Morphological Variability in Genetically Defined Categories of Anuran Hybrids.

Hybridization phenomena in anurans have traditionally been studied through morphological comparisons, under the assumption that various hybrids (e.g., F1 's, backcrosses) are predictably intermediate to parental species. We critically evaluate this assumption by examining morphology in genetically categorized hybrids between the treefrogs Hyla cinerea and H. gratiosa. A total of 202 frogs from a hybridizing population in Alabama were assayed for allozyme and mitochondrial DNA genotype and for a large suite of osteological characters. Discriminant analyses demonstrated distinct morphological separation between the genetically "pure" parental species. Morphometric analyses of genetically identified hybrids showed: 1) an extreme range of phenotypic expression within F1 and backcross classes, and 2) no apparent directional parental bias on the F1 phenotype. Had morphology alone been used as a guide, over 40 percent of the individuals with known hybrid ancestry would have been misclassified as "pure" parental species, and about 25 percent of the backcross individuals would not have been distinguished from F1 's. These results exemplify the utility of joint comparisons of morphology and genotypic constitution in studies of natural hybridization, and they emphasize the limitations inherent in describing hybrid classes solely by morphological criteria.