Covariance matrices supporting: An evolutionary quantitative genetic analysis of the impact of cephalopelvic disproportion on cranial and pelvic co-evolution in anthropoids

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. 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
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. Sex-specific patterns of evolution support
hypotheses that obstetric selection may have impacted multiple primate
lineages. While 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.