Final answer:
The clinal distribution of lighter skin pigmentation is believed to be a selective adaptation for improved vitamin D synthesis in regions with less intense sunlight, particularly farther from the equator and in areas with longer winters.
Step-by-step explanation:
The selective factor proposed as an explanation for the clinal distribution of lighter skin pigmentation is the need for adequate vitamin D synthesis in regions with less ultraviolet radiation. In areas where sunlight is less intense, such as regions farther from the equator or with longer winters, lighter skin allows for more efficient production of vitamin D. Conversely, in tropical areas, darker skin provides protection against the potential for excessive vitamin D synthesis and defends the skin cells from damaging ultraviolet radiation. Physical geography and distance from the equator have significantly influenced human evolution, resulting in adaptations such as skin pigmentation variation. Cultural factors and sexual selection have also played roles in the evolution of physical characteristics, but the primary selective pressure for skin color variation is the level of exposure to ultraviolet rays and the corresponding need for vitamin D.