Hybridization among western whiptail lizards (Cnemidophorus tigris) in southwestern New Mexico: Population genetics, morphology, and ecology in threecontact zones

Cnemidophorus tigris punctilinealis of the Sonoran Desert and C. t. marmora tus of the Chihuahuan Desert contact each other and interbreed in the Anima s Valley of southwestern New Mexico. More than 600 specimens have been exam ined from the contact region, and data on biochemical genetics (mitochondri al DNA haplotypes, protein electrophoresis of nuclear gene products), chrom osomes, external morphology (coloration, size, scalation), reproduction, an d fitness have been compared for three hybrid zones. Habitats in the contac t region were mapped and photographed, and they are discussed in the contex t of vegetational changes during Pleistocene to Recent times, which affecte d the geographic distribution of these animals. Data from mitochondrial DNA, allele frequencies at four protein loci (of 36 analyzed), and body coloration demonstrate that the areas of contact have steep, concordant, and coincident step-dines in which most gene exchange oc curs in hybrid zones that are 3.2-7.8 km wide. Analyses of allele frequenci es, genotype frequencies, and fixation indices (including Hardy-Weinberg eq uilibrium, linkage equilibrium, and cytonuclear equilibrium) indicate a pop ulation structure determined primarily by random mating and an absence of s election against hybrids. Estimates of gene flow indicate that the dines re sulted from neutral secondary contact initiated with the newest reconnectio n of the Sonoran and Chihuahuan Deserts within the present interglacial epi sode, from 1000 to 5000 years ago. This timeframe is consistent with paleoe cological data from packrat middens. Analyses of karyotypes, morphology, reproduction, and physiology also fail to detect differences in fitness among lizards with various genotypes. Alth ough it is possible that there are fitness differences that are too small t o be detected by the sample sizes we employed, the data indicate that repro ductive success, fitness, and the dynamics of populations within the hybrid zones presently are no different from those in nonhybrid populations. Earl ier data, which suggested that one of the step-dines was moving, are not su pported. The clines are located in fragile semiarid habitats that are subject to des ertification. Consequently, we present considerable data and dated photogra phs of habitats, precise locations of sampling sites, and local allele freq uencies, so that future investigators can monitor changes in position, widt h, or dynamics of these hybrid zones. In addition, the population genetics data are discussed in the context of t he following: (1) absence of rare, apparently novel alleles forming in the hybrid zones; (2) genetic comparisons with additional subspecies of C. tigr is (C. t. aethiops and C. t. seprentrionalis); and (3) interspecific hybrid ization between C. tigris and other whiptail lizards of either bisexual or unisexual (parthenogenetic, clonal) species. Cnemidophorus tigris is one of the ancestors of some of the parthenogens, which are of hybrid origin, and our interest in their evolutionary history fuels our efforts to improve un derstanding of hybridization among whiptail lizards.