The evolution of human skin color case study answer key

*Adapted from: • Prud’homme-Généreux, A 2011. The evolution of human skin color: a case study. National Center for Case Study Teaching in Science.

. • Bonetta L, M. Csikari, and S. Keep. 2015. Human skin color: evidence for selection. HHMI BioInteractive.

biointeractive/human-skin-color-evidence-selection>. Case copyright held by the National Center for Case Study Teaching in Science, University at Buffalo, State University of New York. Originally published December 18, 2018. Please see our usage guidelines, which outline our policy concerning permissible reproduction of this work.

by Annie Prud’homme-Généreux

TELUS World of Science – Edmonton, Canada

A Rainbow of Sepia: The Evolution of Human Skin Color*

Part I – Why Are People Different Colors? Nina Jablonski, a biological anthropologist at Penn State University, spent many hours thinking about “the sepia rainbow of human skin color.” She knew that our closest primate relatives have pale skin under dark fur, but human skin comes in a variety of shades from pinkish white to dark brown. How did this variation arise?

Many biological traits have been shaped by natural selection. Could human skin color be one such trait? If so, she would have to find the selective pressure that caused different populations to evolve varying levels of skin pigmentation.

You will now meet Nina in a short five-minute video (https://www.hhmi.org/biointeractive/biology-skin-color). She will take you on a journey that she undertook in the pursuit of this question. After you have viewed the video, answer the following five questions.

Questions 1. Describe the conditions necessary for evolution by natural selection to take place in a population.

2. With regard to the evolution of skin color in humans, which of these conditions is Nina sure about (and why?) and which one(s) is she investigating?

3. What is Nina’s likely hypothesis about the evolution of skin color in humans?

4. If her hypothesis is correct, predict the characteristics of the environment where the most heavily pigmented hu- man populations are likely to be found, and the environment where the least pigmented human populations will be found.

5. How might she test her hypothesis?

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Page 2“A Rainbow of Sepia” by Annie Prud’homme-Généreux

Part II – Is UV Radiation Linked to Skin Pigmentation? Nina has a hunch that melanin evolved in our skin to protect us from the sun’s harmful UV radiation, and that the most heavily pigmented human popu- lations have evolved in areas of high UV intensity. This can account for the variability in human skin pigmentation only if different populations are exposed to different levels of UV light. Is there evidence that the amount of UV light varies across different environments on this planet?

Nina asked her husband, Penn State University ge- ographer George Chaplin, for help. At her request, he created a map showing the UV Index, a standard- ized scale that forecasts the intensity of UV radiation at any given time and location on the globe. Figure 1 shows an example of a similar map to the one George created.

Figure 1. UV Radiation Index across the World. Recorded on July 15, 2017, this map shows the amount of UV light reaching the Earth’s surface throughout the world. The legend at the bottom indicates that reddish areas receive the greatest amount of UV radiation and blue areas receive the least. (Source: European Space Agency, .)

Questions 1. Describe the pattern of UV variability across the globe.

2. If Nina's hypothesis is correct, where on this map do you expect to find the most heavily pigmented human popu- lations? Would it be in Brazil, Greenland, Europe, India, or Australia?

3. Using the graph provided, plot the data Nina expects to find. Each data point represents a human population. The X axis shows how far away from the equator the population lives. The Y axis captures the skin pigmenta- tion of the population. Note that this data is recorded as “skin reflectance,” which is a measure of skin lightness (i.e., high values indicate lighter skin). Two curves should be drawn, one for populations living in the Northern Hemisphere and one for populations in the Southern hemisphere.

15

25

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Southern Hemisphere

Northern Hemisphere

Distance from the Equator (Degrees Latitude)

Sk in

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)

http://www.temis.nl/uvradiation/UVindex.html
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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.)

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Page 4“A Rainbow of Sepia” by Annie Prud’homme-Généreux

Folate is a B vitamin found in dark green leafy vegetables, from which it gained its name (“foliage”). In men, this vitamin helps with sperm production, and in women it is essential during pregnancy. Deficiencies can lead to malformation of the baby’s nervous system.

Questions 1. Name two ways in which a person could become deficient in

folate.