontinuity. This study expands upon Dr. Dowling's previous research by examining eastern locations where the two species co-occur and adjacent areas to the west and south, providing complete characterization of geographic variation in reproductive isolation. Sequence variation of the mitochondrial DNA control region is being used to infer phylogenetic relationships of cornutus and chrysocephalus populations and identify the importance of pre- and post-Pleistocene effects in structuring of genetic variation within these species. Comparisons of these relationships with the pattern of reproductive isolation will allow for tests of the role of ancestry, dispersal, and geological history in geographic variation of reproductive isolation. Likewise, population phylogenies will allow for examination of the role of hydrographic history and dispersal on the distribution of genetic variation within each of these species, providing a test of hypotheses concerning the origin of species diversity in the Central Highlands region, one of the most diverse faunas in North America.
