Replicate hybrid zones suggest a limited role of plumage in reproductive
isolation among subspecies of the Variable Seedeater (Sporophila
corvina)
Abstract
After establishing secondary contact, recently diverged populations may
remain reproductively isolated or hybridize to a varying extent
depending on factors such as hybrid fitness and the strength of
assortative mating. Replicated contact zones between hybridizing taxa
offer a unique opportunity to explore how different factors interact to
shape patterns of hybridization. Here, we used genomic and phenotypic
data from three independent contact zones between subspecies of the
Variable Seedeater (Sporophila corvina), to examine how coloration and
genetic divergence shape patterns of hybridization. We found that
plumage coloration has limited introgression across contact zones, but
the degree of plumage divergence does not explain overall patterns of
introgression. Across two parallel contact zones between populations
with divergent phenotypes (entirely black vs. pied plumage) populations
hybridized extensively across one contact zone but not the other,
suggesting that plumage divergence is not sufficient to maintain
reproductive isolation. Where subspecies hybridized, hybrid zones were
wide and formed by later-generation hybrids, suggesting that hybrids
present similar or higher fitness than parental subspecies. Moreover,
contemporary gene flow has played an important role in shaping patterns
of genetic diversity between populations. Overall, our results
demonstrate that divergence in plumage coloration is important in
reducing gene flow but insufficient in maintaining reproductive
isolation in this clade, and that other factors such as divergence in
song and time since secondary contact may also play an important role in
driving patterns of reduced hybridization and gene flow.