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 XXXXXXXXXXHemispheric 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
y 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 XXXXXXXXXXNational 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?