Biology of Skin Color

Other primates have pale skin. Why isn't this a disadvantage to primates other than humans living in areas with intense UV radiation?

They are less likely to absorb the UV than the light skinned humans.

What did scientists infer from the lack of variation in the MC1R gene among African populations?

They inferred that the MC1R gene changes the color of skin darker and more UV causes more MC1R.

Why is it important when sampling human skin color to know whether an individual is indigenous or not?

To know the origin or amount of exposure to the sun that person has had.

Describe the relationship between folate levels and UV exposure.

UV light reduces folate levels.

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.

Melanocytes

found in the lower epidermis and produce melanin

Keratinocytes

found on surface of epidermis, used as protection against the environment

What does indigenous mean?

native to a place

__________ selection for light skin color in equatorial Africa

negative (the selective removal of rare alleles that are deleterious; traits of a species are selected for)

In human populations that migrated to latitudes with less UV radiation, there was a ________________ selection for lighter skin

positive ( new advantageous genetic variants sweep a population )

Advantage of lighter skin

produce more Vitamin D

MC1R allele

- is a receptor that functions to turn on expression of the darker eumelanin pigment (as opposed to the lighter: pheomelanin) - found in people of African ethnicity is highly active, leading to increased expression of the eumelanin - researchers found very little genetic variation in MC1R allele in people of African ethnicity

How do these data support the hypothesis that the evolution of lighter skin colors was driven by selection for vitamin D production?

------------ lighter skin facilitates vitamin D synthesis better at lower UV radiation levels

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).

Vitamin D and folate levels in the blood are both affected by UV light. Describe the predicted effects of using 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.

Evolution

Change in a kind of organism over time; process by which modern organisms have descended from ancient organisms.

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.

What other traits are highly variable like skin color?

Eye color and Hair color

Why do areas of High Altitude have greater than expected UV intensity and areas of constant cloud cover have less than expected?

High Altitude places have more exposure to UV than low altitude because they are farther away from the sun.

Melanin protects individuals from skin cancer. What is it about the timing of skin cancers that may decrease their importance in causing the evolution of dark skin color?

It depends on the UV levels of the individuals.

Indigenous peoples with diets rich in vitamin D living in high latitudes have dark skin. How does this observation support the hypothesis presented in the film about the selective pressure for the evolution of lighter skin? What other explanations could account for this observation?

It is dependent on the amount of vitamin D recieved.

The enzymes to produce melanin are found in all major taxa of life. What does this suggest about the importance of melanin production for living things?

It tells us without melanin are DNA would be damaged and would result in major issues such as cancer.

What are the risks associated with light skin in equatorial areas? With dark skin in high latitudes?

Light skin in equatorial areas can be exposed to too much UV and Dark Skin people could get not enough UV in high latitude areas.

Dr. Zalfa Abdel-Malek says that the supernuclear caps formed by melanin are like little parasols. Parasols are a type of umbrella. Explain this analogy.

Parasols block out the sun just like melanin does.

Can the effects of UV light on folate explain the full variation of human skin color that exists among human populations today? Explain your reasoning

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?

Explain how scientists could test this hypothesis.

Scientists could measure the average skin color of people at different locations throughout the world and compare that to average annual UV intensity.

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.

What is the connection between DNA and visible traits?

The DNA codes for the traits that we can visibly see.

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.

Describe the relationship between folate levels and UV exposure. Use specific data from the graph to support 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.

Darker skin protects skin cells from UV radiation. So why aren't all humans dark skinned?

We don't all have dark skin because we don't all receive the same amounts of UV radiation.

Folate

a B vitamin vital to the survival of the fetus and destroyed by UV light if melanin is not present to protect it

Functions of melanin in other animals (2)

camouflage and temperature regulation

What is a mutation?

change in DNA sequence

Melanoma

common type of skin cancer that occurs later in life

Melanosomes

contain melanin and protect the nucleus from damaging UV light

What is a adaption?

inheritable traits that help an organism survive

Advantage of darker skin

protection from damaging UV light

Reduced folate(water soluble B vitamin) levels lead to..........

severe birth defects and lower sperm counts

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

the relationship between them is the more intense uv radiation runs along the equator at 0 degrees while the least intense is more north and south poles the further you are from the equator the less intense the uv radiation is