Selection Gradients and Ecogeographic Variance in the Human Post‐Crania

Abstract

Recent work by Roseman and Auerbach (in press) indicates that the ecogeographic distribution of human body proportions is driven by a combination of neutral evolutionary forces and natural selection. This contrasts with assumptions that natural selection produces morphological variance along ecogeographic clines (i.e., Bergmann's and Allen's “rules”). Roseman and Auerbach's approach, however, does not distinguish between direct and indirect responses to the natural selection acting on these morphologies. This study uses retrospectively estimated selection gradients for limb lengths, femoral head size, and body breadth to assess the nature of selective forces on ecogeographic variance in human morphology.We estimated the vectors of selection gradients required to evolve one group into another for pairs of populations across major climate regions, and used parametric bootstraps to determine 95% confidence intervals. Equatorial African populations are assumed to have been drifting atop a plateau on the adaptive landscape, reflecting a tropically‐adapted human state. Results indicate that strong selection on distal limb lengths, femoral head size, and body breadth effected an evolutionary transition to morphologies found in arctic populations. The patterns of selection necessary to effect a transition to proportions found in more temperate regions (i.e. North Africa and Europe) were qualitatively similar but much less strong. These results in part support the findings of Roseman and Auerbach, but suggest that the action of natural selection may be entangled with random genetic drift, gene flow, and neutral mutation.