Exploring Ecogeography, Drift, and Selection on the Macaque Cranium

The human cranium is one of the most studied osteological complexes among primates due to its size, good preservation in the paleontological record, and its wide morphological diversity. As H. sapiens is the last extant member of its genus, we must turn to related genera to answer questions regarding variation among species and the evolutionary forces that produce it. This pilot study uses morphometric data collected from skeletons of three species of macaques ( M. fascicularis , M. nemestrina , M. radiata ) housed at UC Davis. Following the work of Lande, Ackermann and Cheverud the data are used to 1) examine the phenotypic variation present in the cranium of each species by cranial region, 2) to test for deviation from a null model of neutral microevolution (i.e. genetic drift), and 3) to quantify the selection gradients for each trait by species. For the analysis, 15 linear distances were extracted from a series of 16 ectocranial landmarks that were collected using MicroScribe 3DX digitizer. In order to analyze how variance was apportioned within the cranium, a series of PCAs were performed 1) on all measurements combined and 2) by cranial module (basicranium, neurocranium and splanchnocranium). The selection analysis was performed using a linear regression incorporating both between‐species and within‐species variance/covariance matrices following the method outlined in work by Ackermann and Cheverud. Then, using the between‐species phenotypic variance/covariance matrix, pairwise selection gradients were calculated for each macaque species. To assess whether directional selection in these traits was different from zero, a sample distribution of selection gradient values was generated by bootstrapping values for the within‐species phenotypic variance/covariance matrix as well as the between‐species variance/covariance matrix at 1000 iterations each, from which 95% confidence intervals were generated. The results from this study indicate that phenotypic variance in the Macaca cranium has been driven largely by selection since the divergence of the genus from the last common ancestor (LCA) of Papionini some 9–10 million years ago. This preliminary analysis indicates that all three groups have been subject to strong selection in the upper and lower face (including the maxilla). Increases in body size drove a higher degree of flexion of the cranial base in facscicularis and radiata . It is unclear if selection is directly acting on body size, in turn affecting facial morphology, or on aspects of craniofacial variation. In nemestrina , the driver of variation is still unclear, although hybridization is suspected at present. In all cases, a departure from the ancestral pattern of trait covariation is plausible, although how and which traits covary requires further analysis. These results warrant further investigation via process modeling in order to rule out other complicating factors such as hybridization and heterochrony.