Human Pigmentation Variation: Evolution, Genetic Basis, and Implications for Public Health
Esteban J. Parra- Department of Anthropology, University of Toronto at Mississauga
Pigmentation, which is primarily determined by the amount, the type, and the distribution of melanin, shows a remarkable diversity in human populations, and in this sense, it is an atypical trait. Numerous genetic studies have indicated that the average proportion of genetic variation due to differences among major continental groups is just 10–15% of the total genetic variation. In contrast, skin pigmentation shows large differences among continental populations. The reasons for this discrepancy can be traced back primarily to the strong influence of natural selection, which has shaped the distribution of pigmentation according to a latitudinal gradient. Research during the last 5 years has substantially increased our understanding of the genes involved in normal pigmentation variation in human populations. At least six genes have been identified using genotype/phenotype association studies and/or direct functional assays, and there is evidence indicating that several additional genes may be playing a role in skin, hair, and iris pigmentation. The information that is emerging from recent studies points to a complex picture where positive selection has been acting at different genomic locations, and for some genes only in certain population groups. Diffuse reflectance spectroscopy (DRS) is considered the most versatile and accurate noninvasive strategy to measure skin pigmentation (Stamatas et al., 2004).
Skin reflectance curves for individuals of European, East Asian, South Asian, and African ancestry. Skin reflectance was measured on the inner upper arm at 0.5-nm intervals along the visible spectrum (400–700 nm) using a Stellarnet EPP2000 (now BLACK-Comet) reflectance spectrometer. Note the decrease in skin reflectance in the green yellow wavelengths (540–580 nm) due to hemoglobin absorption, which is particularly evident in the curves of the individuals with the highest skin reflectance.
