Analysis of multilocus DNA reveals hybridization in a contact zone between <i>Empidonax</i> flycatchers

Abstract

Contact zones between recently diverged taxa offer unique opportunities to test whether the forms are reproductively isolated and therefore distinct species. The Pacific‐slope flycatcher Empidonax difficilis and Cordilleran flycatcher Empidonax occidentalis are closely related taxa that were officially separated into two species in 1989, a treatment that has been controversial due to reports of phenotypically intermediate birds across the southern interior of British Columbia and Alberta. We present the first analysis of molecular variation across this region, in order to determine whether there is genetic introgression between the taxa. Allopatric populations of Pacific‐slope and Cordilleran flycatchers belong to distinct mitochondrial clades, and all of the individuals sampled in interior southwestern Canada have the Pacific‐slope haplotype. In contrast, variation in nuclear DNA (AFLPs) indicates hybridization between Pacific‐slope and Cordilleran flycatchers in this region. We suggest that the discordance between the mitochondrial and nuclear markers most likely results from stochastic loss of Cordilleran mitochondrial haplotype lineages facilitated by asymmetries in mating due to earlier arrival and greater abundance of Pacific‐slope flycatchers in the contact zone. The discovery of hybridization between Pacific‐slope and Cordilleran flycatchers in southwestern Canada may call into question the decision to split them into two species. On the other hand, allopatric populations are genetically distinct in both mitochondrial and nuclear DNA, and the hybridization might not affect populations outside of the contact zone. This study highlights the importance of employing multiple genetic markers in studies of contact zones between closely related species.