"Because Europeans are so variable, and lighter-skinned, we might also ask whether this is due to selection for light skin or the relaxation of selection in melanin production...it is possible that lighter skin is an adaptation to lower levels of sunlight. No matter which is the case, the evolution of human skin color and the MC1R gene demonstrates that as humans spread out across the globe, the conditions of selection varied in different regions, the amount and quality of sunlight being just one obvious variable," (Carroll 173).
Human skin color has always been a battle of debate. As evolutionary theorists and biologists come to explain the idea of natural selection as the perceived driving force upon skin color, new findings are beginning to occur. How have mutations and natural selection acted upon the specific gene MC1R? Weigh and consider the benefits/costs to each race. Is the MC1R gene the only contributing factor to variation within skin color? What other conditions of selection affect the evolution of human skin color? Finally, what do you think is the cause of the evolution of skin color: Natural Selection, Relaxation of Natural Selection, or Both? Please delineate the reasons for your though process. Again, as always, I urge responders to connect to the various themes of Biology: Structure/Function, Heritable Information, Interaction with the Environment, and Evolution. Please discuss the relationship(s) with mutations and natural selection.
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Human skin color is largely dependent on the amount of melanin in the skin. As we have learned, melanin is a pigment produced by cells called melanocytes. The production of melanin is regulated by the hormone α-melanocyte-stimulating hormone (αMSH) which is produced in the pituitary gland. The melanocytes have a receptor called melanocortin-1 receptor (MC1R), which the hormone αMSH can bind to in order to stimulate the production of melanin. The amount of UV light one is susceptible to can induce the “production of increased levels of melanin” (171), which would darken the hair and skin.
ReplyDeleteMutations in the MC1R gene can also be caused by UV radiation, as the UV light can affect the bases of the DNA code, thus altering the structure of the MC1R receptor. This relates to the theme of structure/function, as the change in the structure of the MC1R receptor could cause the hormone to not bind to the receptor, so production of the melanin cannot be stimulated. If the structure is changed, the receptor does not function in the way it is supposed to. Since the different areas around the earth receive differing amounts of sunlight, the people living in different areas are susceptible to different levels of UV radiation. Thus, the people living in areas that receive weak levels of UV radiation such as those living in northern Europe would have different MC1R mutations than people living in areas with strong levels of UV radiation, like those living in Africa. Natural selection has allowed multiple variants of the MC1R genes in European/Asian populations and in African populations. However, there are 13 variants of the MC1R gene in the European/Asian populations, (10 which “alter the MC1R protein” (172) and three that do not) as opposed to the five variants of the MC1R gene in the African population, all of which do not change the MC1R protein. Natural selection has prevented any changes to the MC1R protein in the African population, which has led the African population to continually produce high levels of melanin. Because of the high levels of melanin, the African population has dark skin, which would protect them from the harsh UV radiation. Because melanin absorbs different wavelengths of sunlight, melanin offers protection in a harsh climate with strong UV radiation, which would benefit the African race. However, the darker skin could cause a deficiency of Vitamin D because the UV radiation is needed to induce the production of Vitamin D, which helps calcium absorption. The relaxation of natural selection seems to be the cause of the lighter-skinned European/Asian races. As the MC1R gene has over 10 different variants, all of which can change the MC1R protein, the variations in the protein shows that natural selection does not select for one particular structure of the protein. The environment of the European/Asian races could be the reason for the relaxation of natural selection, as the weaker levels of UV radiation do not make high levels of melanin production mandatory. The fair skin would be a benefit in areas that do not receive strong amounts of UV radiation, because the lighter skin would be more ideal for the absorption of lower amounts of UV radiation. However, lighter skinned people can suffer in intense sunlight, as they do not have high levels of melanin to absorb the radiation. The UV radiation could cause damage to their DNA through mutation.
Diet can also affect the evolution of skin color. Foods rich in Vitamin D can induce the production of high levels of melanin. For example, even though the Inuit (who live in Arctic regions) live in areas of low sunlight, their skin has stayed dark due to their diet of fish, which have high levels of vitamin D.