The Evolution of the Primate Shoulder: Assessing Constraint and Evolvability

Observable differences among primate taxa closely align with different demands imposed by the wide range of environments they inhabit. This is reflected in the morphology of the primate shoulder girdle, which is the most variable among placental mammals. Previous research has demonstrated the morphology of the shoulder girdle reflects functional demands of the upper limb. Therefore, researchers hypothesize traits of the scapula are adaptations for specific functions related to locomotion, resource acquisition, or habitat; however, these hypotheses remain untested.In this dissertation, I use quantitative genetic approaches to assess the role of shared trait covariances on primate shoulder girdle morphology and directly test hypotheses of how this anatomical region evolved using phylogenetic comparative methods. Traits may evolve through shared responses in genetically, developmentally, or functionally covarying traits, via random genetic drift or responses to selection. In primates, the shoulder girdle shares developmental and functional relationships with traits of the basicranium, pelvic girdle, and humerus. Whether these relationships affect the evolution of the shoulder girdle, and ultimately primate morphological diversification, have not been explored. Using a broad sample of extant primate skeletal material, I estimate evolvability and conditioned covariances between the shoulder girdle, basicranium, pelvic girdle, and humerus, compare integration and evolutionary potential of these regions among primates, and directly test factors influencing shoulder girdle morphological variation. Results indicate the shoulder girdle does not evolve independently, rather its evolution is influenced by traits of the basicranium and pelvic girdle, and vice versa. Further, primate species have different patterns of covariance among these anatomical regions that affect their ability to respond to directional selection, and evolutionary covariance among these anatomical regions is not solely due to size-related variance, but also shape-related functional variance. However, no hypothesized single factor evolutionary model—locomotion, resource acquisition, or habitat—best explains what selection factors motivated evolution across all primates. These findings have implications for understanding how developmentally and functionally covariant anatomical regions may interact during evolution and their effect on morphological variation in primates and other vertebrates. Models of trait evolution should not be limited to single traits, anatomical regions, or single causes, without extreme caution.