Cryptic Genetic Introgression into an Appalachian Sky Island Population of Black-capped Chickadees (Poecile atricapillus)

Abstract

Black-capped chickadees (Poecile atricapillus) hybridize with Carolina chickadees (P. carolinensis) along the species' contact zone across the eastern United States and in the northern Appalachian Mountains. The Great Smoky Mountains harbor the last large breeding population of atricapillus in the southern Appalachians, isolated from the species main range by nearly 200 km. This population is believed to be reproductively isolated from local carolinensis due to a behaviorally-mediated elevational range gap during the breeding season, which may function as an incipient speciation mechanism. I characterized this population genetically, using hundreds of AFLP loci spread throughout the genome as well as cytochrome-b sequence data from the mitochondrial DNA. The Great Smoky Mountain population of atricapillus has experienced genetic introgression from carolinensis, but at a much lower level than other populations near the northern hybrid zone. This population is also differentiated from northern conspecifics, likely due to historically reduced gene flow.