Genetics, taxonomy, and conservation of the threatened California Gnatcatcher

The California Gnatcatcher (Polioptila californica) has become a flagship s pecies in the dispute over development of southern California's unique coas tal sage scrub habitat a fragile, geographically restricted ecosystem with high endemism. One aspect of the controversy concerns the status of the sub species of this bird in southern California coastal sage scrub that is curr ently listed ns threatened under the U.S. Endangered Species Act. To invest igate the recent population history of this species and the genetic distinc tiveness of subspecies and to inform conservation planning, we used direct sequencing of mitochondrial DNA (mtDNA) for 64 individuals from 13 samples taken throughout the species' range. We found that coastal sage scrub popul ations of California Gnatcatchers are not genetically distinct from populat ions in Baja California, which are dense and continuously distributed throu ghout the peninsula. Rather, mtDNA sequences from this species contain the signatures of population growth and support a hypothesis of recent expansio n of populations from a southern Baja California refugium northward into th e southern coastal regions of California. During this expansion, stochastic events led to a reduction in genetic variation in the newly occupied range . Thus, preservation of coastal sage scrub cannot be linked to maintaining the genetic diversity of northern gnatcatcher populations, despite previous recognition of subspecies Our study suggests that not all currently recogn ized subspecies are equivalent to evolutionarily significant units and illu strates the danger of focusing conservation efforts for threatened habitats on a single species.