A persistent question always arises when reading phylogeographic studies of herp distribution: how effective are putative riverine "barriers" (e.g., especially the Mississippi and Apalachicola systems) at limiting gene flow between members of species that are distributed across them?
Such herpetological phylogeographic studies as currently exist tend to indicate that for more terrestrial forms (e.g., Pantherophis obsoletus complex, P. guttatus complex, Coluber constrictor, Lampropeltis getula, etc) these river systems significantly impede the exchange of at least mitochondrial genes between populations located on either side of such drainage systems (regardless of the well-known fact that all known species of snakes possess the innate ability to swim).
As would be expected, however, the evidence of a barrier effect is much more equivocal when semi-aquatic species are investigated. Agkistrodon piscivorus (Guither and Burbrink, 2008) and Nerodia erythrogaster (Makowsky et al., 2010), for example, both display extensive (mitochondrial) gene flow across such river systems, implying that they are less effective barriers for such semi-aquatic species than for more terrestrial forms.
A new study from M.C. Brandley et al. has sought to specifically address this question with the large water snake Nerodia rhombifer, which they characterize as one of the most aquatic North American water snakes, "spending the majority of its time in or near freshwater environments and consuming fish almost exclusively (97% of its diet . . . .)". The authors investigated the phylogeographic structure of this species via a "time-calibrated" phylogenetic analysis of mitochondrial DNA using tissue samples from 388 specimens collected throughout the current range of the species in the USA.
The results of the study indeed indicate the presence of distinctive haplotypes to the east and west of the Mississippi River. However, especially along the river itself, there were several populations composed of a mixture of the eastern and western haplotypes, leading the authors to conclude that, while the Mississippi River did not function as a complete barrier in N. rhombifer, it nevertheless influenced mitochondrial gene flow to the extent that characteristic mt haplotypes managed to develop on both sides of the river.
The authors further suggest that although the Mississippi is - or has been - only a partial barrier (i.e., a "filter") to N. rhombifer, much of the mitochondrial divergence detected in this study occurred relatively recently during the early Pleistocene, likely during interglacial periods when the river was much wider than currently.
The curious failure of the Mississippi River to serve as an impediment to either Agkistrodon piscivorus or Nerodia erythrogaster while exerting at the same time a detectable filter effect on the equally or even more aquatic N. rhombifer remains unexplained by this paper although the authors proffer the possibilities that either 1) mtDNA markers are not sensitive to the signs of vicarience in A. piscivorus and N. erythrogaster, or 2) that these species have invaded the areas west of the river too recently to be detected by mtDNA, or 3) that ultimately the river has not truly acted as an impediment to either of these species.
This paper is a refreshing, relatively noncontroversial application for mtDNA techniques in a venue that is perhaps more appropriate for them than some of the intraspecific "analyses" in which they are currently employed. Fortunately, no traditional subspecies were available to be sunk as a result of this study, but one must wonder, if only cynically, if the authors were philosophically inclined toward the recognition of subspecies, whether they might have described their eastern and western clades of N. rhombifer as some sort of "mitochondrial subspecies"!
Brandley, M.C., Guiher, T.J., Pyron, R.A., Winne, C.T, and F.T. Burbrink. 2010. Does dispersal across an aquatic geographic barrier obscure phylogeographic structure in the diamondback water snake (Nerodia rhombifer)? Molecular Phylogenetics and Evolution 57(2010): 552-560.
PDF = http://www.naherpetology.org/pdf_files/1664.pdf
Guiher, T.J. and Burbrink, F.T. 2008. Demographic and phylogeographic histories of two venomous North American snakes of the genus Agkistrodon. Mol. Phylogenet. Evol. 48: 543-553. PDF = http://www.naherpetology.org/pdf_files/1024.pdf
Makowsky, R., Marshall, J.C., Jr., McVay, J., Chippindale, P.T. and L.J. Risssler. 2010. Phylogeographic analysis and environmental niche modeling of the plainbellied water snake (Nerodia erythrogaster) reveals low levels of genetic and ecological differentiation. Mol. Phylogenet. Evol. 55 (2010): 985-995. PDF = http://www.naherpetology.org/pdf_files/1498.pdf