Contrasting the depths of divergence between gene-tree and coalescent estimates in the North American Racers (Colubridae: Coluber constrictor)

The North American racers (Coluber constrictor) are widely distributed
across the Nearctic and numerous studies have demonstrated extensive
variation in morphology, ecology, and population genetic structure. Here
we take an integrative approach to understand lineage diversification
within this taxon by combining genomic sequence capture data, mtDNA
sequence data, morphometrics, and ecological niche models. Both the
genomic data and mtDNA phylogeographic analyses support five lineages
distributed across the range of this species. However, demographic model
selection based on these two datasets strongly conflict in both the model
of divergence and estimates of timing of lineage divergence. While mtDNA
and concatenated genomic data suggest a Miocene origin of these distinct
groups, coalescent-based demographic models of sequence capture data
suggest lineage diversification occurred at ~33 kya in allopatry without
gene flow. Using linear morphological measurements of head shape we
demonstrate that lineages distributed largely east and west of the
Mississippi River are distinguishable. Furthermore, ecological niche
modelling demonstrates that lineages distributed in subtropical habitats
have environmental niche space that is significantly differentiated from
lineages distributed across the continent. Taken together, these results
suggest that ecology is an important axis of lineage divergence within
this group and that more fine-scale analyses may find even greater
differentiation across the populations identified here.