Abstract
Global variation in skin pigmentation is one of the most striking examples of environmental adaptation in humans. More than two hundred loci have been identified as candidate genes in model organisms and a few tens of these have been found to be significantly associated with human skin pigmentation in genome-wide association studies. However, the evolutionary history of different pigmentation genes is rather complex: some loci have been subjected to strong positive selection, while others evolved under the relaxation of functional constraints in low UV environment. Here we report the results of a global study of the human tyrosinase gene, which is one of the key enzymes in melanin production, to assess the role of its variation in the evolution of skin pigmentation differences among human populations. We observe a higher rate of non-synonymous polymorphisms in the European sample consistent with the relaxation of selective constraints. A similar pattern was previously observed in the MC1R gene and concurs with UV radiation-driven model of skin color evolution by which mutations leading to lower melanin levels and decreased photoprotection are subject to purifying selection at low latitudes while being tolerated or even favored at higher latitudes because they facilitate UV-dependent vitamin D production. Our coalescent date estimates suggest that the non-synonymous variants, which are frequent in Europe and North Africa, are recent and have emerged after the separation of East and West Eurasian populations.
Highlights
Modern humans are genetically less diverse than other living hominoids
The geographical pattern of variation in human skin pigmentation correlates with the latitudinal differences in annual UV radiation (UVR) level [2]
The most well established explanation assumes that the optimal degree of skin pigmentation is a balance between skin dark enough to protect our cells from UV radiation, yet light enough to permit sufficient vitamin D production
Summary
Modern humans are genetically less diverse than other living hominoids. Most of human genetic diversity is found within rather than among populations. The degree of among population variation of this trait has been estimated as 88%, which is high compared to roughly 10-15% observed for genetic loci on average Such high phenotypic differentiation is understood to be due to the effect of natural selection [1]. Several evolutionary mechanisms that are dependent on UV level have been proposed to explain the variation in human skin pigmentation. These include the vitamin D [4], protection from UV-induced folate photolysis [5], sexual selection [6], skin cancer [3] and xeric stress hypothesis [7]. Increased concentration of protective eumelanin is essential to avoid damaging sunburns and extensive folate photolysis in (near-) equatorial areas, while it could be harmful in regions with low annual UV level due to reduced UV-dependent vitamin D synthesis [3,8]
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