Final answer:
Populations in South America have lighter skin due to natural selection and the need to compensate for less ultraviolet radiation exposure.
Step-by-step explanation:
Populations in South America close to the equator have lighter skin due to natural selection. As humans migrated to the Northern Hemisphere, they were exposed to less ultraviolet radiation, which meant less absorption of Vitamin D needed for strong bones and immune functions. To compensate for this, skin pigmentation became lighter.