Integration, Evolvability, and Constraint within the Primate Functional Shoulder Complex

In vertebrates, especially primates, the morphology of the scapula is closely associated with locomotion, and individual traits of the shoulder are hypothesized to be under directional selection to meet these behavioral demands. By ascribing individual traits to variation in locomotor behaviors, however, researchers implicitly assume traits evolve independently. Traits that share mechanical function may covary genetically, and could therefore evolve through correlated responses to selection. The scapula, for example, is part of a complex of functionally related traits, linked both developmentally and mechanically, which includes dimensions of the clavicle, humerus, and basicranium. This study investigates whether the functional relationships among these skeletal regions relate to evolutionary covariance by comparing patterns of integration and evolvability among homologous traits in humans and tamarins. Three‐dimensional landmarks were taken from the scapula, humerus, clavicle and basicranium of 109 humans ( Homo sapiens ) and 100 tamarins ( Saguinus sp. ). Twelve interlandmark distances were calculated from these landmarks for analysis, representing a complex of traits relating to mechanical function from both the basicranium and the shoulder girdle. Analyses aimed at examining evolutionary covariance were applied using both raw and data scaled by the geometric mean. The strength of integration was quantified using the analysis of eigenvalues and a measure of trait variational autonomy. Evolvability was estimated by applying 1000 simulated random selection gradients to the phenotypic variance/covariance matrices and comparing evolutionary responses among taxa. Statistical significance of differences among taxa was determined by recomputing each value of integration and evolvability for 1000 iterations of the bootstrapped sample. A t‐test was used for pairwise comparisons of the bootstrapped values among taxa (α = 0.05). Results indicate when traits from the basicranium and shoulder girdle are evaluated as a complex, the ability of these regions to respond to directional selection is decreased compared to their responses were they independent. Evolutionary independence among these traits is not supported, and analyses instead indicate evolutionary constraint among these regions. Morphological differences in the nuchal region and shoulder complex between humans and tamarins further reflect differences in the ability to respond to directional selection in these two genera, which may have had functional implications for evolution of the shoulder in apes and monkeys. Namely, the traits in humans are more tightly integrated, resulting in weaker evolvability. This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal .