Influence of evolutionary allometry on rates of morphological evolution and disparity in strictly subterranean moles (Talpinae, Talpidae, Eulipotyphla, Mammalia)

The adaptation to a particular function could directly influence the morphological evolution of an anatomical structure as well as its rates. The humeral morphology of moles (subfamily Talpinae) is highly modified in response to intense burrowing and fully fossorial lifestyle. However, little is known of the evolutionary pathways that marked its diversification in the two highly fossorial moles tribes Talpini and Scalopini. We used two-dimensional landmark-based geometric morphometrics and comparative methods to understand which factors influenced the rates and patterns of the morphological evolution of the humerus in 53 extant and extinct species of the Talpini (22 extant plus 12 extinct) and Scalopini (six extant plus 13 extinct) tribes, for a total of 623 humeri. We first built a synthetic phylogeny of extinct and extant taxa of the subfamily Talpinae based on all the available information from known phylogenies, molecular data, and age ranges of fossil records. We tested for evolutionary allometry by means of multivariate regression of shape on size variables. Evolutionary allometric trajectories exhibited convergence of humeral shape between the two tribes, even when controlling for phylogeny, though a significant differences in the evolutionary rates was found between the two tribes. Talpini, unlike Scalopini, seem to have reached a robust fossorial morphology early during their evolution, and their shape disparity did not change, if it did not decrease, through time. Furthermore, the basal Geotrypus spp. clearly set apart from the other highly fossorial moles, exhibiting a significant acceleration of evolutionary shifts toward higher degree of fossorial adaptation. Our observations support the hypothesis that the evolution of allometry may reflect a biological demand (in this case functional) that constrains the rates of evolution of anatomical structures.