Evolution of Skin Color
Describe the relationship between the UV Index (the colored bar in Figure 1) and latitude (y-axis)
UV radiation is most intense near the equator and least intense toward the poles. Students may also say it is most intense at lower latitudes and increasingly intense as latitude increases.
Compare and contrast the cell size and amount of melanin in cells from the wild-type zebrafish and the golden zebrafish seen in Figure 1
The cells (melanophores) in the wild-type zebrafish are larger and contain larger and more abundant melanosomes and much more melanin compared to the melanophores of the golden zebrafish
Based on Figure 6, how long ago do you think the SLC24A5 gene evolved? Explain your answer.
The gene must have evolved in the most recent common ancestor of all the groups on the tree, which existed at least 400 million years ago. As a result, the gene must have been present at least 400 million years ago (though it may be much older).
your answer
The group exposed to UV radiation has less serum folate. The mean concentration for the "normal" group was about 7 ng/mL and the mean concentration for the "patient" group was about 4 ng/mL.
Compare and contrast the zebrafish cells in Figure 1 with the human cells shown in Figure 2
Answers may vary. An acceptable answer would be that the melanin color in the golden zebrafish melanosomes looks like the melanin from the woman with light skin.
Why do you think that reflectance data are collected from a subject's inner arm
The inner arm is not usually affected by environmental factors (e.g., it doesn't tan).
How do you explain the relationship between the UV Index and latitude (In other words, why does UV intensity change with latitude
) The answer has to do with the angle of Earth relative to the sun. Latitudes at the equator receive direct sunlight year-round. Latitudes toward the poles receive sunlight at an oblique angle, which means that the same amount of radiation is spread out over a larger area than at the equator. Find your approximate location on the map. What is the primary UV Index value of your state on thi particular day in September? Answers will vary depending on location. Most states in the U.S. have a UV index between 4 and 6.
Do the data in Figure 8 match your prediction in Question 11? Explain why or why not.
Answers will vary, but most students will notice that Europe has a high frequency of the A allele but Asia does not; this will not match their predictions.
In the film, Dr. Jablonski explains that melanin, located in the top layer of human skin, absorbs UV radiation, protecting cells from the damaging effects of UV. Genetics determines the type of melanin (i.e., brown/black eumelanin or red/brown pheomelanin) and the amount of melanin present in an individual's cells. Based on this information, write a hypothesis for where in the world you would expect to find human populations with darker or lighter skin pigmentation (i.e., different amounts of melanin)
Answers will vary, but students may predict that populations with darker skin color (or more eumelanin) would be found in regions with more intense UV radiation. Thus, populations found in equatorial areas will have the darkest skin (most eumelanin) and populations at higher latitudes will have lighter skin (least eumelanin). Scientists could measure the average skin color of people at different locations throughout the world and compare
Support the claim that zebrafish can serve as a model organism for studying the biology of human skin color with three pieces of evidence from the reference figures provided and discuss why this makes sense using scientific reasoning.
Answers will vary, but students should mention for example that Figure 4 shows that zebrafish and humans share a common genetic code and are made up of the same conserved amino acid sequences. In Figure 5, scientists were able to take a messenger RNA (mRNA) fragment from the human DNA sequence for the SLC24A5 G allele and inject it into golden zebrafish embryos and the zebrafish was able to translate the human mRNA into a protein. The cladogram in Figure 6 shows that zebrafish and humans share a common ancestry.
more or less probable hypothesis than when you first proposed it
Answers will vary. If the student had hypothesized that protection from skin cancer provided the selective pressure, based on this information they might want to revisit their hypothesis.
Use Figure 7 to predict the geographic region(s) where you'd expect the A allele for SLC24A5 to be most common. Explain your reasoning.
Asia, Northern North America, Australia, Europe, Southern South America. These are the areas with the lightest native skin tones.
Why does Dr. Jablonski dismiss the hypothesis that protection from skin cancer provided selection for the evolution of darker skin in our human ancestors
Because skin cancer does not usually arise until after an individual's peak reproductive years. To be affected by natural selection, a trait must have an effect on an individual's ability to survive and pass on its genes.
a tanning booth (which exposes skin to UV light) on the blood levels of these two vitamins
Being in a tanning booth would increase the amount of circulating vitamin D and decrease the levels of folate. It can also put the individual at greater risk of developing skin cancer.
Review the information in the Table 1 reading, then briefly explain how a single change in a DNA sequence could alter the function of a protein
DNA codes for a messenger RNA (mRNA), which can be translated into a protein. The mRNA is read in groups of three to code for a specific amino acid. A change in a DNA sequence will change the mRNA that gets formed, which could result in a code for a different amino acid and ultimately a different protein.
Based on these data, describe the populations least likely to synthesize sufficient levels of vitamin D. Explain your answer with data from the figure
Dark-skinned people are least likely to have sufficient vitamin D. They cannot produce enough vitamin D regardless of where they live. Moderately dark-skinned people can synthesize enough vitamin D if they live near the equator.
Based on everything that you have learned so far, provide an explanation for how the different shades of skin color from pinkish white to dark brown evolved throughout human history
Darker skin colors evolved because they provided increased fitness in early human populations living in equatorial Africa. Darker skin protects circulating folate from being broken down. Some human populations migrated out of Africa to places where UV radiation was less intense. Here there was selection for lighter skin which let more UV radiation through for vitamin D synthesis. Thus the evolution of variation in human skin color is due to the balance between needing protection from UV to maintain circulating folate levels and needing some UV to prevent vitamin D deficiency.
Dr. Jablonski describes learning that low folate levels are linked to severe birth defects as a "eureka moment." Explain what she means by this
Dr. Jablonski saw a connection between phenotype (skin color), environment (UV intensity), and fitness (folate levels and the risk of severe birth defects and low sperm counts). This connection provides an alternative hypothesis for the selective pressure that drove the evolution of darker skin.
Use evidence in Figures 7 and 8 to support or contradict the claim that light-skinned phenotypes evolved only once in human history. (Hint
Focus on European and Asian populations. Compare their phenotypes and allele frequencies.): The data contradict the claim because clearly there is more than one "way" to get light skin. The A allele evolved and spread among European populations, but not in Asian populations, so some other genetic pathway for forming light skin must have evolved.
For a person living farther away from the equator, would the risk of vitamin D deficiency be uniform or vary throughout the year
If it would vary, how would it vary? UV intensity varies with the seasons. A person would be at a higher risk for vitamin D deficiency in the winter, when UV radiation is less intense.
How do these data support the hypothesis that the evolution of lighter skin colors was driven by selection for vitamin D production
Light-skinned individuals are better able to synthesize sufficient vitamin D, especially at higher latitudes. That means that light skin increases fitness away from the equator.
Describe the trends visible in the data. Which subpopulation (gender, race/ethnicity) is at the greatest risk for vitamin D deficiency and which subpopulation is at the least risk for vitamin D deficiency
Non-Hispanic blacks have the lowest mean vitamin D levels overall and among males and females living in the United States. Non-Hispanic whites have highest mean vitamin D levels overall and among males and females. The
What is one of the consequences of recent human migrations on human health
One consequence is that people's skin color may not be a good match for the UV radiation intensity where they live.
Can the effects of UV light on folate explain the full variation of human skin color that exists among human populations today
Protection of folate from destruction can explain the selective pressure for the evolution of darker skin. However, it does not explain why there is variation in human skin color. What is the selection for the evolution of lighter skin
What does it mean for a trait, such as light skin coloration, to be under negative selection in equatorial Africa
Relate negative selective pressure to what we know about MC1R allele diversity among African populations. It means that there is selection against that trait. Researchers found that among people of African ethnicity, there is very little variation in MC1R alleles; almost everyone has the allele associated with the darker skin trait. There is selection against any MC1R alleles that do not code for darker skin.
According to the graphs in Figure 3, is the Δ melanin index a good way to predict the SLC24A5 genotype? Explain your answer.
Sample answer: Not really; although people who are homozygous for the G allele have the highest amount of melanin and the darkest skin color and people who are homozygous for the A allele have the lowest amount of melanin and the lightest skin color, there is a large amount of overlap among the groups. Someone with a Δ melanin index value of 0, for example, could belong to any of the three genotypes.
and steepness of the lines' slopes
Skin reflectance increases as you move north and south from the equator. That means that skin is darker near the equator and lighter as you move north or south. This graph indicates that darker-skinned individuals (individuals with more eumelanin in their skin that reflects less visible light) tend to live around the equator, where UV intensity is highest. Student responses may vary about whether the findings support their hypothesis from question 5.
Based on what you know about skin pigmentation so far, suggest a mechanism by which UV intensity could provide a selective pressure on the evolution of human skin color. In other words, propose a hypothesis that links skin color to evolutionary fitness
Students may propose that melanin protects an individual from skin cancer. While this is true, it may not account for the selection for dark skin, as they will learn in the upcoming film segment. Melanin also protects circulating folate from being broken down by UV radiation.
Look at the regions that receive the most-intense UV (light pink). Site a specific piece of evidence from the map that a factor other than latitude was contributing to UV intensity on this day
The Andes and Himalayas have higher UV Index values than you'd expect, which is evidence that UV intensity increases with higher altitude. Students may also say that there might be decreased cloud cover or greater humidity. All these answers would be acceptable.
What do the results in Figure 5 suggest about the similarity or dissimilarity of the function of the protein encoded by the SLC24A5 gene in different species?
The fact that the messenger RNA (mRNA) from the human SLC24A5 gene restored the high melanin levels (and dark color) in the zebrafish larvae strongly suggests that the proteins have a similar function in the different species.
revise your hypothesis to explain the selective pressure on the evolution of human skin color
The greater amount of eumelanin in darker skin protects folate from being broken down by UV radiation and thus increases fitness among populations in high-intensity UV areas.
From Figure 4, it's clear that the SLC24A5 gene sequences are highly similar among different species. What can you conclude from this about the importance of the function performed by the protein encoded by the SLC24A5 gene? What is the simplest explanation for the similarity across species?
The high degree of similarity in sequences suggests that the proteins have a similar function across the different species, and this function is critical for survival and/or reproduction. The simplest explanation for the similarity in the sequences is that the gene was inherited from a common ancestor in all the species.
How is the molecule melanin related to human skin color?
The images in Figure 2 show that the size and number of melanosomes, and the amount of melanin, are different in people with different skin colors. Remind students that the film mentioned that a person's skin color is determined by the type and amount of melanin in their skin. "The reddish-yellow pheomelanin is more abundant in lightly pigmented people. More darkly pigmented people have more of the brown-black eumelanin, and the more eumelanin, the darker the skin."
KEY CONCEPTS of evolution of skin color
Within a population, heritable traits that provide a survival and reproductive advantage in a particular environment are more likely than other traits to be passed on to the next generation and thus tend to become more common over time. These traits are known as adaptations. Human populations living in different parts of the world have different sets of evolutionary adaptations. These include wide-ranging variations in the way people look, especially with respect to skin color. Evidence from different disciplines can inform what makes a human trait beneficial or harmful in a particular environment. Evolution involves tradeoffs; a change in a gene that results in an adaptation to one aspect of the environment may be linked to a disadvantage with respect to another aspect of that same environment.
To develop an explanation based on natural selection, biologists look for evidence of adaptation. Look at Table 1. Is the A allele frequency difference between Europeans and West Africans evidence of selection? Why or why not?
Yes. The differences in allele frequency for the SLC24A5 A allele are so dramatic between West Africans (0.09) and Europeans (0.96) that selection must be at play. Almost all Europeans have the A allele, while it is extremely rare among West Africans. This is very similar to the MC1R variation seen (or not seen!) among individuals in these populations, which was caused by selection.
a. other genes and environmental factors b. There is significant overlap among the groups. If it accounted for all of the differences, then there would be no overlap at all; each genotype would result in a unique phenotype.
a. other genes and environmental factors b. There is significant overlap among the groups. If it accounted for all of the differences, then there would be no overlap at all; each genotype would result in a unique phenotype.
