Systematics of the Polymorphic North American Rat Snake (Elaphe Obsoleta).

Abstract

The North American rat snake (Elaphe obsoleta) complex is composed of seven uniquely colored subspecies of E. obsoleta and the sister species E. bairdi. Maximum parsimony (MP) and maximum likelihood (ML) analysis of two mitochondrial DNA genes from 73 E. obsoleta and E. bairdi, produced well-supported trees that do not conform to the currently accepted subspecies. Both ML and MP trees were significantly shorter than the trees in which each subspecies was constrained to be monophyletic. The subspecies of E. obsoleta do not represent genetic lineages. Instead, molecular evidence revealed the existence of four well-supported mtDNA clades confined to particular geographic areas in North America. Lack of genetic variability in each clade of E. obsoleta may indicate that the lineages expanded from southern glacial refugia recently. Univariate and multivariate analysis of 67 morphological characters scored from 1006 specimens provided additional statistical support for the existence of the same four evolutionary lineages identified in the molecular phylogeographic study. Specimens could be classified using canonical discriminant function analysis into the four molecular clades better than they could be partitioned into subspecific categories. Moreover, the identification of these subspecies proved difficult when using the traditional characters ascribed to them. In light of the corroborating molecular and morphological evidence, it is suggested that the recognition of the subspecies of Elaphe obsoleta be discontinued. Instead, the four molecular clades should be recognized as four species: E. alleghaniensis, E. spiloides, E. obsoleta, and E. bairdi. This research demonstrates that the recognition of subspecies from one or two characters may be detrimental to understanding evolutionary history.