What mode of natural selection does skin color fall under?

Directional / Diversifying / Stabilizing ?





Also, what are the selection pressures associated with changing skin color?What mode of natural selection does skin color fall under?
In humans there is arguably many different kinds of selection going on, depending on specifically which facet of skin colour you're looking at.





For example, production of ANY amount of melanin is arguably undergoing stabilizing selection. People who are albinoes and cannot produce melanin usually have a pronounced disadvantage in any environment, so production is 'stabilized'. The same might be said for the ability to vary melanin production with sun exposure to account for seasonal variation and the like - those who can't do this have more problems (though a lot less than an albino).





In certain ecologies, the selection seems to be directional. When the sun beats down constantly, it is an advantage to always be producing lots of melanin... and the more you produce the more of an advantage it is (which is a contrast with stabilizing selection). And vice versa in regions where there is little sunlight, as it may interfere with other skin functions. This is a small pressure compared to the above one, though, because many people with the 'wrong' skin colour do just fine in those environments (technology certainly lowers the impact of it!).





At no point can I think of diversifying selection to be important in human skin colour. There is no reason why it would be an advantage to have different colours, nor is there a location where people naturally have many, many colours unless they just happened to inherit them.What mode of natural selection does skin color fall under?
Since there are multiple skin colors, I would guess the mode of selection to be Diversifying.





One suspected pressure involves the amount of ultraviolet light exposure the population receives.and the need to synthesize vitamin D.