Data from: Cross-decades stability of an avian hybrid zone

Hybrid zones are particularly valuable for understanding the evolution of
partial reproductive isolation between differentiated populations. An
increasing number of hybrid zones have been inferred to move over time,
but in most such cases zone movement has not been tested with long-term
genomic data. The hybrid zone between Townsend’s Warblers (Setophaga
townsendi) and Hermit Warblers (S. occidentalis) in the Washington
Cascades was previously inferred to be moving from northern S. townsendi
southward toward S. occidentalis, based on plumage and behavioral patterns
as well as a 2000-km genetic wake of hermit mitochondrial DNA (mtDNA) in
coastal Townsend’s Warblers. We directly tested whether hybrid zone
position has changed over 2-3 decades by tracking plumage, mtDNA, and
nuclear genomic variation across the hybrid zone over two sampling periods
(1987-94 and 2015-16). Surprisingly, there was no significant movement in
genomic or plumage cline centers between the two time periods. Plumage
cline widths were narrower than expected by neutral diffusion, consistent
with a “tension zone” model, in which selection against hybrids is
balanced by movement of parental forms into the zone. Our results indicate
that this hybrid zone is either stable in its location or moving at a rate
that is not detectable over 2-3 decades. Despite considerable gene flow,
the stable clines in multiple phenotypic and genotypic characters over
decades suggest evolutionary stability of this young pair of sister
species, allowing divergence to continue. We propose a novel biogeographic
scenario to explain these patterns: rather than the hybrid zone having
moved thousands of kilometers to its current position, inland Townsend’s
met coastal Hermit Warbler populations along a broad front of the British
Columbia and Alaska coast and hybridization led to replacement of the
Hermit Warbler plumage with Townsend’s Warbler plumage patterns along this
coastline. Hence hybrid zones along British Columbia and Alaska moved only
a short distance from the inland to the coast, whereas the Hermit Warbler
phenotype appears stable in Washington and further south. This case
provides an example of the complex biogeographic processes that have led
to the distribution of current phenotypes within and among closely related
species.