Comparing Hierarchical Patterns of Constraint and Functional Trait Complexes in the Primate Shoulder and Hip Regions Among New World and Old‐World Monkeys

Abstract

The scapula shares developmental and functional relationships with the basicranium, clavicle, vertebral column, and humerus. Its characteristics are also analogous with the pelvis as a limb girdle. Despite these relationships, in studies of primate shoulder evolution, the scapula is often examined in isolation. However, genetic covariances underlie functional and developmental relationships that result in non‐independent evolution of the scapula from these other skeletal elements. To understand evolution in the scapula, then, accurate models of trait responses to selection must account for the constraints imposed by these covariances.In this study, we work from the position that genetic covariances among morphological traits of the shoulder girdle and basicranium reflect the developmental and functional relationships between them. We use this premise to examine if the genetic covariances constrain the ability of these elements to respond to selection. Due to the developmental and functional parallels among limb girdles, pelvic dimensions are included in our analysis. We obtained cranial and postcranial linear dimensions from a sample of 105 adult common marmosets (Callithrix jacchus) and 66 adult macaques (Macaca mulatta). These measurements are associated with the basicranium, the shoulder, and the pelvis. Measurements were mean‐standardized prior to analysis. We calculated the evolvability and conditional evolvability of these traits in R following the equations of Hansen and Houle; these indices were estimated using 1000 simulated random selection gradients. Greater constraint among traits is indicated by lower evolvability. Lower conditional evolvability reflects constraint on a trait when all other traits' means did not change under selection. Analyses were performed on the following configuration of traits: shoulder girdle and basicranium, shoulder and pelvic girdles, and basicranium and pelvic girdle. Differences in evolvability measures were evaluated by recomputing each value for 1000 iterations of the bootstrapped sample, and performing a pairwise comparison of the bootstrapped values.Our results show that evolvability indices, especially conditional evolvability, are lower among traits of the shoulder girdle and basicranium when compared to those of the shoulder girdle and pelvis, and the basicranium and pelvis. This indicates a tiered pattern of constraint among anatomical regions, with a privileged relationship existing between the basicranium and shoulder girdle. These results are consistent for both macaques and marmosets, suggesting a deep phylogenetic origin among primates for genetic covariances among these anatomical regions. We conclude that the basicranium and shoulder girdle constrain each other in response to directional selection, forming a functional trait complex, and 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).This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.