Nina Jablonski has been interested in evolution of human skin and skin color for a while and have wrote some interesting articles on this topic. This is one of her recent articles and in the article, Jablonski and Yuen review recent medical literatures showing association between hypovitamin D and health status. They use these lines of evidence to suggest that lighter skin color is selected outside of Africa to increase the production of vitamin D.
Followings are diseases and other health issues that are associated with low Vitamin D level;
Bone metabolism and factures
Dental health problems
These potentially reduce reproductive success of individuals with low vitamin D level. People with dark skin living in the area without enough daily sunlight in high latitude usually have hypovitamin D and they have higher risk of developing these diseases and health issues. Some of these conditions decrease the chance of successfully having children, while others decrease the chance of surviving until reproductive age or increase the chance of death before the end of reproductive age. Also, longevity is important for reproductive success of individuals, since in many societies, the elderly individuals usually have a role taking care of young children.
However, this article does not explain the mechanism of natural selection. For example, which genes were involved to produce light skin? Which genetic variants were selected? How about genes involved in vitamin D pathway? Human geneticists began to understand the genes and genetic variants for skin pigmentations, but I have found only a few literatures that address the evolutions of genes in vitamin D metabolism, or vitamin D pathway, and it is poorly understood. Some genetic variants that are associated with vitamin D level have been found, but only a few of them have been conformed in multiple studies. Many studies are conducted with White or Europeans populations, but African Americans or other minority groups are rarely included.
If the efficiency of vitamin D metabolism and utilization is important in Europe or other areas in higher latitude, genetic variants that increase the efficiency were positively selected, and genetic variants that decrease the efficiency were negatively selected, so there should be little variation on the vitamin D pathway genes among the Europeans and other populations from higher latitude. On the other hand, natural selection should not have acted on or should be relaxed in the populations from the areas with enough daily sunlight or populations, in which people consume vitamin D rich food. In these populations, more variation on the vitamin D pathway genes should be observed and some of mutations may reduce the efficiency.