Skin color is historically the locally adaptive trait most commonly considered by European cultures as a “racial trait” in humans. Skin color is an adaptation to the amount of ultraviolet (uv) radiation in the environment: dark skins are adaptive in high uv environments in order to protect from radiation damage that can kill and burn cells and damage DNA if not protected by melanin, and light skins are adaptive in low uv environments in order to make sufficient vitamin D, which requires uv (Hochberg & Templeton, 2010; Jablonski & Chaplin, 2010). The geographical distribution of skin color follows the environmental factor of uv intensity. Skin color differences do not reflect overall genetic divergence. For example, the native peoples with the darkest skins live in tropical Africa and Melanesia. The dark skins of Africans and Melanesians are adaptive to the high uv found in these areas. Because Africans and Melanesians live on opposite sides of the world, they are more highly genetically differentiated than many other human populations (Figure 2) despite their similar skin colors. Europeans, who are geographically intermediate between Africa and Melanesia, are likewise intermediate at the molecular genetic level between Africans and Melanesians, even though Europeans have light skins that are adapted to the low uv environment of Europe. Skin color differences in humans are not a reliable indicator of overall genetic differentiation or evolutionary history. Moreover, skin color varies continuously among humans in a clinal fashion rather than categorical ecotypes (Relethford, 2009). Hence, there is a compelling biological reason to exclude skin color as the racially-defining adaptive trait under the ecotype concept of race.
Alan R. Templeton, “Biological races in humans,” Studies in History and Philosophy of Science Part C: Studies in History and Philosophy of Biological and Biomedical Sciences, Volume 44, Issue 3, September 2013. 262–271. http://dx.doi.org/10.1016/j.shpsc.2013.04.010.