Explain why there are a wide variety of phenotypes for skin color in humans.

There is a wide variety of phenotypes for skin color because there is a wide variety of environments with differing UV radiation for humans AND there is a wide variety of disease which effect humans that are linked to the level of melanism in the skin.

Darker skin has an increased amount of melanin in the epithelial cells, which prevents damage from UV radiation. Humans that evolved in environments with high amounts of UV radiation (i.e. the African continent, Australian aboriginal populations, and certain Asian and South American/Central American regions) were naturally selected for high melanin in their epithelial cells due to high levels of skin cancer in these locations for people with low amounts of melanin in their epithelial cells. High rates of skin disease led to high death rates, which pushed natural selection towards darker skin color.

At the same time, there is an alternative selection in areas with lower UV radiation (areas near the poles such as Scandanavia). These areas had low amounts of UV radiation, and therefore their problem was the lack of sunlight. Sunlight provides essential nutrients to the body (I believe it is Vitamin A or E, I can't remember - I think it's A) through absorption of photons via the skin. Darker skin with high levels of melanin end up with a lack of Vitamin A (E?) which can lead to a disability called rickets - a bone disorder. Humans with this disorder were selected against as it may lead to death and loss of genetic prowess in the population. Therefore, less melanin in the skin was selected for in areas with less UV radiation.