CONCLUSIONS
The taxonomy of Golden-winged and Blue-winged warblers has been debated
for well over a century, since the initial description of Brewster’s and
Lawrence’s warblers (Brewster 1874, Herrick 1874). The intensity of this
debate has heightened recently, in light of genomic analyses (Toews et
al. 2016) whereby genetic differences between Golden-winged and
Blue-winged warblers are associated primarily with genes that control
feather attributes. Indeed, the low level of genome-wide divergence has
been cited as evidence that Golden-winged and Blue-winged warblers may
be plumage morphs of a single-species complex (Kramer et al.
2018). Alternatively, our work suggests these genomic differences relate
to plumage divergence that in turn lead to high levels of reproductive
isolation. We demonstrate a strong, positive correlation between the
frequency of pairing and plumage similarity (Fig. 3) so that
hybridization occurred with only 1.2% of the phenotypically distinct
Golden-winged and Blue-winged warblers. We document strong behavioral
isolation and significant sexual selection against hybrids, which
together provide a value of 0.966 for reproductive isolation. These
attributes support the view that Golden-winged and Blue-winged warblers
are distinct species under the biological species concept.
The taxonomic treatment of these lineages is especially important given
the conservation challenges that Golden-winged and Blue-winged warblers
face (Sauer et al. 2017). Extensive population decline in the
Golden-winged Warbler has resulted in a petition for listing on the
Endangered Species Act (Sewell 2009) and the
decision regarding listing will be highly influenced by one’s
interpretation of speciation in the Vermivora spp. complex.
Ecological and genetic interactions between Blue-winged and
Golden-winged warblers as the former moves into sympatry with the latter
appear to be a major cause of the decline of Golden-winged warblers
(Gill 1980; Confer at al. 2011; Rohrbaugh et al. 2016; Rosenberg et al.
2016). The final genetic outcome of this expansion is unclear. On the
one hand, cryptic introgression (Vallender et al. 2009; Wood et al.
2016) may prevent further divergence between lineages (Strasburg and
Rieseberg 2008; Nosil et al. 2009; Sambatti et al. 2012; Karrenberg et
al. 2019), while sexual selection against hybrids may enhance isolation.
Despite the high levels of reproductive isolation between Golden-winged
and Blue-winged warblers we document, we suggest that their continued
existence may require conservation efforts that maintain or repair
eco-geographic isolation (Roth et al. 2012).