People who live in the tropics are exposed to more of the Sun's harmful UV radiation. As a result, their skin tends to become darker as the body produces more melanin to counteract the effects of the Sun's rays. Over successive generations, genes are passed down from parents to their children, including the tendency to produce a certain amount of melanin given the area where they live.
Likewise, northern peoples tend to have lighter skin colors, because they don't receive as many of the Sun's harmful UV rays. As a result, their bodies do not need to produce as much melanin, which makes their skin color lighter.
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