Data from: Reproductive isolation and cryptic introgression in a sky island enclave of Appalachian birds

Reproductive isolation is central to the speciation process, and cases
where the strength of reproductive isolation varies geographically can
inform our understanding of speciation mechanisms. Although generally
treated as separate species, Black-capped chickadees (Poecile
atricapillus) and Carolina chickadees (P. carolinensis) hybridize and
undergo genetic introgression in many areas where they come into contact
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 an unusual,
behaviorally mediated elevational range gap, which forms during the
breeding season and may function as an incipient reproductive isolating
mechanism. We examined the effectiveness of this putative isolating
mechanism by looking for genetic introgression from carolinensis in Great
Smoky Mountain atricapillus. We characterized this population and parental
controls genetically using hundreds of amplified fragment length
polymorphism (AFLP) loci as well as mitochondrial DNA (mtDNA) sequence
data from cytochrome-b. Great Smoky Mountain atricapillus have experienced
nuclear genetic introgression from carolinensis, but at much lower levels
than other populations near the hybrid zone to the north. No mitochondrial
introgression was detected, in contrast to northern contact areas. Thus,
the seasonal elevational range gap appears to have been effective in
reducing gene flow between these closely related taxa.