What is the selective pressure for darker skin color for lighter skin color?

UV radiation from the sun

Explanation:

Dark skin has more melanin production than light skin. Melanin is a pigment that protects skin from UV-radiation, which can damage cell nuclei and cause skin cancer. It was reported that darker skin color individuals have lesser risks of cancer than lighter skin color individuals. In geographic zones with high UV-radiation levels the individual adquired a more pigmented skin, so the selective pressure was the level of UV radiation in solar radiation.