Choosing among Hypotheses of Rattlesnake Phylogeny: A Best-Fit Rate Test for DNA Sequence Data

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Abstract
Large head scales distinguish the three species of Sistrurus from other rattlesnakes assigned to the speciose genus Crotalus, but this trait is plesiomorphic and therefore not indicative of monophyly. Do rattlesnakes comprise two clades, one with large head scales and one with fragmented head scales, or must the traditional taxonomy of this group be revised? Evidence from comparative DNA sequence analysis of portions of the 12S and 16S mitochondrial ribosomal RNA genes supports the monophyly of Sistrurus. These data also suggest that rattlesnakes are monophyletic with a mid-Cenozoic origin. A phylogenetic hypothesis is proposed that is congruent with morphological and DNA sequence data. Alternative hypotheses were close in length, and the monophyly of Sistrurus was rather unstable, as determined by bootstrap resampling. Maximum parsimony failed to identify the correct tree topology (as indicated by morphology). Character-state differences, as determined by patristic and pairwise measures, were observed between an outgroup (Agkistrodon) and each rattlesnake species. The tree that agreed best with relative rates of evolution as determined by pairwise comparisons was chosen. A monophyletic Sistrurus was favored over competing alternatives by this best-fit rate test.

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