Patterns of correlation between genetic ancestry and facial features suggest selection on females is driving differentiation. D. K. Liberton1, K. A. Matthes1, R. Pereira2, T. Frudakis3, D. A. Puts1, M. D. Shriver1 1) Department of Anthropology, Pennsylvania State University, University Park, PA; 2) Catholic University of Brazil, Brasilia, Brazil; 3) DNAPrint Genomics, Sarasota, FL.
Human facial features show extensive variation within and among populations. By investigating the relationship between dimorphism in facial features and genetic ancestry in different populations, we can explore the roles of sexual and natural selection on the human face. We measured sexual dimorphism in facial traits while controlling for the effects of overall size differences and then tested for interactions between sex and genetic ancestry. The study sample consists of 254 subjects (n=170 females, n=84 males), ages 18-35, showing West African and European genetic ancestry sampled in the United States and Brazil. Maximum likelihood genetic ancestry estimates were determined from 176 ancestry informative markers (AIMs), which allowed for the proportional estimation of genetic ancestry from four parental populations (West African, European, East Asian, and Native American). Three-dimensional photographs of faces were acquired using the 3dMDface imaging system (Atlanta, GA). 22 standard anthropometric landmarks were placed on each image and XYZ coordinates were collected. All 231 possible pairwise inter-landmark distances were calculated and then log transformed. Using the pairwise distances, we tested whether some distances were larger in one sex than the other, having taken size into account, in a) African Americans sampled in the United States, b) Brazilians sampled in Brazil, and c) the combined African American and Brazilian sample. We found that several pairwise distances differed between the sexes. For example, the distance from the brow to nasal bridge was found to be more than 5% larger in females than males. We then tested for an interaction between sex and genetic ancestry by testing for differences in the slopes of the ancestry association between males and females. Although the pattern differed slightly between samples, after Bonferroni correction many correlations were the found to be same in both sexes. However, females in all three samples had many additional significant correlations that were not seen in males, while males had very few correlations that were not found in females. The results of these analyses suggest that selection on females is driving the differentiation in facial features among populations.