The subsequent spread of light skin was thought to be caused by assortive mating[2] and sexual selection contributed to an even lighter pigmentation in females.[33][34] Doubt has been cast on this hypothesis, as a more random patterns of skin coloration would be expected in contrast to the observed structural light skin pigmentation in areas of low UV radiation.[5] The clinal (gradual) distribution of skin pigmentation observable in the Eastern hemisphere, and to a lesser extent in the Western hemisphere is one of the most significant characteristic of human skin pigmentation. Increasingly lighter skinned populations are distributed across areas with incrementally lower levels of UV radiation.[35][36]