Final answer:
Darker skin in tropical regions is an evolutionary adaptation to protect against high UV radiation, with melanin dispersing UV rays and preventing excessive vitamin D synthesis.
Step-by-step explanation:
People indigenous to tropical regions tend to have darker skin because of an evolutionary adaptation that serves to protect against the high levels of ultraviolet (UV) radiation found near the equator. Dark skin contains more melanin, which provides protection by absorbing and dispersing UV radiation. This adaptation served to protect skin cells from damage and also to prevent the synthesis of excessive amounts of vitamin D, which could be harmful.
This evolutionary process, known as directional selection, indicates that in sun-rich environments, dark skin was beneficial and therefore became more prevalent. Conversely, as humans migrated away from the equator, they encountered less UV radiation, and lighter skin evolved to optimize UV absorption for vitamin D production. The natural variation in skin pigmentation reflects thousands of years of human adaptation to differing UV radiation levels around the world.