Data from: Hybridization between Townsend’s Dendroica townsendi and black-throated green warblers D. virens in an avian suture zone

Hybrid zones between species provide natural systems for the study of
processes involved in divergence, reproductive isolation and speciation.
Townsend’s Dendroica townsendi and black-throated green D. virens warblers
are phenotypically and genetically divergent groups that occur in western
and eastern North America respectively, with potential for range contact
in the Rocky Mountains of British Columbia, where other west-east avian
pairs come into contact. Although one potential hybrid (a phenotypic
Townsend’s warbler with the black-throated green mitochondrial DNA) has
been previously reported, there have been no studies of interactions
between the taxa in potential areas of sympatry. To determine whether
interbreeding between these species is a regular occurrence we examined
variation in individuals across the area of putative range overlap.
Analysis of plumage, morphology, and mitochondrial (COI) and nuclear
molecular markers (CHD1Z and numt-Dco1) shows surprisingly extensive
hybridization between these species, with at least 38% of individuals in
the hybrid zone being either hybrids or backcrosses. Each of the traits
displays a sigmoidal cline centred along the eastern slope of the Rocky
Mountains (molecular cline centres averaging 50 km east of the crest of
the Rockies, ranging from 41 to 56 km). The clines are narrow (average
molecular cline width is 60 km, ranging from 40 to 87 km) relative to the
dispersal distance of related warbler species, suggesting that selection
is maintaining the hybrid zone; we discuss possible sources of selection.
Given the narrowness of the zone we recommend the two forms should
continue to be treated as separate taxonomic species. Townsend’s warblers
also form an extensively studied hybrid zone with their more closely
related southern relative, the hermit warbler D. occidentalis. The
combined system of three discrete forms separated by narrow hybrid zones
provides an excellent system for the study of hybridization, reproductive
isolation and speciation.