Cascading Chromosomal Speciation in Lizards: A Second Look

The extent of Robertsonian chromosomal variation in the iguanid lizard Sceloporus grammicus of Mexico is exceptional among lower vertebrates, and this case has been the basis for the cascading chromosomal speciation hypothesis. This paper examines some of the population genetic assumptions of this model by comparing allozyme variability within and among 13 samples of S. grammicus with an equal number of samples of the chromosomally monotypic congener S. graciosus. Only homologous enzyme loci resolved in both species are used in the comparison. Estimates of such parameters as mean levels of heterozygosity, average number of alleles per locus, genetic distances, and F statistics are generally inconsistent with assumptions of strong population subdivision and/or recent bottlenecks associated with extinction-colonization events in S. grammicus.. We tentatively conclude that the population structure of at least some chromosome races in this complex is sufficiently panmictic to retard the fixation of electromorphic variants. Problems of making inferences about speciation mechanisms from population genetic correlates are discussed