White People

The main hypotheses which attempt to account for white skin suggest it is an adaptation to inadequate ultraviolet radiation. As humans moved out of the tropics, a conspicuous latitude gradient of skin tones follows the out of Africa dispersion, it is argued natural selection for sufficient ultraviolet penetration to enable vitamin D production gave rise to the evolution of skin pigmentation by the mechanism of evolution by natural selection. Deleterious health effects of insufficient vitamin D are also pointed to as confirmation that skin lightening was in response to strong selection pressure for maximizing vitamin D.[34][35][36] A variation of the vitamin D argument is that humans lived in Europe for several thousand years without their skin lightening and that it only became white after they adopted agriculture.[34][37] It is suggested that in Europe the latitude permitted enough synthesis of vitamin D combined with hunting for health, only when agriculture was adopted was there a need for lighter skin to maximize the synthesis of vitamin D, therefore it is suggested the elimination of game meat, fish, and some plants from the diet resulted in skin turning white several thousand years after modern human settlement in Europe.[38][39]
Skin color is a quantitative trait that varies continuously on a gradient from dark to light, as it is a polygenic trait, under the influence of several genes. KITLG[40] and ASIP[41] have been found responsible for skin colour variation between sub-Saharan Africans and non-African populations. SLC45A2,[42] TYR[43] and SLC24A5[44] have been positively shown to account for a substantial fraction of the difference in melanin units between Europeans and Africans, while DCT,[45] MC1R[46] and ATRN[42] have been statistically indicated as possible sources for skin tone differences in East Asian populations.
A 2006 study concluded that light pigmentation in European and Asian populations evolved independently of each other via different genetic mechanisms, meaning that light skin probably arose some time after the initial migration out of Africa after the separation of the M and N mtDNA haplogroups.[47] The mutation resulting in the light skin version of the SLC24A5 gene has been estimated to have originated in Europe between 6,000 and 12,000 years ago, indicating that at least one of the genes responsible for pale skin colour in Europeans arose relatively recently.[48]

(http://en.wikipedia.org/wiki/White_people)