Phylogeography of the Timber Rattlesnake (Crotalus Horridus) Based on mtDNA Sequences

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Journal of Herpetology





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The Timber Rattlesnake (Crotalus horridus) is beset by a variety of conservation problems, including habitat loss and persecution. Effective management plans require an understanding of rangewide population structure and intraspecific evolutionary subdivisions. Northern and southern populations have been recognized as distinct subspecies, but this classification remains controversial. A proposed alternative arrangement recognizes southern, northern, and western morphotypes. To resolve intraspecific partitions, we examined a 319 base-pair (bp) fragment of mtDNA cytochrome b in 123 specimens of C. horridus. Neighbor-joining and parsimony analyses reveal a shallow gene genealogy (dmax = 0.024) and sharing of haplotypes among putative subspecies. Analysis of molecular variance demonstrates that traditional subspecific divisions explain only 3.5% of variation, whereas the alternative geographic classification (southern, northern, and western regions) explains 18.6% of genetic variation. The superior performance of the regional grouping can be attributed to an east-west phylogeographic partitioning at the Appalachian and Allegheny Mountain ranges, which were probably uninhabitable at higher elevations during glacial intervals. Distribution of haplotypes and climatic data suggest that a radiation into more northern areas occurred after the most recent (Wisconsinan) glaciation. Hence, the mtDNA data indicate distinct population segments across the range of C. horridus but do not show evolutionary separations that would support subspecific designations.

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