Sumner Lake State Park, De Baca County, New Mexico, is the only known locality where three pattern classes of diploid, parthenogenetic Aspidoscelis tesselata (Sumner C, Sumner D, and Sumner E) coexist in syntopy. Reciprocal skin transplants confirmed that the pronounced phenotypic differences between Sumner C and Sumner E represent postformational genetic changes rather than separate hybridization origins. Sumner D is meristically indistinguishable from Sumner C and is considered to be a recent mutational derivative of the latter. In contrast, Sumner E is distinctly different from Sumner C in multivariate meristic characters and several important life-history characteristics. Discordant patterns of phenotypic variation characterize many geographically disjunct groups of A. tesselata classified as pattern class E, thus defying a cohesive diagnosis. Therefore, based on the evolutionary species concept (ESC), we consider Sumner C and Sumner E to be divergent clonal groups in the same species. We contrast this example with a parthenogenetic complex on the Yucatán Peninsula in which formal recognition of Aspidoscelis maslini and Aspidoscelis cozumela can be accommodated under the ESC.