INTRODUCTION
Ecotypic variation within a species often corresponds to selection
pressures that give rise to local adaptation, and studies of local
adaptation are important for understanding how selection may lead to
evolution in natural systems. These kinds of population-level ecotypic
differences have been commonly described in freshwater fishes (stream
and lake stickleback, Gasterosteus aculeatus ; Hendry et al.,
2002), coastal and inland annuals (Mimulus guttatus ; Lowry et
al., 2008), and island finches (large- and small-billed Geospiza
fortis ; Hendry et al., 2009). Among vagile, widely distributed,
continental birds, however, local adaptation at a broad scale is less
commonly described (but see Alcaide et al., 2014). Specifically, in
avian taxa, geographic variants—described variably as populations,
races, or subspecies depending on the magnitude of differentiation—are
often expressed through plumage or song, with a less obvious connection
to their environments (Toews et al., 2016; Toews, Taylor, et al., 2016).
These phenotypic differences can be further obscured by overlap with
respect to ecological affinities and have presented significant debate
to their contribution to biodiversity and conservation science (Mayr,
1982; Zink, 2004).
Wood warblers of North, Central, and South America (Parulidae) are
particularly notable among bird species for lacking strong ecological
disparities: the classic study of niche partitioning by MacArthur (1958)
was in fact initiated because “ecologists studying them have concluded
that any differences in the species’ requirements must be quite
obscure”. Indeed, although MacArthur observed fine-scale habitat
partitioning, it is still generally appreciated that some of the most
distinct differences among warbler species occur within plumage and song
(Kent et al., 2021; Price et al., 2000). Moreover, any morphological and
ecogeographic differences within most species are not discrete, but
clinal in nature, such as the gradual increase in body size with
increasing latitudes (Jones et al., 2003; Youtz et al., 2020).
One exception to the general pattern—where there appears to be
discrete, non-clinal morphometric variation within a warbler
species—occurs within the black-throated green warbler
(Setophaga virens ). This Neotropical-Nearctic migrant is a common
breeder in upland, mixed-hardwood and evergreen forests of the eastern
USA and much of Canada (Morse & Poole, 2020). Its occurrence is much
rarer in several disjunct breeding locations scattered across the
southern periphery of its breeding range (Haggerty, 2009; Mumford et.
al., 1984; Rodewald, 1997), including the south Atlantic Coastal Plain
where one formerly recognized subspecies, S. virens waynei , was
first described (Bangs, 1918). Colloquially referred to as Wayne’s
warbler, S. v. waynei is uniquely associated with swamp and
non-riverine wetland habitats (Sprunt, Jr., 1953; Watts et al., 2011;
Worm & Carpenter, 2021) and separated by more than 400 km (and up to
1200 m elevation) from the closest conspecifics in the southern
Appalachian Mountains. While plumage differences from the nominate form
may be subtle, S. v. waynei is reported to be smaller, especially
in bill dimensions, even when compared to individuals from the same
latitude (Morse & Poole, 2020). Despite these reported differences in
morphology and habitat use—as well as continued recognition ofS. v. waynei as a Species of Greatest Conservation Need in three
U.S. state Wildlife Action Plans (North Carolina Wildlife Resources
Commission, 2015; South Carolina Department of Natural Resources, 2014;
Virginia Department of Game and Inland Fisheries, 2015)—this
population is now considered “non-diagnosable” (Morse & Poole, 2020),
leaving the status of S. v. waynei as a meaningful subspecies in
doubt.
Genetic diagnosability is important for conservation to appropriately
delineate management units and prioritize limited resources; however,
differences between S. virens and S. v. waynei as
reflected in the genome remain unknown. This uncertainty may delay
critical conservation action for S. v. waynei, which has
experienced recent declines (Watts et al., 2011) likely related to loss
and degradation of its breeding habitat (Davis et al., 1997; Richardson,
1983).
To address this taxonomic debate and fill a critical knowledge gap, we
apply whole-genome resequencing to quantify, for the first time, genetic
difference across the S. virens complex, including samples
obtained from putative S. v. waynei individuals. Our goals are to
determine: 1) whether S. v. waynei cluster independently from
other S. virens populations, 2) if there are genetic differences
among the groups, including the extent and distribution of the
divergence (i.e., is differentiation clustered into regions of the
genome), and 3) approximately when and where S. v. wayneiseparated from the nominate form. This information is essential to
better understand the unique evolutionary history of the species
(Bermingham et al., 1992) and to increase our ability to conserve and
manage a putative subspecies of concern within the complex.