The role of hybrid vigor in the replacement of Pecos pupfish by its hybrids with sheepshead minnow

Many species are jeopardized by hybridization and genetic introgression with closely related species. Unfortunately, the mechanisms that promote or retard gene flow between divergent populations are little studied and poorly understood. Like may imperiled fish species, the Pecos pupfish (Cyprinodon pecosensis) is threatened with replacement by its hybrids with a close congener. We examined swimming performance and growth rate of hybrid pupfish to determine the role of hybrid vigor in the genetic homogenization of C. pecosensis by its hybrids with sheepshead minnow (C. variegatus). The F1 hybrids, backcross hybrids, and purebred C. variegatus displayed greater swimming endurance than purebred C. pecosensis. In addition, F1 hybrids and C. variegatus grew more rapidly than C. pecosensis. The ecological superiority of hybrids probably promoted their rapid spread through and beyond the historic range of C. pecosensis. These results indicate that eradication of hybrids and restoration of C. pecosensis to its native range is unlikely. Extinction of unique species via genetic homogenization can result from human activities that increase gene flow between historically fragmented populations; conservations managers must weigh the potential for such a catastrophe against the presumed benefits of increased interpopulation gene flow. This example illustrates how, after hybridization has occurred, conflict my arise between formerly complementary conservation goals.