Comments: Analysis of historical biogeography based on mtDNA data (Pook et al. 2000) revealed two main clades, one including populations from east and south of the Rocky Mountains and the other consisting of populations west of the Rocky Mountains. The conventionally recognized subspecies do not fully correspond to the phylogenetic pattern, and a review of the systematic status of several populations is needed (Pook et al. 2000).
Ashton and de Queiroz (2001) examined mtDNA variation among 26 populations of C. viridis and also identified two main clades: eastern, including subspecies viridis and nuntius (low levels of genetic divergence), and western, including all other subspecies. However, Ashton and de Queiroz (2001) differed from Pook et al. (2000) with respect to the relationships among members of the western clade, although Ashton and de Queiroz studied only a few individuals from each member of the western clade and stated that the relationships within the western clade are largely unresolved and that none (except possibly cerberus) appeared to deserve recognition as separate evolutionary species. Ashton and de Queiroz suggested that the two main clades be regarded as distinct species, C. viridis (eastern clade) and C. oreganus (western clade). The historical biogeographic scenario described by Ashton and de Queiroz (2001) suggests secondary contact between C. viridis and C. oreganus in northern Arizona, southwestern and northwestern Colorado, and southeastern Utah.
Douglas et al. (2002) examined mtDNA variation in C. viridis, with emphasis on the populations on the Colorado Plateau. As did Pook et al. (2000) and Ashton and de Queiroz (2001), they identified eastern and western clades, with the former including the nominal subspecies viridis and nuntius and the latter encompassing all of the other subspecies. Douglas et al. (2002) argued that all of the western subspecies should be recognized as species, but they did not effectively indicate details of distributional relationships in the contact zones among the proposed species. Douglas et al. (2002) concluded that the taxon nuntius should be regarded as a synonym of viridis.
Crother et al. (2003) considered all of the foregoing evidence and adopted the two-species taxonomy (Crotalus oreganus, Crotalus viridis) that is supported by the congruence among all three studies cited above. Campbell and Lamar (2004) also recognized only the two species. However, further clarification of the distributions of C. viridis and C. oreganus is needed, particularly in the contact zones in northern Arizona, southwestern and northwestern Colorado, and southeastern Utah. For example, populations in northwestern Colorado (Moffat County) identified by Douglas et al. (2002) as C. viridis were mapped as C. oreganus concolor by Campbell and Lamar (2004).
Venom characteristics indicate hybridization between C. viridis and C. scutulatus in New Mexico (Glenn and Straight 1990).