Modern human phenotypic variation: Exploring patterns of differentiation within and between continents

Abstract

Current consensus is that modern human cranial phenotypic variation is a result of isolation-by-distance and neutral evolutionary processes, with natural selection acting most no tably in some anatomical regions of populations living in extreme environmental conditions. Under this model, there is an underlying assumption that the rate of morphological differentiation in the past was uniform across the planet. The goal of this study is to test this assumption by comparing the morphological differentiation of human groups between and within geographic regions. We analyzed a large sample of male individuals from 135 human series. Craniometric variation was assessed through Fst estimates calculated from 33 linear measurements for each pair of series. Series were grouped into 15 geographic regions and the average Fst values within regions were then compared with the average linear geographic distances between series. Our results show a very strong linear correlation between average Fst values and geographic distances within each geographic region. However, between-regions analyses generally show lower correlations with distances, suggesting that geographic distance is not a good predictor for the Fst values between continents. These preliminary results indicate that the processes associated with the morphological differentiation within and between continents may have differed considerably, and probably a more significant influence of bottlenecks and natural selection. With the exception of the differences between North America and the Old World, isolation by distance may not be a good predictor of morphological differentiation between groups located in different continents. Alternative models should be taken into account to explain this pattern.