Genetic structure and biogeographic history of the Bicknell’s Thrush/ Gray-cheeked Thrush species complex

Abstract We examined species limits, admixture, and genetic structure
among populations in the Bicknell’s Thrush (Catharus
bicknelli)–Gray-cheeked Thrush (C. minimus) species complex to establish
the geographic and temporal context of speciation in this group, which is
a model system in ecology and a high conservation priority. We obtained
mitochondrial ND2 sequences from 186 Bicknell’s Thrushes, 77 Gray-cheeked
Thrushes, and 55 individuals of their closest relative, the Veery (C.
fuscescens), and genotyped a subset of individuals (n = 72) at 5,633
anonymous single nucleotide polymorphic (SNP) loci. Between-species
sequence divergence was an order of magnitude greater than divergence
within each species, divergence was dated to the late Pleistocene (420
kbp) based on Bayesian coalescence estimation, and a coalescent model
(IMa) revealed almost no gene flow between species based on ND2. SNP data
were consistent with mitochondrial results and revealed low levels of
admixture among species (3 of 37 Bicknell’s Thrushes, no Gray-cheeked
Thrushes, and no Veeries were >2% admixed). Species
distribution models projected to the Last Glacial Maximum suggest that
Bicknell’s Thrush and Gray-cheeked Thrush resided in primarily allopatric
refugia in the late Pleistocene, consistent with the genetic data that
support reproductive isolation over an extended period of time. Our
genetic data suggest that both species underwent demographic expansions,
possibly as they expanded out of Pleistocene refugia into their current
ranges. We conclude that Bicknell’s Thrush and Gray-cheeked Thrush are 2
distinct species-level lineages despite low levels of Gray-cheeked Thrush
introgression in Bicknell’s Thrushes, and divergence has been maintained
by a long history of allopatry in subtly different habitats. Their unique
phylogeography among boreal forest birds indicates that either cryptic
species breaks in eastern North America are still undiscovered, or another
factor, such as divergent natural selection, high migratory connectivity,
or interspecific competition, played a role in their divergence.