|Agkistrodon cf bilineatus. JCM|
The idea that a single species could occur in several different forms, varieties, or subspecies can be traced to Ernst Mayer’s book Systematics and the Origin of Species. The idea advanced evolutionary thought but also added confusion to the concept of species and concealed the concept of cryptic species. Molecular analysis often supports polytypic species species as complexes of cryptic species suggesting squamate diversity is significantly greater than commonly assumed.
In a new paper Porras and colleagues (2013) describe the cantil (Agkistrodon bilineatus) as a polytypic species of North American pitviper with a variably fragmented distribution extending from extreme southwestern Chihuahua and southern Sonora, Mexico, to northwestern Costa Rica, on the Pacific versant, and parts of the Yucatan Peninsula, northern Belize, Guatemala, and extreme western Honduras on the Atlantic versant; the snake also occurs in Las Islas Marías, an archipelago of four islands located about 100 km west of the state of Nayarit, Mexico . The cantil usuall inhabits dry forest, deciduous forest, thorn scrub, and savanna, primarily areas of low relief that have been exploited heavily by agriculture and areas where this species has become a rare snake; the species elevational range extends from near sea level to about 1,500 m . Along the Pacific coast of Mesoamerica, tropical dry forests were reported as the most endangered of the major tropical ecosystems, with only 0.09% of that region afforded official conservation status . Based on multiple lines of evidence, a phylogeographic analysis of the cantils A. b. taylori was raised to the rank of full species, emphasizing that the loss of forested areas in the habitat of this species underscored the need for its conservation. More recently, an extensive conservation assessment for the entire Mesoamerican herpetofauna, in which numerous authorities provided information on the status of cantils. Although the methodological approaches of these authors varied, it was clear from the outcome that the conservation status of A. bilineatus showed dramatic differences when analyzed on a country by country or regional basis, Porras et al. note several lines of evidence suggest that numerous populations of cantils (Agkistrodon bilineatus, A. taylori) are in rapid decline. They examined the IUCN conservation status for A. bilineatus, assessed for the entire range of the species, as well as the Environmental Vulnerability Scores (EVS) provided for certain countries along its distribution. Because of pronounced disparities in these conservation assessments and notable phenotypic differences that coincide with the geographic distribution of certain cantil populations. They conducted a taxonomic reassessment of the common cantil, Agkistrodon bilineatus (Günther 1863), to determine if the recognized subspecies of A. bilineatus merit specific status. Based on their morphological assessment, biogeographical evidence, and the results of previous DNA-based studies, we elevate the three previously recognized subspecies of A. bilineatus to full species (A. bilineatus, A. russeolus, and A. howardgloydi).
The article can be found on-line.
Porras, LW, Wilson LD, Schuett GW, Reiserer RR. 2013. A taxonomic reevaluation and conservation assessment of the common cantil, Agkistrodon bilineatus (Squamata: Viperidae): a race against time. Amphibian & Reptile Conservation 7:48-73.