Describe the relationship between the uv index (the colored bar in figure 1) and latitude (y-axis).

Human Evolution Revised April 2018 www.BioInteractive.org Page 1 of 7

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Human Skin Color: Evidence for Selection

INTRODUCTION 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. To determine whether the variation in human skin color is the result of evolution by natural selection, scientists look for patterns revealing an association between different versions of the trait and the environment. Then they look for selective pressures that can explain the association.

In this lesson, you will explore some of the evidence for selection by analyzing data and watching the film The Biology of Skin Color (http://www.hhmi.org/biointeractive/biology-skin-color), featuring anthropologist Dr. Nina Jablonski. In Part 1 of this lesson, you’ll discover the particular environmental factor correlated with the global distribution of skin color variations. In Parts 2 and 3, you’ll come to understand the specific selective pressures that have shaped the evolution of the trait. Finally, in Part 4, you’ll investigate how modern human migration is causing a mismatch between biology and the environment.

PROCEDURE Read the information in Parts 1–4 below, watching segments of the film and pausing as directed. Answer the questions in each section before proceeding to the next.

PART 1: Is There a Connection Between UV Radiation and Skin Color? Watch the film from the beginning to time stamp 5:49 minutes. Pause when Dr. Nina Jablonski asks the question, “Is there a connection between the intensity of UV radiation and skin color?”

In this segment of the film, Dr. Jablonski explains that the sun emits energy over a broad spectrum of wavelengths. In particular, she mentions visible light that you see and ultraviolet (UV) radiation that you can’t see or feel. (Wavelengths you feel as heat are in a portion of the spectrum called infrared.) UV radiation has a shorter wavelength and higher energy than visible light. It has both positive and negative effects on human health, as you will learn in this film. The level of UV radiation reaching Earth’s surface can vary depending on the time of day, the time of year, latitude, altitude, and weather conditions.

The UV Index is a standardized scale that forecasts the intensity of UV radiation at any given time and location in the globe; the higher the number, the greater the intensity. Examine Figure 1 on the next page and answer Questions 1–6.

1. Describe the relationship between the UV Index (the colored bar in Figure 1) and latitude (y-axis).

2. How do you explain the relationship between the UV Index and latitude? (In other words, why does UV intensity change with latitude?)

http://www.hhmi.org/biointeractive/biology-skin-color

http://www.hhmi.org/biointeractive/biology-skin-color

http://www.hhmi.org/biointeractive/biology-skin-color

Human Skin Color: Evidence for Selection

Human Evolution Revised April 2018 www.BioInteractive.org Page 2 of 7

Activity Student Handout

3. Find your approximate location on the map. What is the primary UV Index value of your state on this particular day in September? _________

4. Look at the regions that receive the most-intense UV (light pink). Site a specific piece of evidence from the map that a factor