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A rainbow of sepia NATIONAL CENTER FOR CASE STUDY TEACHING IN SCIENCE Page 3“A Rainbow of Sepia” by Annie Prud’homme-Généreux Questions 1. Does this graph confirm or refute Nina’s hypothesis? 2. Propose a mechanism by which UV radiation exerts a selective pressure on the human population that leads to the evolution of skin pigmentation. Specifically, what is affecting fitness (the number of descendants that an individual contributes to the next generation)? 3. When Nina turned her attention to this question, a leading hypothesis was that UV radiation causes DNA muta- tion, sometimes leading to melanoma, a form of skin cancer with a high mortality rate. Skin cancer was proposed to remove individuals who did not have sufficient pigmentation in their skin from the population and exert a strong selective pressure for dark skin in high UV areas. But Nina didn’t buy it. Why might Nina suspect that this proposed mechanism is not accurate? Part III – How Does UV Radiation Exert its Selective Pressure? Nina searched the literature and discovered that another anthropologist had collected data on exactly what she wanted. John Relethford, a Distinguished Teaching Professor at the State University of New York at Oneonta, had searched the literature for in- formation on the skin pigmentation of indigenous men and graphed it against the latitude where the population evolved. The data is shown in Figure 2. Figure 2. Relationship between skin reflectance and latitude. (Source: Relethford, J.H. 1997. Hemispheric difference in human skin color. American Journal of Physical Anthropology 104: 449–57. Figure 2. ©Wiley Periodicals, Inc., used with permission.) NATIONAL CENTER FOR CASE STUDY TEACHING IN SCIENCE Page 6“A Rainbow of Sepia” by Annie Prud’homme-Généreux Part VI – What Happens Now? In the next two-minute video clip, Nina will take you through some of her thinking on vitamin D. What Nina discovered is that the needs to pro- tect folate and to synthesize vitamin D exert opposing pressures on the evolution of skin color. Melanin levels in the skin represent a compromise between these two needs, struck by evolution through natural selection. After thousands of years of evolution, how well adapted is our skin color? Consider the data in Figure 5, which shows the blood levels of vitamin D in Americans grouped according to age, sex, and ethnic background. Levels between 20–50 ng/mL are considered adequate for health. Figure 5. Levels of vitamin D in the blood of Americans. In the race- ethnicity section, “Hisp” refers to individuals of Hispanic background, “NHB” refers to non-Hispanic blacks, and “NHW” refers to non-Hispan- ic whites. Note that “Serum total 25(OH)D” means the levels of vitamin D in the blood. (Source: Schleicher, R.L., et al. 2016. National estimates of serum total 25-hydroxyvitamin D and metabolite concentrations mea- sured by liquid chromatography-tandem mass spectrometry in the US population during 2007–2010. The Journal of Nutrition 146(5): 1051–61, Figure 1A. Reproduced by permission of American Society for Nutrition.)Questions 1. How do you explain the differences between ethnic groups in vitamin D levels? 2. Which population is at greatest risk of developing rickets, a disease of vitamin D and calcium deficiency that leads to bone malformation? What can be done about it? 3. For several decades, vitamin D has been added to milk to help increase intake of this nutrient. But in some ethnic groups, this strategy isn’t working. Why might this be? 4. Who is most at risk of developing folate deficiency and fertility problems? What can be done about it? 5. Based on what you have learned in the case, what actions will you take to maximize your vitamin D and folate levels? 6. Is natural selection still acting on the trait of skin color in human populations? If so, how would you expect human skin color to change in human populations in the future?