The Evolution of Morphological Integration in the Ruminant Skull

Patterns of morphological integration have the potential to influence evolutionary trajectories. However, this influence depends on the stability of the integration pattern relative to the rate of evolutionary change. Studying the evolution of integration over large phylogenetic scales is complicated by its multivariate nature and the need to have both large sample sizes and a comprehensive taxon coverage. As a result, the question of how integration evolves over long time scales is still poorly understood. In this study I examined the evolution of integration across the phylogeny of extant ruminants, as reflected in their within-population covariation structure. I analyzed interlandmark distances from 2,054 skulls, representing 47 out of the 200 extant species of ruminants, including all major subfamilies of bovids and cervids. I estimated the within-population covariance matrix for each species, and compared them using multidimensional scaling and phylogenetic comparative methods. Results show that closely-related taxa differ substantially from each other in their integration pattern. However, the differences among higher-level clades still reflect their history of common descent. Differences between bovids and cervids involve mainly the oral and nasal regions, in accord with their different feeding and locomotion adaptations. Thus, the effect of both natural selection and phylogenetic history can be detected in the ruminant skull even though integration varies considerably among closely-related species.