Morphological Evidence for Adaptive Radiation in Western Rattlesnake (Crotalus viridis)

Morphological data are the standard by which species are recognized and described by taxonomists. Their inferential power has been strengthened by the advent of geometric morphometric (GM) approaches, such that compelling arguments for reevaluation of long-standing taxonomic designations are now possible, particularly when contrasted with a molecular perspective. Here we employ the Western Rattlesnake (Crotalus viridis) complex as the evolutionary context within which this juxtaposition is explored. Nine subspecies are recognized in the complex based on traditional morphological analysis, while molecular assessments reveal two distinct lineages with additional but unresolved divergences within each. Assessment (via MANOVA) of combined dorsal and lateral GM landmarks revealed that shape differed significantly among subspecies. Post hoc pairwise comparisons further determined that all subspecies differed one from another in head shape with the exception being a single subspecies pair (i.e. abyssus x cerberus). Discriminant Analysis was effective at 70% in correctly assigning individuals to their identified subspecies, and suggests that shape means in Western Rattlesnake exhibit some overlap in morphospace. Disparity Through Time analysis determined that shape divergence among subspecies was significantly greater than expected by chance, indicating adaptive divergence. Finally, Phylogenetic Generalized Least Squares revealed that both phylogeny and ecology contributed significantly to variance in head shape among subspecies. Thus, lack of congruence among phylogenetic hypotheses may stem from ecological adaptation. Results from our shape analyses reinforce morphological, venom, and molecular genetic data and may aid in solidifying taxonomic revisions.