Western Massasauga (Sistrurus tergeminus tergeminus). Photo by Michael Smith, Wikimedia Commons
A recent paper by Laura S. Kubatko and associates (Kubatko et al. 2011) analyzed the entire genus Sistrurus by subspecies, using 18 independent nuclear genes and one mitochondrial gene to compare both "gene trees" (the usual kind of DNA analysis we are accustomed to seeing, typically based on one mitochondrial gene) and the more informative "species trees" (based on information from multiple nuclear genes).
In the past phylogeneticists have avoided the "subspecies problem" by ignoring or completely disavowing them. This "subspecies denial" arose no doubt in part simply due to the fact that their preferentially employed techniques, based entirely (or almost so) on the sequencing of mitochondrial DNA (mtDNA), appear to be, regardless of their other useful properties, generally incapable of detecting subspecies. In an age when we see a constant emphasis on the recognition of biodiversity, it is remarkable that so many phylogeneticists are unwilling (some adamantly so) to take official notice of biodiversity below the species level.
Accordingly, it is rare to see phylogenetic trees (cladograms) published in taxonomic papers where the terminal branches represent both species and subspecies. In the present paper, however, subspecies are regarded as valid entities, a decision supported by the nuclear DNA (nDNA) results presented. As expected, the genetic distances determined between subspecies are less than those between the catenatus and tergeminus/edwardsii clades, supporting the authors' proposal that these two groups represent distinct species.
An additional criterion advanced in proposing full species status for the Eastern Massasauga is that its distribution is completely allopatric to that of the Western form. Potential hurdles to that claim lay in the existence of three isolated, relictual, and allegedly hybrid or intergradient populations in northern Missouri, which could weaken the argument from allopatry by providing evidence of geologically recent contact between the Eastern and Western taxa. Consequently, two of these problematic populations were genetically examined in tandem with the present study, using similar techniques, and found to be unequivocally referable to the Western Massasauga (S. c. tergeminus) with little or no indications of the eastern form (Gibbs et al. 2011).
Of course while allopatric distribution, in itself, is not necessarily an indicator of speciation, it does represent a form of de facto reproductive isolation which, taken along with morphological diagnosability and significant genetic differentiation, supports the assertion that full species status for catenatus is warranted.
Ordinarily I am particularly skeptical when phylogenetics papers propose elevation of populations that are extremely similar morphologically to full species status (so-called "cryptic" species), especially so when such proposals are based on little more than mtDNA sequences. In this case, however, the preponderance of the evidence has convinced me that the proposed solution is the correct one.
One surprising aspect of this paper was the authors' retention of the Desert Massasauga (S. t. edwardsii) as a valid subspecies. I had always considered edwardsii to be a poorly defined subspecies since in many parts of Texas, at least, it is sometimes difficult to determine whether a given specimen is tergeminus or edwardsii; the intergradation zone is apparently broad and, given the spotty distribution of the species, is difficult to delimit. The authors' nDNA evidence, however, strongly suggests that edwardsii is equally as valid as any other Sistrurus subspecies. Their rationalization for this revolves around observations that while tergeminus is adapted to grassland habitats, edwardsii has evolved to exploit more xeric desert-grassland habitat. Since the boundaries between these two usually adjacent life zones are diffuse and even shifting with climate changes, this scenario fits the imprecise intergrade zone existing between the two subspecies.
Somewhat surprisingly, Kubatko et al. failed to even mention the extensive taxonomic maneuvering that was required (involving a highly unorthodox decision by the ICZN) beforehand to preserve and restrict the name "catenatus" for the Eastern species (Rafinesque's type specimen was apparently a tergeminus!), as I discussed several years ago [Another Fine "Messasaugus"]
Gibbs, H.L., Murphy, M., and J.E. Chiucchi. 2011. Genetic identity of endangered massasauga rattlesnakes (Sistrurus sp.) in Missouri. Conserv. Genet. 12: 433-439. [PDF]
Holycross, Andrew T., Anton, Thomas G., Douglas, Michael E., and Darrel R. Frost. 2008. The Type Localities of Sistrurus catenatus and Crotalus viridis (Serpentes: Viperidae), with the Unraveling of a Most Unfortunate Tangle of Names. Copeia 2008 (2): 421-424.
Kubatko, Laura S., Gibbs, H. Lisle, and Erik W. Bloomquist. 2011. Inferring Species-Level Phylogenies and Taxonomic Distinctiveness Using Multilocus Data in Sistrurus Rattlesnakes. Syst. Biol. 60(4): 393-409. [PDF]