Final answer:
The latitudinal gradient of skin color is called clinal variation, which refers to gradual changes in skin color across geographic regions.
Step-by-step explanation:
The latitudinal gradient of skin color is called clinal variation. Clinal variation refers to gradual changes in a particular trait, such as skin color, across geographic regions. Skin pigmentation is influenced by pigments like melanin, which is produced by melanocytes and transferred to keratinocytes in the epidermis.
The variation in skin color is believed to be an adaptation to different levels of ultraviolet (UV) radiation from the sun. Closer to the equator, where UV radiation is stronger, people tend to have darker skin to protect against the harmful effects of UV radiation. Conversely, in regions farther from the equator, where UV radiation is weaker, people tend to have lighter skin to allow for better absorption of vitamin D from sunlight.