48 - Morphological Integration and Evolvability of the Human Hyoid, Vertebrae, and Cranium

Poster Board Number: 48
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Abstract Body : While the human hyoid does not directly articulate with other skeletal elements, it is invested within its surrounding musculoskeletal tissues with which it shares mechanical function and development. While the role of the hyoid’s morphology and relative position have been explored with respect to the function of hyolingual and hyolaryngeal tissues, how these adjacent tissues may affect the evolution of the hyoid remains unknown. As the hyoid and basicranium share developmental origins, and the hyoid forms muscular and cartilaginous attachments to surrounding bony anatomy, we hypothesize that it cannot evolve independent of these adjacent anatomical regions.
In our study we examine morphological integration, which arises from underlying genetic covariation, and the ability of morphological traits to respond to directional selection (evolvability). As potentially integrated structures, the vertebrae and basicranium, as well as other adjacent skeletal tissues, have the capacity to impose evolutionary constraint on the hyoid. Our study of the evolutionary relatedness among these elements builds on decades of research that show that selection on one trait may yield correlated responses in other traits. Among recent such studies, researchers have described evolutionary constraints between traits based on shared development and function (e.g., the shoulder girdle and basicranium). Following this precedent, this study further investigates skeletal traits that are hypothesized to developmentally and functionally relate to assess if such connectivity manifests as evolutionary integrated traits.
We placed landmarks on the basicranium, cervical vertebrae, and hyoids from a sample of skeletal remains of 59 donated humans. From these data we calculated five interlandmark distances each for the basicranium, C5 vertebra, and hyoid. Using a custom script in R, we accounted for body size differences between the sexes using MANOVA residuals and then body size-scaled the data using eigenvectors. From the resulting covariance matrices, we calculated evolvability, respondability, and conditional covariance following the equations of Hansen and Houle (2008) using 1000 random selection gradients between zero and one.
Results indicate that the basicranium and the hyoid are potentially more genetically covariant than either is with C5; the hyoid and basicranium are less evolvable than either is with C5. Conditional covariance further shows that the basicranium imposes more constraint on the hyoid and on C5 than these bones impose on each other, and the hyoid imposes more constraint on the ability of C5 to respond to selection than the reverse relationship. Together with the evolvability estimates, this indicates that the hyoid and basicranium are less evolvable than either is with C5, and that the basicranium and hyoid both impose potential evolutionary constraint on the cervical vertebra. This is expected given developmental relationships among these elements, where the basicranium and hyoid share some similar developmental origins, while the cervical vertebra—although functionally related to both the hyoid and basicranium—is largely developmentally independent. Future investigation will incorporate other adjacent skeletal elements and a larger sample size.