Homoplasy and the evolution of ontogeny in papionin primates.

Recent advances in developmental biology reveal that patterns of morphological development, even during early phases, may be highly susceptible to evolutionary change. Consequently, developmental data may be uninformative with regard to distinguishing homology and homoplasy. The present analysis evaluates postnatal ontogeny in papionin primates to test hypotheses about homology and homoplasy during later periods of development. Specifically, the analysis studies the allometric bases of craniometric resemblances among four papionin genera to test the hypothesis that homoplasy in adult cranial form, particularly of baboons (Papio) and mandrills (Mandrillus), is underwritten by divergent patterns of development. Bivariate and multivariate allometric analyses demonstrate that the developmental patterns in Papio baboons diverge markedly from ontogenetic allometric trajectories in other papionin species. The resemblances between Papio and Mandrillus (assuming that patterns of development in smaller papionins are ancestral) are largely consequences of perinatal increases in relative brain size in juvenile Papio. Postnatal growth to large size and strong negative allometry of neurocranial form results in shape similarities because developmental pathways for large papionin genera intersect. Analyses show that allometric data may not be particularly informative in revealing homoplasy. However, placed into proper phylogenetic context, such data illustrate derived patterns of development that may reflect critically important life-history or ontogenetic adaptations.