Structure, Demographics, and Phenotype
Results from DAPC and sNMF are highly concordant in estimating three major lineages (Fig. 3). These correspond primarily to populations in the Piedmont (‘CHR’), Cohutta Mountains (‘COH’) in north-central Georgia, and Nantahala mountains in southwestern North Carolina and adjacent north-east Georgia (‘NTA’). In contrast, conStruct estimates two distinct lineages (CHR and NTA+COH) when accounting for spatial distance, with the second layer accounting for ~80% of allelic covariance (Fig. S7). Estimates accounting for admixture in sNMF and conStruct suggest widespread introgression at the contact zones between these lineages (montane/Piedmont in the latter), with substantial genomic ancestry (>10%) from each lineage inferred for multiple individuals from multiple populations across most of the species’ range.
The GADMA estimates converged on a model with relatively small ancestral population sizes (~240k) originating in the mid-Pleistocene (~1.2Ma), with recent exponential increases (~3–19x) during the latest glacial cycles ~630Ka–present (Fig. 3). Migration was very high between the two ancestral lineages (estimated at the upper bound of 2Nm = 20) and decreased towards the present; remaining >1 for NTA<–>COH (1.24 and 1.85), CHR–>NTA (1.40), and CHR–>COH (1.70). Populations are not phenotypically differentiated based on the 17 linear morphometric measurements taken here (Fig. 2). Specimens from all 3 lineages overlap in linear-discriminant space, where the heaviest-loading size-corrected variable of trunk length (AG; axilla-groin) does not differ significantly between them. Overall, these results are congruent with the hypothesis of a single deeply structured species, wherein the constituent phylogeographic lineages are not phenotypically differentiated and maintain migration sufficient to overcome drift, local adaptation, and divergent ecological selection.