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The Biology of Skin Color
OVERVIEW
In The Biology of Skin Color, Penn State University anthropologist Dr. Nina Jablonski walks us through the evidence
that the different shades of human skin color are evolutionary adaptations to the varying intensity of ultraviolet
(UV) radiation in different parts of the world. Our modern human ancestors in Africa likely had dark skin, which is
produced by an abundance of the pigment eumelanin in skin cells. In the high-UV environment of sub-Saharan (or
equatorial) Africa, darker skin offers protection from the damaging effects of UV radiation. Dr. Jablonski explains
that the variation in skin color that evolved since some human populations migrated out of Africa can be
explained by the trade-off between protection from UV and the need for some UV absorption for the production
of vitamin D.
KEY CONCEPTS
A. Biological traits are not inherently good or bad. Some traits can provide an advantage to an organism in
certain environments but be a disadvantage in other environments.
B. Inherited traits that provide a survival and reproductive advantage in a particular environment are more likely
to be passed on to the next generation and thus become more common over time.
C. Different human populations living many generations in a particular part of the world may have different
variations in certain traits. In spite of these differences, all humans are very closely related and share most
traits.
D. Evidence from different disciplines, such as anthropology, developmental biology, physiology, genetics, and
cell biology, can inform what makes a human trait beneficial or harmful in a particular environment.
E. Variations in genes can lead to differences in biological traits. By studying the DNA sequences of large
numbers of people from different populations, scientists can estimate when and where those variations
arose.
F. Evolution involves trade-offs; 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.
G. Cells in multicellular organisms specialize to meet particular functions in an individual.
H. Molecules in living organisms absorb or reflect certain wavelengths of light from the sun. When a molecule
absorbs light, the energy is transformed into other forms of energy.
CURRICULUM CONNECTIONS
Standards
Curriculum Connections
NGSS (2013)
LS3.A, LS3.B, LS4.A, LS4.C
AP Biology (2015)
1.A.1, 1.A.2, 1.C.3, 3.A.1, 3.C.1, 4.C.1, 4.C.2
AP Environmental Science (2013)
I.A, III.B
IB Biology (2016)
1.2, 2.6, 3.4, 5.1, 10.2
IB Environmental Systems and Societies (2017)
8.1
Common Core (2010)
ELA.RST.9-12.2, WHST.9-12.4
Vision and Change (2009)
CC1, CC2
PRIOR KNOWLEDGE
Students should
• be familiar with the tree of life and know that humans are part of the primate group and that chimpanzees
are our species’ closest living relative;
• know that modern humans evolved in Africa and then migrated throughout the world; and
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• know that scientists have methods to analyze and compare DNA from different individuals within species and
across species.
PAUSE POINTS
The film may be viewed in its entirety or paused at specific points to review content with students. The table
below lists suggested pause points, indicating the beginning and ending times in minutes in the film.
Begin
Content Description
Review Questions
1
0:00
• Biological traits aren’t good or bad. They are
features that have evolved within populations
because they enhance an organism’s odds of
surviving and passing on its genes.
• Skin color is an easily visible marker of
variability. Our lack of body hair and our
variable skin color are some of the traits that
set us apart from our closest primate relatives.
• Wavelengths of light are reflected or absorbed
by pigment in the skin called melanin. Melanin
is synthesized in structures called melanosomes
that are produced by cells called melanocytes.
• There are two primary types of melanin in
humans: pheomelanin, which is reddish yellow,
and eumelanin, which is brown black.
• Can you think of other traits that
are highly variable like human skin
color?
• What is an adaptation?
• What is the connection between
DNA and visible traits?
2
3:55
• UV radiation can penetrate living cells and
cause mutations in DNA.
• Melanin protects human cells from the
damaging effects of UV radiation by absorbing
UV.
• There is a clear correlation between the
intensity of UV radiation and latitude. UV
radiation is most intense along the equator and
is weakest at the poles.
• UV intensity predicts the skin color of
indigenous populations. Stronger UV radiation
is correlated with darker skin color.
• Data suggest that variation in human skin
melanin production arose as different
populations adapted biologically to different
solar conditions around the world.
• What is a mutation?
• 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.
• 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?
• Why do areas of high altitude (e.g.,
on the Tibetan plateau) have
greater than expected UV intensity
and areas of constant cloud cover
(e.g., Congo Basin) have less than
expected?
• What does “indigenous” mean?
• Why is it important when sampling
human skin color to know whether
an individual is indigenous or not?
3
9:08
• Early in human history, our ancestors lost most
of their body hair and increased melanin
production in skin.
• Evidence of natural selection can be found in
the genome.
• MC1R is a gene that codes for a protein
involved in the production of eumelanin.
• Other primates have pale skin. Why
isn’t this a disadvantage to
primates other than humans living
in areas with intense UV radiation?
• What did scientists infer from the
lack of variation in the MC1R gene
among African populations?
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BACKGROUND
“Melanin” is the collective term for a family of pigment molecules found in most organisms, from bacteria to
humans, suggesting that melanin has a long evolutionary history and a broad range of important functions.
In humans, melanin pigments are found mainly in human skin, hair, and eyes, and they include reddish-yellow
pheomelanin and brown and black eumelanins. A related molecule called neuromelanin is found in brain cells.
In human skin, melanin pigments are synthesized in organelles called melanosomes that are found in specialized
cells called melanocytes in the skin epidermis. Once the melanosomes are filled with a genetically determined
amount and type of melanin, they migrate to other skin cells called keratinocytes.
Melanin synthesis involves a series of chemical reactions that begin with the amino acid tyrosine. An enzyme
called
tyrosinase promotes the conversion of tyrosine into DOPA, and then into dopaquinone. Dopaquinone can
either be converted into eumelanin or combined with the amino acid cysteine to produce pheomelanin. Whether
eumelanin or pheomelanin is produced depends partly on the activity of the
melanocortin 1 receptor (MC1R)
protein (
Figure 1).
E
umelanin is a remarkable molecule that can absorb a wide range of the wavelengths of radiation produced by
the sun, in particular, the higher-energy UV radiation. UV can damage biological molecules, including DNA. When
UV radiation strikes eumelanin, the pigment absorbs the radiation and mostly transforms the energy into thermal
energy, without breaking down, making it a powerful sunscreen that protects against UV damage. Pheomelanin is
less effective as a sunscreen than eumelanin and can, in fact, produce damaging molecules, known as free
radicals, when it interacts with UV radiation.
• Worldwide human genome sampling revealed
that among African populations, the vast
majority of individuals have an MC1R allele that
results in darker skin.
• Fossil and genetic evidence suggest that all
humans were dark-skinned about 1.2 million
years ago.
• UV breaks down circulating folate in the skin’s
blood vessels.
• 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?
4
13:33
• UV-B absorption is critical for the synthesis of
vitamin D, a process that starts in the skin.
• Weaker UV-B intensity and greater UV-B
variability throughout the year in areas toward
the poles put dark-skinned individuals at risk for
vitamin D deficiency.
• Toward the poles, selective pressure for dark
skin (to protect folate) decreases and selection
for lighter skin shades (to enable vitamin D
synthesis) increases.
• Selection for light-skin gene variants occurred
multiple times in different groups around the
world.
• Today, human migration does not take
generations. So there is a lot of mismatch
between skin color and geography.
• Skin color is a flexible trait that is inherited
independently of other traits.
• Darker skin protects skin cells from
UV radiation. So why aren’t all
humans dark skinned?
• 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?
• What are the risks associated with
light skin in equatorial areas? With
dark skin in high latitudes?
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A person’s skin color is determined primarily by the proportion of eumelanin to pheomelanin, the overall amount
of melanin produced, and the number and size of melanosomes and how they are distributed. People with
naturally darkly pigmented skin have melanosomes that are large and filled with eumelanin. Those with naturally
paler skin have smaller and fewer melanosomes that contain varying amounts and kinds of eumelanin and the
lighter-colored pheomelanin.
Figure 1. The melanin
biosynthesis pathway. Melanin
synthesis occurs in organelles
called melanososomes and
starts with the conversion of
the amino acid tyrosine to
DOPA by the enzyme
tyrosinase. The melanosome
sits inside a specialized skin cell
called the melanocyte. A
protein in the membrane of
the melanocyte, called MC1R,
receives messages from other
cells. MC1R can be activated by
the melanocyte-stimulating
hormone (MSH), which is
produced in response to
damage by ultraviolet light
(UVR) and other stimuli. Other
molelcules, such as the agouti
signaling protein (ASIP), inhibit
the activation of MC1R. When
a functional MC1R is activated,
it stimulates the production of cyclic adenosine monophosphate (cAMP), which is a second messenger
important in transferring the effects of hormones into cells. This cAMP production in turn triggers a
biochemical pathway that results in eumelanin production. Certain mutations in the MC1R gene that
prevent MC1R activation or binding of ASIP and other inhibitors to MC1R result in pheomelanin production.
Genetics of melanin production
Constitutive pigmentation, or the pigmentation we are born with, is a polygenic trait, and many of the genes
involved have been identified. These genes code for the enzymes that affect melanin synthesis and for the
packaging, distribution, and degradation of melanosomes. Mutations in some of these genes cause an absence of
melanin, as seen in human oculocutaneous albinisms and related disorders. For example, one form of albinism is
caused by mutations that inactivate the tyrosinase gene.
The HHMI film mentions the importance of the MC1R gene. This gene codes for a protein that sits in the
melanocyte membrane. It is activated by a variety of stimuli, such as by the
melanocyte-stimulating
hormone (MSH), and is responsible for determining whether eumelanin or pheomelanin is produced. People of
African descent have a version of the MC1R gene that is associated with eumelanin production. As mentioned in
the film, there is very little variation in the MC1R gene in African populations, compared to populations
indigenous to Europe and Asia. This lack of diversity at a genetic locus is evidence of selection, suggesting that
eumelanin production provides an advantage to people living in equatorial Africa. (Differences in the MC1R gene
are also responsible for coat-color variations in the rock pocket mouse. For resources related to MC1R activity in
this model system, search
BioInteractive.org using the key words “MC1R” and “pocket mouse.”)
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S
cientists have looked for evidence of selection in other parts of the genome and have identified genes involved
in skin color in different populations. For example, one allele of a gene called OCA2 results in lighter skin colors
and is almost exclusively found in East and Southeast Asian populations. On the other hand, alleles of two genes
called SLC24A5 and SLC45A2 are also associated with lighter skin colors and are much more frequent in
Europeans than in other populations. These and other data suggest that lighter skin color evolved more than once
by different mechanisms. Interestingly, the SLC24A5 and SLC45A2 genes were first discovered in zebrafish and are
responsible for differences in the stripe colors.
An i
mportant concept to make sure students understand is that genetic evidence indicates that similar skin colors
and tanning abilities evolved independently as different human groups dispersed into distant places with similar
UV conditions.
DISCUSSION POINTS
• The Biology of Skin Color film offers an opportunity to distinguish between negative and positive selection. Dr.
Jablonski says that the lack of diversity among the MC1R alleles in people of equatorial African descent is due
to negative selection. Tell your students that negative selection works to remove deleterious alleles from a
population—another term that describes this is “purifying selection.” Then ask them to infer a definition for
positive selection (selection for alleles that increase fitness). Positive selection results in directional selection.
Positive selection for a beneficial allele can increase the frequency of neutral variants neighboring the
beneficial allele; such genetic hitchhiking may result in a large area of homozygosity (i.e., a loss of diversity in
a large area of a chromosome) due to a highly advantageous mutation and is referred to as a selective sweep.
• Students may be curious about how eumelanin and pheomelanin produce hair colors that range from blond
to black, and why hair turns gray with age. Explain that the ratios of the two types of melanin pigments are
responsible for all of the hair colors. For example, yellow (or blond) hair is produced by a small amount of
brown eumelanin. Red hair results from a small amount of brown eumelanin mixed with mostly red
pheomelanin. Gray or white hair result from a lack of melanin that occurs when melanocytes in hair follicles
stop producing melanin as part of the aging process.
• Students may ask why, if melanin controls pigmentation of both skin and hair, there are people with light skin
and dark hair and eyes or dark skin with light hair. The colors of a person’s skin, hair, and eyes are controlled
by different sets of genes. Certain melanin-related genes get activated in one set of cells, like the cells that
make hair, whereas other genes get activated in other cells, like the cells of the iris. Some genes affect
pigmentation globally, as in some extreme forms of albinism, and certain mutations in the MC1R gene are
associated with light skin and red hair. But for the most part, because the genes are not linked, pigmentation
genes are inherited and function independently of one another. These genes are mostly independent of
genes controlling other aspects of the human phenotype. Your students might be interested to learn that
blond hair is found in parts of the South Pacific, such as the Solomon Islands, Vanatu, and Fiji (Figure 2).
Indigenous Solomon Islanders have naturally dark skin, which is to be expected given the intense UV radiation
in the South Pacific. However, a significant proportion has blond hair. Researchers have confirmed that the
trait is not due to gene flow with Europeans but instead has its own origin among the indigenous population.
The blond hair in people from the Solomon Islands is due to mutations in the TYRP gene that are unique to
this group and occur at a frequency of 26% in the population.
• Students may be confused about how we know that millions of years ago our ancestors were covered in hair
and had pale skin. We don’t have any fossilized skin from our ancestors. Researchers use comparative
anatomical and genetic evidence from modern humans and modern African apes to construct what our
ancestors were like. The human lineage originated in equatorial Africa 6–7 million years ago when it split from
the lineage that led to our closest living relative, the chimpanzee. The last common ancestor modern humans
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shared with modern chimpanzees was not a chimpanzee but probably shared many features with modern
African apes. All African apes have pale skin under their fur, and we can infer that the same was probably the
case for the last common ancestor we shared with them.
• Humans are the only primates whose bodies are not completely covered in thick hair. Students may ask about
the benefit of losing this hairy covering. The earliest fossils of the genus Homo (which is our genus) were
found in Africa and date back to about 2 million years ago. From these fossils, we know that by this time our
ancestors walked on two legs and would have been capable of walking long distances and even running. A
running body produces a lot of heat. One hypothesis is that having less hair would have helped keep bodies
cool when running and provided an advantage to these early humans. Over many generations, their bodies
lost most of their hair and our ancestors became extensively covered in one type of sweat gland that makes
dilute sweat. Walking long distances and running also suggest that early humans were spending less time in
dense forests and more time in open areas with more intense sunlight. Ask your students why the evolution
of more-heavily pigmented skin may have provided an advantage in this environment.
• Students may wonder if a suntan confers the same protective benefit as a darkly pigmented baseline skin
color. Explain that while two similar skin tones (one natural, one tanned) may absorb and scatter damaging
radiation in the same way at the surface, two notable differences are present. First, the UV-absorbing
melanin in baseline (or constitutively) dark skin is present in cells deeper in the epidermis, rather than just
near the surface as in temporarily tanned skin, and thus provides a greater overall protective benefit. Second,
the continuous UV exposure that is required to maintain tanned skin can lead to premature aging due to the
long-term damage to the structural proteins that give skin its strength and resiliency. Also explain that DNA
damage occurs long before the tanning response can be observed and that recent evidence suggests that
DNA damage may in fact be required to initiate the melanin production that causes tanning. After presenting
the connection between cell damage and tanning, see if your students can infer why skin peels after a severe
sunburn. (The reason is that the cells affected by UV become so damaged that they die, causing them to
slough off to be replaced by new skin cells.)
• Dr. Jablonski suggests in the film that skin cancers may not have had a major impact on fitness. Why is that?
It’s true that more exposure to UV radiation leads to a higher risk of cancer, and pigmented skin protects
against damage from UV. However, most skin cancers act later in life, after people are past reproductive age.
By that time, most people would have had children and passed their genes on to them. This is why Dr.
Jablonski argues that protection from skin cancer may not explain the evolution of darker skin color. Point out
to students that this is an area of active scientific discussion and study. Some scientists have pointed out that
melanomas can be fatal and some types, although rare, do strike younger people. Furthermore, other
scientists argue that factors that affect people later in life can affect fitness. They note the importance of
elders, including grandparents, for collecting food in hunter-gatherer societies, helping people achieve social
status, and as sources of knowledge in preliterate societies. Dr. Jablonski, however, offered an alternative
hypothesis for why darker skin is advantageous in high-UV environments. It is based on the observation that
melanin not only protects DNA, it also protects an essential biological compound called folate (vitamin B9)
from degrading. In females, folate is required for adequate egg cell production, implantation of the embryo in
the uterus, and in the growth of the placenta. Once an embryo begins developing, folate protects the embryo
from various abnormalities like spina bifida. In males, a deficiency in folate contributes to improper sperm
development and infertility. Ask students how protecting folate from UV degradation would provide
increased fitness in certain environments. Why is this a more likely explanation for the selective pressure on
increased pigmentation in skin than skin cancer? Are the two explanations mutually exclusive?
• Students may be surprised to learn that vitamin D is synthesized in the body. Foods such as salmon and
swordfish are high in vitamin D. Milk, some brands of cereal, orange juice, and yogurt are also fortified with
vitamin D. However, many people don’t get enough vitamin D from food alone and need to produce vitamin
D in the body. Vitamin D synthesis in the body starts in the skin when a compound called 7-
dehydrocholesterol (7-DHC) is converted into a compound called previtamin D in the presence of UV-B
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radiation. The previtamin D then undergoes further modification to become vitamin D. Vitamin D is essential
for sufficient absorption of calcium and potassium to build and maintain our bones and to support our
immune systems. A diet that includes foods that are naturally rich in vitamin D, coupled with a small amount
of direct sun exposure (5–10 minutes per day on arms and legs, depending on latitude, altitude, and time of
day) may be enough to provide the body with all the vitamin D it needs.
• As you show and discuss this film about human skin color, students may have questions and want to discuss
race. When the biologist Carolus Linnaeus began to classify organisms in earnest in the early 1700s, he used
skin color to identify what he thought were four different groups of people. Today, biologists recognize that
there are differences among human populations both in terms of visible traits, like skin and eye color, and
other traits, like susceptibility to disease. But when the full spectrum of variations across many traits is
considered, there is no evidence for the existence of discrete human races. In other words, there are no set
“packages” of traits that constitute a human race. For further background, consider visiting the
Race: Are We
So Different? website (http://www.understandingrace.org/home.html) from the American Anthropological
Association. In addition, Dr. Jablonski has written several excellent articles and books on this topic; a recent
interview with her appeared in Nautilus (Paulson, Steve, “About Your Skin: What You Should Know about Your
Body’s Biggest Organ,” July 2, 2015,
http://nautil.us/issue/26/color/about-your-skin).
STUDENT HANDOUT
We designed the student handout as a learning assessment that probes students’ understanding of the key
concepts addressed in the film, which can be used before or during the film to assess students’ prior knowledge
and to guide students as they watch the film. We encourage you to choose the use that best fits your learning
objectives and your students’ needs. Moreover, because the vocabulary and concepts are complex, we encourage
you to modify the handout as needed (e.g., reducing the number of questions, explanations of complicated
vocabulary for English learner students).
ANSWERS
1. (Key Concept A) True / False. Biologists classify specific forms of traits as good or bad. For example, long tails
in cats could be classified as good and short tails as bad.
False. Biologists do not assign a moral value,
judgment, or statement of worth to particular traits, as this is outside of the domain of science. They may study
the impact of traits on fitness, but even then the effect of a particular trait may have a positive impact on fitness
in one environment but a negative effect on fitness in a different environment.
2. (Key Concept A) Explain the reasoning or evidence you used to answer Question 1.
After watching the film, students should realize that different shades of skin color have had different effects on
fitness depending on where indigenous people lived. Dark skin had a positive effect on fitness in high-UV
environments, whereas lighter skin had a positive effect on fitness in low- or variable UV environments. As Dr.
Jablonski says in the film, “. . . biological traits aren’t good or bad. They’re features that have evolved because they
enhance an organism’s odds of surviving and passing on its genes.”
3. (Key Concept D) If you travel north from the equator, what generally happens to the intensity of ultraviolet
(UV) light?
a. The intensity increases.
b.
The intensity decreases.
c. The intensity stays the same.
d. It is impossible to predict.
The pattern of decreasing UV with increased latitude is clearly shown in the film at time mark 7:17.
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4. (Key Concepts D and F) Who would you expect to be most at risk for developing the bone disease rickets?
a.
Children born to mothers with dark skin, living far from the equator
b. Adults with dark skin who live close to the equator
c. Children born to mothers with light skin, living close to the equator
d. Adults with light skin who live close to the equator
e. Anyone who eats a diet that includes a lot of fish
5. (Key Concepts D and F) Explain the reasoning or evidence you used to answer Question 4.
Dark-skinned mothers living in high latitudes are at risk for vitamin D deficiency because environments far from the
equator have lower levels of UV. If they do not produce enough vitamin D, they cannot pass it to their children in
breast milk. Low levels of vitamin D interfere with the absorption of calcium and can lead to the bone disease rickets.
6. (Key Concept D) When Dr. Nina Jablonski describes her discovery of the UV data collected by NASA, a
headline is visible that reads, “Ozone Depletion Raising Risk of Skin Cancer, Scientist Says.” Use this headline
and your understanding of what causes skin cancer to infer a beneficial feature of the ozone layer for
humans. Why would a depleted ozone layer increase the risk of skin cancer?
The film discusses how UV damage to skin cell DNA can lead to cancer. Because ozone depletion raises the risk of
skin cancer, one may infer that a depleted ozone layer is associated with a larger amount of UV reaching Earth’s
surface. The ozone layer is involved in blocking a substantial amount of the UV from the sun. An increase of UV in
areas with depleted ozone would lead to more damage to skin cell DNA and ultimately higher rates of skin cancer.
7. (Key Concepts B and H) Ultraviolet light can cause mutations and other damage within cells, which can hurt
an individual’s chance of surviving and leaving offspring. Some molecules can protect cells from damage by
UV. The amount of these molecules is determined by genes. Within a population, some individuals make
more of these UV-protection molecules than others. What do you predict would happen to the frequency of
the genes that cause more of the molecules to be made in a population over time? Assume all other factors
are equal.
a. The frequency would increase because individuals need the genes.
b.
The frequency would increase because individuals with the genes for more molecules would leave more
offspring.
c. The frequency would decrease because molecules are types of chemicals, and having more chemicals in
the body is harmful.
d. The frequency would stay the same because populations do not change over time.
8. (
Key Concepts B and H) Write down the evidence or reasoning you used to answer Question 7.
The problem states that increased UV can negatively affect a person’s fitness. People with more of the molecules
that protect cells from UV would leave relatively more offspring. Because the amount of these molecules is
determined genetically, the number of individuals in the next generation with the genes that cause more of the
molecules would increase. This is an example of natural selection.
9. (Key Concepts A, B, C, and F) Describe your ideas about why indigenous groups of people in different parts of
the world have different skin colors from other groups of people.
In high-UV environments, darker skin offers protection from the damaging effects of UV radiation, especially to DNA
and the valuable nutrient folate. In low-UV environments, there is a trade-off between protection from UV and the
need for some UV absorption for the production of vitamin D. These low-UV environments favor lighter skin.
10. (Key Concept D) Describe at least three different types of evidence that support your ideas for Question 9.
Evidence to support the claims in Question 9 could include
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•
data from anthropology documenting the skin colors of indigenous people,
•
data from physical science showing the pattern of UV across the globe,
•
mathematically showing a relationship between skin color and UV levels,
•
cell biology studies showing the effect of UV on DNA or folate,
•
cell biology studies showing the protective effect of melanin, and
•
data showing that in the past indigenous people with darker skin color in high-UV areas and people with lighter
skin color in low-UV areas had higher fitness.
U
se the following scenario to answer Questions 11 and 12.
A biologist was studying two indigenous groups of people from different areas of the world. The first population
was from equatorial Africa. The second population was from northern Europe. The biologist was studying a gene
that affects skin color. The biologist examined the gene in 100 people from each population. She kept track of how
many different forms (or alleles) of the gene she found in each population. The results are in the graph in Figure 1.
1
1. (Key Concept E) Describe the major pattern in the data in Figure 1.
In population 1, there are only three different alleles, whereas in
population 2 there are 16 different alleles for the same gene.
12. (Key Concept E) Make a claim about the strength of stabilizing
natural selection on this gene in the two populations. Use
evidence from the graph (Figure 1) to support your claim.
In the film, Dr. Jablonski explains that “the absence of MC1R
diversity in African populations indicates that, in that part of the
world, there is strong negative selection against any alleles that
would alter dark skin.” This means that there was a selective
pressure to remove alleles from the population that caused a
different skin color. The removal of harmful alleles is called
negative selection. Another term for negative selection is
“stabilizing selection.”
Using this reasoning, a valid claim is that population 1 seems to have strong stabilizing selection. The evidence is the
much lower number of alleles in the population (which means a lower diversity). Population 2 shows weaker
stabilizing selection as there are many different alleles for the gene.
13. (Key Concepts A, C, and F) Describe how having dark skin may have provided an advantage in survival and
reproduction to people thousands of years ago in some places in the world but not in others.
As explained in Question 9, in high-UV environments, darker skin offers protection from the damaging effects of UV
radiation, especially on DNA and the valuable nutrient folate. Thus, people with dark skin in these high-UV
environments would have had an advantage in survival and reproduction. But in low-UV environments, there is a
trade-off between protection from UV and the need for some UV absorption for the production of vitamin D. People
with lighter skin in these environments would have higher rates of survival and reproduction.
Figure 2. This graph appears as Figure 1
on the student handout.
0
2
4
6
8
10
12
14
16
18
Population 1
(equitorial
Africa)
Population 2
(Northern
Europe)
Number of Different
Alleles of the Gene
The Biology of Skin Color
Evolution Revised January 2018
www.BioInteractive.org Page 10 of 11
Film Guide
Educator Materials
14. (Key Concept A) Biologists sometimes say that “natural selection depends on the specific environment where
a species lives.” What does this statement mean?
a. If populations of a species are in different environments, traits that individuals need to meet their needs
in each environment will appear.
b.
Traits can be helpful or harmful. If populations of a species are in different environments, some traits that
are helpful in one environment might be harmful in another environment.
c. Traits are always either helpful or harmful, and the environment of a population does not matter. If
populations of a species are in different environments, the same traits will always be helpful.
d. Species were formed to perfectly match their environment. The traits of individuals in a species depend
on the specific environment in which they were created.
1
5. (Key Concept F) Describe how UV light is harmful to people but can also be necessary.
UV causes damage to DNA and to folate and can be harmful in both cases. However, UV is also needed to start the
synthesis of vitamin D, which is necessary for bone and immune health.
16. (Key Concept G) How does the synthesis of melanin by melanocytes help these cells with their major function
in skin?
One of the main functions of the skin is to act as a protective barrier against the harmful elements of the
environment, such as ultraviolet radiation. Melanocytes manufacture melanosomes, which in turn synthesize
melanin. Melanin protects the DNA of skin cells by forming protective coverings over the nucleus of skin cells. The
absorption of UV by melanin protects folate in the circulatory system under the skin.
17. (Key Concept B) The graph in Figure 2 summarizes the age at which people are diagnosed with melanoma, the
most serious form of skin cancer. Use the graph to explain why protection from skin cancer may not explain
the strong selective pressure for dark skin in high-UV areas.
The graph demonstrates that most people contract melanoma after their childbearing years. Therefore, most people
who contract melanomas would have already had children and passed their genes on to them, reducing the strong
selective pressure for darker skin. The role of skin cancer as a factor that may have influenced the evolution of skin
color is still an area of active scientific discussion and study.
Figure 3. This graph appears as Figure 2
on the student handout. (Source:
National Cancer Intitute’s Surveillance,
Epidemiology, and End Results Program
http://seer.cancer.gov/statfacts/html/me
lan.html).
ADDITIONAL RESOURCES
The Smithsonian Institution website at http://humanorigins.si.edu/education/teaching-evolution-through-human-
examples provides additional resources on the biology of skin color.
The Biology of Skin Color
Evolution Revised January 2018
www.BioInteractive.org Page 11 of 11
Film Guide
Educator Materials
REFERENCES
Barsh, Gregory S. “What Controls Variation in Human Skin Color?“ PLoS Biology 1, no. 1 (October 13, 2003): 19–22.
Dooley, Christopher M., Heinz Schwarz, Kaspar P. Mueller, et al.
“Sl
c45a2 and V
-ATPa
se Are Regulators of Melanosomal pH
Homeostasis in Zebrafish, Providing a Mechanism for Human Pigment Evolution and Disease.” Pigment Cell & Melanoma
Research 26, no. 2 (March 2013): 205–217.
Hol
ick, Michael F. “Vitamin D Deficiency.” New England Journal of Medicine 357, no. 3 (July 19, 2007): 266–281.
Jablonski, Nina G. “The Evolution of Human Skin and Skin Color.” Annual Review of Anthropology 33 (2004): 585–623.
Jablonski, Nina G. Living Color: The Biological and Social Meaning of Skin Color. University of California Press, 2012.
Kaidbey, Kays H., Patricia Poh Agin, Robert M. Sayre, and Albert M. Kligman. “Photoprotection by Melanin—A Comparison of
Black and Caucasian Skin.” Journal of the American Academy of Dermatology 1, no. 3 (September 1979): 249–260.
Kenny, Eimear E., Nicholas J. Timpson, Martin Sikora, et al. “Melanesian Blond Hair Is Caused by an Amino Acid Change in
TYRP1.” Science 336, no. 6081 (May 4, 2012): 554.
Lin, Jennifer Y., and David E. Fisher. “Melanocyte Biology and Skin Pigmentation.” Nature 445, no. 7130 (February 22, 2007):
843–850.
Praetorius, Christian, Christine Grill, Simon N. Stacey, et al. “A Polymorphism in IRF4 Affects Human Pigmentation through a
Tyrosinase-Dependent MITF/TFAP2A Pathway.” Cell 155, no. 5 (November 21, 2013): 1022–1033.
Raghavan, Maanasa, Michael DeGiorgio, Anders Albrechtsen, et al. “The Genetic Prehistory of the New World Arctic.” Science
345, no. 6200 (August 29, 2014): 1255832.
Sturm, Richard A., and David L. Duffy. “Human Pigmentation Genes under Environmental Selection.” Genome Biology 13
(2012): 248.
Thomas, Daniel B., Kevin J. McGraw, Michael W. Butler, et al. “Ancient Origins and Multiple Appearances of Carotenoid-
Pigmented Feathers in Birds.” Proceedings of the Royal Society B: Biological Sciences 281 (June 25, 2014).
doi:10.1098/rspb.2014.0806.
AUTHORS
Written by Paul Strode, PhD, Fairview High School
Edited by Laura Bonetta, PhD, HHMI, and Stephanie Keep, Consultant
Reviewed by Paul Beardsley, PhD, Cal Poly Pomona and Nina Jablonski, PhD, Penn State University