An Evolutionary Quantitative Genetic Analysis of the Impact of Cephalopelvic Disproportion on Cranial and Pelvic Co-Evolution in Anthropoids

Objectives: Evolutionary quantitative genetics methods are increasingly applied to studies of human skeletal evolution, with a growing emphasis on investigating postcranial evolution and the evolution of multiple skeletal elements. Here, we apply a commonly used method from evolutionary quantitative genetics, the drift-rate test, to test whether broad patterns of cranial and pelvic co-evolution within male, female, and pooled-sex samples of anthropoid primates follow those expected under the long-standing hypotheses of obstetric selection via cephalopelvic disproportion. Materials and Methods: Using interlandmark distances from the cranium and articulated pelvis from samples of four platyrrhine, four cercopithecoid, and five hominoid primate genera, we tested cranial, pelvic, and craniopelvic traits for evidence of deviation from neutral evolutionary patterns using both regression tests of within- on between-group eigenvalues and correlation tests of principal component scores. Results: Results for analyses of shape data indicate that patterns of non-neutral evolution are different in male and female samples at multiple taxonomic levels, and that cranial and pelvic shape are co-evolving. Rejection of neutral evolution was pervasive for tests of form, but inconclusive regarding sex-specific selection or whether the cranium and pelvis appeared to covary or evolve independently. Discussion: Sex-specific patterns of evolution support hypotheses that obstetric selection may have impacted multiple primate lineages. Although size may play a role, it does not appear to be the dominant factor in evolution for either element. This study highlights the usefulness of using methods from evolutionary quantitative genetics to test long-standing hypotheses by incorporating multiple skeletal elements simultaneously.