Apparently skin colour evolution is linked to folate and vitamin D. The most amusing part of this excellent article is that people with dark skin in higher latitudes can develop rickets if they don't get enough sun. I didn't know that people actually got that disease anymore since the fortification of milk. I also thought that vitamin D deficiencies were related to seasonal affective disorder (SAD), but I haven't found anything about darker skinned individuals being more prone to this disorder. In fact they seem to be just as prone as the general population. strange.
Related to the podcast discussion (part III of episode 6 yet to come out) on continuing human evolution I wonder if there is any selective pressures on skin colour due to vitamin D and/or folate in any parts in the world today.