One of my students correctly pointed out that many physical traits, for example, skin color, represent human adaptations to different environments. A biological adaptation is a trait that has evolved over time because it increases the likelihood that a person will survive long enough to reproduce, thus allowing the survival of a population. A cline is a gradual change in a trait over a geographic area. The above map shows a cline for skin color.
Our skin must:
1. Absorb enough ultraviolet radiation from the sun to manufacture Vitamin D, which our bodies need to keep our bones strong and healthy.
2. Protect us from the damages of too much ultraviolet radiation from the sun. Too much UV radiation destroys folate, a B vitamin. Folate deficiency is linked to neural tube birth defects during pregnancy.
Melanin is the pigment in our skin that determines how much UV radiation will be absorbed by our bodies. Melanin also gives skin its color. The darker the skin, the more melanin it contains. Note the regions where skin tone is the darkest. These areas are closer to the equator where ultraviolet radiation from the sun is most intense. Melanin acts as a filter to protect the body from absorbing excessive amounts of UV radiation. If your skin tone is darker, your ancestors most likely lived closer to the equator. In regions with low UV radiation, such as Scandinavian countries, the skin contains low levels of melanin, making skin lighter. Light skin allows the absorption of enough UV radiation to make Vitamin D.
If a person with dark skin moves to Sweden, he/she should take Vitamin D supplements to prevent conditions such as rickets. This is why milk is usually fortified with Vitamine D. If a light-skinned Scandinavian woman were to move to a country in central Africa, she would likely need to take higher doses of the B vitamin, folic acid, particularly during pregnancy.
Hence, visible traits are biological adaptations to environmental conditions and not determinants of a person's identity.
In the following video clip, Nina Jablonski, author of Skin: A Natural History, discusses human adaptation to different climates and levels of ultraviolet radiation.