Historical biogeography of the western rattlesnake (Serpentes : Viperidae : Crotalus viridis), inferred from mitochondrial DNA sequence information

We infer the phylogeography of the Western Rattlesnake (Crotalus viridis) u sing phylogenetic analysis of mitochondrial DNA sequences from 1345 bp of t he genes for cytochrome b and NADH dehydrogenase subunit 4. Two main clades are revealed: one includes populations from east and south of the Rocky Mo untains (conventionally referred to as Crotalus viridis viridis and C. v. n untius), and the other consists of populations west of the Rocky Mountains. Within the western clade, a population from southern Arizona (C. v. cerber us) represents the sister taxon to the remaining western populations. The c onventional subspecies recognized in this species do not fully correspond t o the phylogenetic pattern, and a review of the systematic status of severa l populations is needed. Our data allow the inferences that small body size evolved twice and that the ability of one population (C. v. concolor) to s ecrete highly lethal toxins related to Mojave toxin arose within the comple x. Our phylogeny should represent the basis for further studies on the caus es of geographical variation in this complex. (C) 2000 Academic Press.