PHYLOGENETIC-RELATIONSHIPS OF THE GARTER SNAKES BASED ON DNA-SEQUENCE AND ALLOZYME VARIATION

We estimated phylogenetic relationships among 26 species of garter snakes (genus Thamnophis) using allozyme and mitochondrial cytochrome b gene nucleotide sequence variation. Parsimony analyses of the two data sets give substantially different estimates of phylogeny. Several lines of evidence indicate that much of this conflict is due to error associated with the restricted number of characters in each data set. Such sampling error may be reduced by combining all the characters; we therefore present an estimate of phylogeny based on parsimony analysis of all the data combined. All our analyses support several conclusions in conflict with previous views: a very distant relationship between T. errans and T. elegans, non-monophyly of the elegans group (even excluding T. errans), and nesting of the form validus (previously considered a member of the genus Nerodia) within Thannophis. The combined analysis gives an almost fully resolved tree. However, bootstrapping indicates only weak support for many clades in this tree. Furthermore, paraphyly of the assemblages of cytochrome b gene lineages within T. elegans and T. radix indicate the potential for discordance between the mitochondrial DNA (mtDNA) and species phylogenies through the sorting of ancestral mtDNA polymorphisms. These problems suggest the need for assaying additional characters, especially ones likely to be independent of those used in the present study.