EVOLUTIONARY DIVERGENCE, SHAPE CHANGE, AND GENETIC CORRELATION STRUCTURE IN THE RODENT MANDIBLE

Divergence in a set of area traits describing morphological form of the mandible in rats and mice was examined by finite element scaling analysis (FESA) and conventional statistical analyses. The area traits correspond to the skeletal derivatives of mesenchymal condensations involved in the production of the rodent mandible. Three major questions are addressed. (1) Can differences in the form of the adult rodent mandible be explained by changes in specific morphogenetic components? To answer this question, four specific hypotheses that might account for differences between mice and rats in morphological form of the mandible were evaluated: isometric size-scaling, mesenchymal condensation differentiation, muscle hypertrophy, and differential tooth growth. The results support all hypotheses except differential tooth growth, suggesting a complex and multidimensional differentiation in mandibular form between these taxa. The FESA and statistical analyses give qualitatively the same results for all of the hypotheses tested. (2) Has morphological change in mandible form been accompanied by significant changes in the genetic correlation structure? Genetic correlation patterns among traits are different between these species, and these differences persist after the genetic correlation between mandible dimensions and body weight is accounted for. This result differs from those of previous reports on pelvic dimensions in these animals, where no significant change in genetic correlation structures was found, suggesting mosaic evolution of the skeletal system in these taxa. (3) Does using different extrinsic and intrinsic covariates to scale animals to the same size produce equivalent effects on the covariance structure of the traits? Results show that different extrinsic and intrinsic covariates in analysis of covariance often produce very different results.