INTRODUCTION HUMAN SKIN COLOR: EVIDENCE FOR SELECTION 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 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 and answer Questions 1-6.
The Correct Answer and Explanation is:
Correct Answers to Questions 1–6 (based on the description provided and reference to Figure 1, which typically shows global UV Index distribution):
1. The UV Index tends to be highest near the equator and decreases toward the poles.
2. Regions near the equator, especially central Africa, northern South America, and parts of Southeast Asia, experience the highest UV Index levels.
3. The pattern shows a clear correlation between latitude and UV radiation, with UV intensity decreasing as distance from the equator increases.
4. Darker skin colors are generally found in populations living in areas with high UV exposure, while lighter skin tones are more common in populations living at higher latitudes with lower UV exposure.
5. The pattern suggests that human skin color varies with UV radiation intensity, indicating an adaptive trait in response to environmental conditions.
6. The environmental factor most likely correlated with the global distribution of skin color variations is ultraviolet (UV) radiation.
Explanation:
Human skin color variation strongly aligns with patterns of ultraviolet (UV) radiation intensity around the globe. The sun emits UV radiation, a form of energy that, while invisible to the eye, significantly affects human biology. UV intensity is not uniformly distributed; it is highest near the equator, where sunlight strikes Earth more directly, and decreases with distance from the equator toward the poles. The UV Index, a standardized measurement, reflects this trend, with equatorial regions consistently experiencing high values.
Darker skin colors are most common in equatorial regions, such as sub-Saharan Africa and parts of South America and Southeast Asia, where UV radiation is intense year-round. These populations have higher melanin levels in their skin. Melanin acts as a natural sunscreen, absorbing and dispersing UV radiation and thereby protecting the skin from damage, including DNA mutations that can lead to skin cancer.
Conversely, lighter skin tones dominate in populations at higher latitudes, such as Northern Europe and parts of Asia. In these regions, lower UV radiation reduces the risk of overexposure, making heavy melanin protection less critical. Light skin allows more efficient synthesis of vitamin D, which requires UV exposure for its production in the skin.
The relationship between skin color and UV radiation reveals that skin pigmentation is not random but a trait shaped by natural selection. Variations in skin color evolved as adaptive responses to differing UV environments, ensuring a balance between protection from UV damage and sufficient vitamin D synthesis. Thus, UV radiation is the primary environmental factor associated with the evolution of human skin color.
