Evolutionary Potential and Morphological Integration of the Shoulder Girdle Among Primates

Abstract

The morphology of the shoulder girdle is highly varied among primates, possibly reflecting an array of functional demands placed through the range of locomotor and behavioral movements exercised in the upper limb. Variation in the shoulder girdle, and especially the scapula, is hypothesized to be the result of directional selection for different functional demands. Evolutionary analyses of the shoulder girdle often assess this anatomical region and its traits individually, which implicitly models the evolution of the girdle as independent of other traits. However, the primate shoulder girdle has developmental and functional covariances with the basicranium, pelvis, humerus, and vertebral column that we have recently demonstrated have potential to influence its evolution through correlated responses to directional selection. Whether these relationships are similar across primate taxa and how these relationships may affect morphological variation among primates is unknown.Here we evaluate the strength of the covariance and evolutionary potential among four anatomical regions: shoulder girdle, basicranium, pelvis, and humerus. We obtained cranial and postcranial linear dimensions from Alouatta, Colobus, Homo, Hylobates, and Papio. Measurements were mean standardized prior to analysis. We estimated morphological integration using Wagner's variance of the eigenvalues (VE), and estimated evolutionary potential using Hansen and Houle's evolutionary flexibility (EF) with 1000 simulated random selection gradients. EF captures how closely the evolutionary response in a group of traits follows the direction of the selection. We evaluated differences in VE and EF among taxa by performing a pairwise comparison of the bootstrapped values for each. Analyses were performed using the R statistical environment.Our results show that patterns of morphological integration and evolutionary flexibility among anatomical regions differ across primate taxa, with humans having the greatest evolutionary potential and Papio the lowest. Broadly, the patterns revealed among taxa do not strictly correspond to the functional demands of the upper limb experienced by each; taxa with different locomotor demands on the upper limb have the same kinds of covariance or evolutionary flexibility. Thus, evolutionary hypotheses of primate shoulder girdle morphological variation that evaluate functional demands alone may not account for the effect of these relationships. We conclude that different trait covariances among anatomical regions may manifest as morphological differences in the primate shoulder girdle, which has important implications for models of shoulder evolution.Support or Funding InformationThis research is supported by a National Science Foundation Doctoral Dissertation Improvement Grant (NSF BCS‐1825995).