Comments: Sceloporus undulatus is morphologically highly variable (Stebbins 1985, Conant and Collins 1991, Hammerson 1999). Leaché and Reeder (2002) examined range-wide mtDNA variation and identified at least four apparently monophyletic (but morphologically highly variable) groups, which they proposed as species under the evolutionary species concept (Eastern group: east of Mobile Bay; Central group: east of the Rockies and west of Mobile Bay; Western group: southern Wyoming to central Arizona and northern New Mexico; Southwestern group: eastern Arizona and central New Mexico to northern Mexico and western Texas). All of the groups are discordant with recognized subspecies circumscriptions. For example, the Central group encompasses six nominal subspecies ranging from the foothills of the Rocky Mountains in Colorado to the Gulf Coast of southern Mississippi. Populations of the morphologically distinctive subspecies erythrocheilus in central Colorado grouped with subspecies garmani (Central group) rather than with populations of erythrocheilus in south-central Colorado (Western group). Leaché and Reeder (2002) tentatively proposed the following names: eastern group, S. undulatus; central group, S. consobrinus; western group, S. tristichus; southwestern group, S. cowlesi. However, Leaché and Reeder identified no diagnostic characters for any of the proposed species, and the distributions of proposed species were only coarsely mapped and do not correspond closely with the distributions of previously recognized subspecies, leaving in doubt the specific identities of many Sceloporus populations.

Miles et al. (2002) examined allozyme variation (24 loci from 12 populations, 6 of the 11 recognized subspecies, plus 3 additional species and an outgroup species). Phylogenetic trees were inconsistent with current subspecific designations. Additionally, S. occidentalis, S. virgatus, and S. woodi arose within S. undulatus. The subspecies S. u. hyacithinus and S. u. undulatus are polyphyletic, and S. u. garmani and S. u. tristichus are paraphyletic. Two major lineages were identified: (1) a midwestern grasslands group (includes a population from St. Louis County, Missouri) and (2) various populations from eastern woodlands and western canyonlands.

Further integrated study of morphological and genetic variation, and more detailed genetic examination of various geographic areas, are needed before a well-founded, geographically comprehensive taxonomic revision of the S. undulatus group can be made (e.g., see Niewiarowski et al. 2004).

Specific distinctness of S. occidentalis and S. undulatus is confirmed by their sympatric reproductive isolation in southwestern Utah (Cole 1984, Smith and Chiszar 1989).

See Sites et al. (1992) for a review of phylogenetic hypotheses for lizards of the genus Sceloporus.