For example, people in southern China have very slim body types with more pronounced eyelids. There is a greater likelihood of epicanthic eye folds, whereas as one examines northern Asians, Koreans and Mongolians, there are more pronounced Mongoloid features with stockier body types. The Mongoloid eye fold which is a single fatty layer is more common among these northern populations. One of the reasons why this may occur is the difference in climate. When ancient populations migrated more northerly, the law of natural selection may have allowed the development of Mongoloid features as a protection against the harsher climate.
Humans can also adapt to different substances in foods.
The complex sugar in milk, lactose, is a disaccharide that needs to be broken down by lactase in order for absorption to take place. Lactose is poorly absorbed by the adults of most populations because lactase is only found in juvenile mammals. In non- Western cultures, people are not heavily dairy oriented, thus their intestinal lactase levels diminish due to the absence of that gene. This is the condition known as being lactose intolerant. When this particular individual consumes dairy products, diarrhea and flatulence may occur.