The term race is used to describe skin color and other related genetic traits that are expressed, such as black, course hair and dark brown eyes. Race classification among humans is a somewhat controversial topic because race infers that genetic divergence exists among humans and that these races are subspecies of Homo sapiens.
Mutation and Adaptation
Genetic divergence occurs when a population undergoes a genetic mutation. A mutation occurs when DNA is permanently altered. Some mutations result in the death of an organism, often before birth, but mutations that prove to be beneficial, or at least compatible with life, are passed on to offspring and can result in a new species or subspecies or other group, depending on which level of the phylogenetic tree the mutation takes place.
Adaptation is change in a population through the process of natural selection. If a trait is more desirable and is better for survival in a given environment, then organisms with that trait have a better chance of survival and will live to pass on that trait. If the environment changes to favor a different trait, then that newly-favored trait will prevail. Adaptation favors existing traits, whereas mutation randomly creates new traits.
Skin Pigmentation is an Adaptation to UV Light
The research of Nina G. Jablonski and George Chaplin of the Department of Anthropology of the California Academy of Sciences in San Francisco supports the theory that skin pigmentation is an adaptation to UV light exposure. According to Jablonski and Chaplin, humans living closer to the equator, where there is more direct and increased UV light exposure, have dark pigmentation. Those living closer to the poles are exposed to less UV light and have light pigmentation. Those living between these areas "have the potential for developing varying degrees of facultative pigmentation (tanning)."
Vitamin D3 Production and Folate Photolysis
These differences in pigmentation are adaptations in response to the need for vitamin D3 and protection from skin damage resulting from UV light exposure. Closer to the equator, the skin receives more than enough UV light for vitamin D3 production, but the exposure also contributes to the photolysis, or destruction, of folate, an important B vitamin that is necessary for fetal development as well as bone marrow and red blood cell creation. Therefore, there is greater skin pigmentation to protect against skin and folate damage.
Humans living closer to the poles receive much less UV light exposure, so less pigmentation makes the skin more susceptible to UV light absorption that is necessary for adequate vitamin D3 production. Vitamin D3 can be stored in body fat to accommodate for low UV light exposure during the winter.
Skin Pigmentation Adaptation Frequency
Jablonski and Chaplin proposed that human populations have undergone multiple adaptations from light to dark and back to light pigmentation as they migrated between areas of greater UV light exposure and less UV light exposure. These adaptations have taken place over relatively short, geological time periods.
Jablonski and Chaplin explain, "As the pace of human migrations has quickened in recent centuries, more and more populations are finding themselves living under UV irradiation regimes to which they are inherently poorly adapted (e.g., the English who settled in Australia in the nineteenth and twentieth centuries), with major public health consequences."
It is questionable how modern culture and transportation will affect skin color in the future, if there will be adaptation to frequent environmental changes; however, it can be argued that current migration is contributing to a greater incidence of disease related to UV light exposure and that greater attention to dietary needs is necessary to ensure adequate vitamin D3 and folate levels.
Resources
Brooker, Robert J., Widmaier, Eric P., Graham, Linda E., and Stiling, Peter D. Biology. New York: McGraw-Hill, 2008.
Jablonski, N.G. & Chaplin, G. (2000). "The evolution of human skin coloration." Journal of Human Evolution. 39(1):57-106. Accessed June 22, 2010.
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Diane Ursu - Diane Ursu joined Suite101 as a contributing writer in August 2009 and became a Feature Writer in January 2010. She is a freelance writer ...
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