condensed version
Evolution of Human Skin Color
student workbook
Developed as a Part of the
Teaching Evolution through Human Examples Project
Smithsonian
National Museum of Natural History
Name: _____________________________________________________
Period: _________________
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Paul M. Beardsley, California State Polytechnic University, Pomona, Center for Excellence in Mathematics
and Science Teaching, Biological Sciences Department, Lead Author
K. David Pinkerton, Independent Education Consultant
Barbara Resch (Copyediting)
National Museum of Natural History
Briana Pobiner, Human Origins Program, Project Principal Investigator
Rick Potts, Human Origins Program, Project Co-Principal Investigator
Advisory Committee
Constance Bertka, Science and Society Resources, LLC
Juliet Crowell, National Science Resources Center, Smithsonian Institution
Jay Labov, National Academy of Sciences
Dennis Liu, Howard Hughes Medical Institute
Sharon Lynch, The George Washington University, Graduate School of Education and Human Development
Lee Meadows, University of Alabama at Birmingham, Department of Curriculum and Instruction
Bill Watson, Office of Catholic Schools at the Diocese of Camden, Director of Curriculum and Assessment,
Senior Personnel and Data Analyst
Design Team
Nikki Chambers, West High School (Torrance, CA)
Chelsea Crawford, Fremont Union High School District (Northern CA)
Holly Dunsworth, Department of Anthropology, University of Rhode Island
Mark Terry, The Northwest School (Seattle, WA)
Jennifer Clark, Human Origins Program, Illustrator
Norma Oldfield, Human Origins Program, Illustration Research Assistant
Anna Ragni, Human Origins Program, Illustration Research Assistant
Reviewer
Nina Jablonski, Penn State University, Department of Anthropology, Evan Pugh Professor of Anthropology
© 2015 Smithsonian Institution. Permission to copy and distribute is freely granted for educational, noncommercial use only.
This material is based upon work supported by the National Science Foundation under Grant No. 1119468.
Any opinions, findings, and conclusions or recommendations expressed
in this material are those of the author(s) and do not necessarily reflect the
views of the National Science Foundation.
Contents
Student Workbook Pages by Lesson1
Lesson 1: Can Everybody Tan? . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Lesson 2: The Melanin Connection . . . . . . . . . . . . . . . . . . . . . 14
Lesson 3: Developing a Fuller Explanation for Skin
Color Evolution in Humans . . . . . . . . . . . . . . . . . . . . 30
Lesson 5: Explaining Human Skin Color Evolution . . . . . . . . . 42
Credits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
1. A
lthough Lesson 1, Can Everybody Tan?, and Lesson 2, The Melanin Connection, were modified for the condensed
version, all the original lesson, figure, table, and master numbers have been retained. Also, Lesson 4, More Evidence
for Skin Color Evolution in Humans, does not appear in the condensed version.
Evolution of Human Skin Color
Contents
3
L ess o n 1
Can Everybody Tan?
1. Do animals tan? Explain your thinking, using examples of specific animals that support
your answer.
2. Briefly describe your ideas for designing an experiment to test the question, “Does exposure to ultraviolet (UV) or visible light affect a shark’s skin color? In other words, can
sharks get a suntan?”
3. Write your predictions for the shark experiment described in class.
4. Write down answers to the following questions.
a. What evidence is there that hammerhead sharks can get a tan?
b. What advantage does the melanin increase give to sharks that tan?
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c. Why might studying tanning in shark species be important for understanding
processes in humans?
5. Record your initial ideas to the questions below. You will share your answers with a partner in the next step while following a specific protocol.
a. How much do you tan? How about your friends?
b. Are there differences in humans’ ability to tan? What is your evidence?
c. Do you think the ability to tan is affected by a person’s genes? Why or why not?
d. At some point, the ability to tan developed in the lineage that led to humans.
What do you think first caused differences in the ability to tan?
e. How might the ability to tan (or lack of the ability to tan) have affected survival or how many surviving children people had in the past?
f. Do you think the ability to tan is linked to the historical geographic region of
different people’s ancestors? Explain your answer.
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Master 1.2, The Skinny on Skin
In the pages that follow, you’ll be introduced to the fascinating organ of skin, using both images
and text. First read the tasks you’ll need to accomplish. Knowing these tasks will help you focus
your efforts as you read and analyze the following images and text. When you are finished reading,
complete the tasks.
Tasks
1.2.1. Make a sketch of the epidermis. Indicate where melanocytes are found and briefly explain
why melanosomes are important for the topic of this unit.
1.2.2. Make a diagram that defines and compares the following terms: “melanocyte,” “melanin,”
“melanosome,” “melanin granule,” and “melanogenesis.”
1.2.3. Briefly describe the role of melanin in shaping human skin color.
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1.2.4. Create a flowchart to summarize the tanning process.
1.2.5. On Master 1.3, add to the Venn diagram that summarizes the similarities and differences
between facultative and baseline skin color.
Skin is a complex organ that performs several important functions, and it is composed of many
cell and tissue types. One of the main functions of the skin is to act as a protective barrier to keep
moisture in and other things out, like parasites. Skin also absorbs and filters ultraviolet (UV) light,
which can damage cells and mutate DNA.
One way the skin protects against UV
light is by developing a thick outer layer of
dead skin cells, called the stratum corneum.
Find (1) on the diagram (Figure 1). Cells
in this layer contain a large amount of the
protein keratin. These layers of cells constantly flake off, to be replaced by new cells,
pushing up from below, in a 30-day cycle.
Prolonged exposure to UV often results in a
thicker stratum corneum.
Interspersed among other cells in the
bottom layer of living cells underneath
the dead skin are melanocytes. The layer of
cells that contain melanocytes is sometimes Figure 1. Detailed image of human skin. The layer that is critical
called the pigment layer (2) and is more
for skin color and tanning is the epidermis.
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formally called the Malpighian layer after the famous Italian scientist Marcello Malpighi from the
17th century. Among other discoveries, Malpighi found the capillaries that others had predicted
but never saw. The stratum corneum and the living layer of cells make up the epidermis (3). Melanocytes make the pigment melanin, which gives skin its color and protects against UV damage.
Figure 2. Close-up view of the region of skin cells that contain melanocytes and the process by which melanosomes
are formed.
Melanin is a common pigment in a wide range of living organisms. In humans, melanin is
found in skin, hair, the iris, and many other places. Melanin absorbs a wide range of wavelengths
of light, including UV. The most common form of biological melanin is eumelanin (black-brown).
People who produce mostly eumelanin tend to have brown or black hair and dark skin that tans
easily. Eumelanin also protects skin from damage caused by UV radiation in sunlight.
A second form of melanin is pheomelanin (red and yellow). People who produce mostly pheomelanin tend to have red or blond hair, freckles, and light-colored skin that burns easily. Because
pheomelanin does not protect skin from UV radiation, people with more pheomelanin have an
increased risk of skin damage caused by sun exposure.
Melanin is formed in a reaction involving the amino acid tyrosine, catalyzed by the enzyme
tyrosinase. Tyrosinase is synthesized in the rough endoplasmic reticulum, and then it accumulates
in the Golgi apparatus. The tyrosinase-filled vesicles that bud off from the Golgi are called melanosomes. Melanosomes go through stages of development, and melanin is eventually produced (Figure
2). Melanosomes that contain melanin migrate to the tips of the melanocyte and are transferred to
other cells in the Malpighian layer of the skin. Eventually, tyrosinase activity ceases and the resulting structure is called a melanin granule.
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Melanin and Baseline Skin Color
Although some people’s skin color can change with exposure to UV light, everyone has a baseline skin color that is seen in the absence of sun exposure. Biologists typically measure this skin
color under the armpit. This baseline skin color is called constitutive skin color. The root for the
word “constitutive” means “to be a part of; to compose.” In biology, constitutive often denotes an
enzyme or a biochemical pathway in which the default state is “on.”
A person’s skin color is determined predominantly by the amount and distribution of melanin.
These varying amounts and distributions of melanin in skin affect the amount of UV radiation
absorbed by skin. In general, the more pigment, the more light that is absorbed and the darker the
skin appears. To learn more about the role of melanin and melanosomes in affecting skin color,
watch the video How We Get Our Skin Color from the Howard Hughes Medical Institute at http://
www.hhmi.org/biointeractive/how-we-get-our-skin-color.
Figure 3. This cartoon shows the different distribution and structure of melanosomes
(solid black, red, or pink ovals) in three generic types of human skin. Heavily pigmented
skin (far left) has more total melanin, and the melanin is aggregated in larger particles than
it is in lightly pigmented skin (far right). Source: Redrawn from Barsh, 2003.
Figure 4. There is near infinite variation between
each “color” of skin.
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Melanin and Facultative Skin Color (Tanning)
Human skin can change color when exposed to UV light or other environmental factors. The
resulting skin color is called facultative skin color. The root for “facultative” means “ability.” In
biology, facultative means something that the organism has the ability to do, but certain circumstances are needed for the process to begin. Another example is facultative anaerobes (microbes that
have the genetically determined biochemical processes to live in environments with low oxygen,
but the default mode for these processes is “off.”) In this case, facultative skin color is the skin color
the organism is able to produce under certain conditions that lead to tanning. Tanning changes the
chemical composition of melanin in the skin and increases the amount and size of melanin produced by melanocytes. Thus, facultative skin color is darker than baseline skin color.
To better understand how UV light interacts with skin, we need to look at the properties of different wavelengths of light. The light from the Sun that reaches Earth can be categorized as infrared (heat), visible light, and UV. Ultraviolet is energetic enough to break some chemical bonds in
molecules commonly found in living organisms. Ultraviolet light is further broken down into three
categories, also based on wavelength.
• UV-A (315–400 nanometers, or nm) is lower energy, and at sea level, 99 percent of
UV is in this lower-energy form.
• UV-B (280–315 nm) is higher energy, causes damage to DNA, and causes
sunburns.
• UV-C (100–280 nm) is filtered out by the atmosphere and does not affect us.
Melanin helps protect cells from damage by UV because it absorbs UV light and transforms
about 99.9 percent of UV radiation into heat. A higher concentration of melanin in skin provides
greater protection from UV.
Figure 5. UV-A and UV-B both affect melanocytes and melanin.
The different types of UV affect the skin differently in the tanning process. UV-A is lower in
energy compared to UV-B and causes less damage. UV-A oxidizes and redistributes melanin that is
already present in the skin, causing it to darken within hours. This effect is temporary and does not
lead to increased production of melanin, so the total amount of melanin does not change.
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UV-B is higher in energy than UV-A and causes damage to cells. One particular type of damage
is direct damage to the DNA double helix. In response to damaged DNA, cells in the human body
can increase the production of melanin (melanogenesis). Melanogenesis leads to delayed tanning,
which is facultative skin color. The effects of facultative tanning become visible about 72 hours after
exposure. This change in skin color lasts much longer than the temporary change caused by UV-A
(oxidation of existing melanin). Facultative skin color changes protect against UV-caused skin
damage and sunburn because of increased melanin density in the skin. Though UV-B is damaging
to cells, it plays an essential role in the production of vitamin D in the skin.
In addition to tanning, some people develop sunburn in response to excessive UV-B. Sunburns
appear as reddish and painful skin, and the skin can blister. The sunburn process is an inflammatory and protective immune response.
Tanning: What Are the Risks?
Though some skin tones can tan, there is still a substantial risk associated with tanning. Evidence
suggests that tanning greatly increases your risk of developing skin cancer; leathery, wrinkled skin;
and eye damage. It is also not true that getting a tan will protect your skin from sunburn or other
skin damage. “Tanned” skin gives a sun protection factor (SPF) between 2 and 4, whereas dermatologists recommend an SPF of 15 for full protection.
Still, some people seek the changes in skin color that accompany the tanning process and look
for convenient ways to “get a tan” other than direct sun exposure. Maybe you have seen a tanning
salon in your area. Many of your peers around the country use these facilities. A national study in
2011 called the Youth Risk Behavior Surveillance System, conducted by the Centers for Disease
Control and Prevention and other local agencies and tribal governments, found that 13 percent of
all high school students and 32 percent of girls in the 12th grade reported using indoor tanning
salons.
There are significant risks to indoor tanning, and it is not safer than tanning in the sun. A 2002
study demonstrated a 50 percent increase in the risk for basal cell carcinoma and a 100 percent
increased risk of squamous cell carcinoma associated with indoor tanning. Another study in 2007
showed that people who start tanning before the age of 35 have a 75 percent increased risk of
developing melanoma, the deadliest type of skin cancer. Wrinkles, eye damage, and changes in skin
texture also accompany indoor tanning.
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Master 1.3, Venn Diagram
Figure 1. This master helps you compare and contrast facultative and baseline (constitutive) skin color. Use Master
1.2 to elaborate on each term in the Venn diagram and to add additional terms in the correct section of the diagram.
Make margin comments that explain why terms are placed where they are.
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6. Examine Master 1.4. Use the information in Figure 1 to complete the following tasks.
a. Briefly discuss how defining skin “types” relates to the ways skin color actually
varies among people.
b. Use the data in Figure 1 to make a claim about skin type and the ability to tan.
Master 1.5, Scoring Rubric for Tanning Research Plan
Table 1.
Task
High performance
Medium performance
Low performance
Explain the role of tanning in
the evolution of human skin
color. (50%)
Explanation offers an
unambiguous and clear
connection between tanning
and the evolution of human
skin color. The explanation
contains a claim, evidence,
and a rationale that links evidence supporting the claim
to one or more overarching
scientific principles.
Explanation offers a connection between tanning and
the evolution of human skin
color. The explanation contains a claim and evidence.
Explanation makes a claim,
but it has weak evidence to
support the claim. It exhibits
lapses in logic.
Design and write a research
plan to validate your explanation. (50%)
Plan includes a justification of the research, which
includes references, a scientific question, a step-by-step
procedure for the proposed
experiment, and a discussion
of how the results will be
analyzed. Also included are
suggestions of evidence that
would falsify the explanation.
Plan includes a scientific
question and a step-by-step
procedure for the proposed
experiment.
Plan only contains a step-bystep procedure.
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L ess o n 2
The Melanin Connection
Part 1: Melanin and Color—Genetic Variation
1. Quietly reflect on the following question: “What determined your baseline (constitutive)
skin color?” Record your thoughts below.
2. What data would you collect to support or refute the claim that baseline skin color is
strongly inherited?
3. Read Master 2.2 and compare your experimental design to the design used by researchers
in the study. As you read, consider the focus question: “What causes the variation in skin
color of different groups of humans?”
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Master 2.2, Skin Color: A Strong Genetic Component?
Most traits vary in a population, and all traits have both genetic and environmental influences. A
measure of the degree to which the difference in a trait is controlled by genetics is called heritability, which has the symbol h2. Values for h2 range from 0 to 1. A value of 0 means that the differences in traits are caused by the environment. A value of 1 means that the differences in traits are
only caused by genetic differences. Heritability usually falls somewhere between these two extremes.
If a trait is at least somewhat heritable, it can be affected by natural selection.
Biologists estimate heritability by comparing a trait among close relatives, for example, in parents and offspring. If genetically close relatives have similar values for the trait, heritability is likely
higher, and this can be the foundation for research aimed at identifying the specific genes associated
with the trait.
Scientists measured skin reflectance in a group of people with a broad range of skin colors in eastern Peru to estimate the heritability of this trait. The sample included people who ranged in age from
2 to 64 years and included 43 father-son, 42 father-daughter, 62 mother-son, and 70 mother-daughter
pairs. The sample also consisted of 57 brother-brother, 60 sister-sister, and 139 brother-sister pairs.
People in the study lived in the same geographic area and had similar sun exposure.
Results: The study indicated a heritability of 0.55 for skin reflectance.
Analysis Questions
2.2.1. What does the phrase “human skin color has a strong genetic component” mean?
2.2.2. How did the evidence researchers collected compare to the evidence you would have
collected?
2.2.3. The heritability for skin reflectance is not 1.0. What do you think this means?
2.2.4. A common misconception about evolution says that organisms evolve because they
acquire traits during their lifetimes and pass them along to their offspring. Make a statement that is a clear example of this misconception. The statement will involve either
tanning or baseline (constitutive) skin color differences.
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4. As you participate in a brainstorming review, record notes about major concepts related
to genes and alleles.
You will explore a simple model to examine how many genes may be involved in causing
differences in skin color among different people. The assumptions of the model follow.
• Each gene involved only has two alleles.
• One allele of a gene codes for a specified amount of a dark color, the
other allele for no color.
• Each allele acts in an additive fashion. In other words, one allele is not
dominant over the other.
If a person is homozygous for the “no color” allele, the person’s skin color
will be white.
If the person is homozygous for the dark allele, the person will have twice
the specified amount of the dark color.
If the person is heterozygous, the person will be a shade in between that of
the two homozygotes.
• Each different gene affects color to the same degree.
n
n
n
5. Table A describes an increasing number of genes that affect differences in skin color. For
each, make a prediction of the number of shades or classes of color that would result.
Record the results of the demonstration.
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Table A.
Scenario
Prediction
One gene, with two alleles
Two genes, each with two alleles
Three genes, each with two alleles
Four genes, each with two alleles
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Results
Master 2.3, The Genetic Basis of Skin Color
Data Set 1: Variation in Skin Color in Humans Is Affected by Many Genes
Initial studies of the genetics of skin color focused on genes that code for proteins in the biosynthetic pathway to produce melanin. Because melanin is the primary pigment responsible for skin
color in humans, genes that influence melanin also influence skin color. Studies in families showed
that some of the genes in this pathway correlate with skin tone. With the advent of rapid DNA
sequencing and genotyping technologies, biologists have been able to compare thousands of genes
and even whole genome scans across different groups of humans. These studies identified at least
six genes that have a major influence on skin color.
More-recent scans that examined a much broader selection of genes have identified even more
genes that are involved in skin color variation (at least 14, but likely many more).
Analysis Question
2.3.1. In what ways do these data support or refute the predictions you made about the number
of genes involved in differences in skin color?
Data Set 2: Some Genes Have a Large Impact on Variation in Skin Color in Some Populations
Evidence in the Data Set 1 confirms that many genes influence the difference in skin colors across
different groups of people. Studies of variations of particular genes have been useful in identifying
particular alleles that account for large pigmentation differences between groups of people whose
ancestors come from different geographic areas.
Recent studies explored a gene called SLC24A5, which is found on human chromosome 15
(Figure 1). (The gene’s full name is solute carrier family 24 [sodium/potassium/calcium exchanger],
member 5.) The gene has nine exons and encodes a protein that has 513 amino acids. Bioinformatic analysis of the protein suggests that it functions as a membrane transport protein that is
involved in the exchange of sodium and calcium ions.
Figure 1. The location of the SLC24A5 gene on chromosome 15. Note that all segments of the chromosome are connected, but some areas have uncondensed DNA which is not visible when the chromosome is condensed. The gene
is found between nucleotides 48,120,971 and 48,142,391. Source: Image adapted from Genetics Home Reference,
2013, SLC24A5.
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In people of African descent, one allele
for this gene, called the G allele, is commonly
found. An alternative allele, called the A allele,
is common among people of European descent.
The A allele has an adenine at a specific nucleotide, whereas the G allele has a guanine instead.
This difference in DNA results in an alanine at
amino acid 111 for the G allele and a threonine
for the A allele.
The differences in the frequency of these
alleles are dramatic. In Europeans, the frequency
of the A allele is 0.96, whereas the frequency in
West Africans is 0.09. This difference in allele
frequency is unusual. For many other genes,
West Africans and Europeans have similar allele
frequencies. People with ancestors from even
very distant geographic regions have far more
genetic similarities than differences.
Scientists measured the relative amount of
melanin in a group of people with a range of
skin colors with both African and European
ancestries (Lamason et al., 2005). The melanin
data were then sorted by the individuals’ genotype for SLC24A5 (Figure 2).
a.
b.
c.
Figure 2. Relative melanin values versus genotype for
SLC24A5. A higher “Melanin index” value means that
the individual had darker skin. Source: Image adapted
from Lamason et al., 2005.
Analysis Question
2.3.2. Use the data in Figure 2 to make a claim about how a person’s genotype at SLC24A5
affects the amount of melanin in the person’s skin and the person’s skin color.
Data Set 3: Scientists Learned about the Function of SLC24A5 by Studying Zebrafish, Part 1
Scientists who study skin color were not initially focused on the SLC24A5 gene. The possible
role of this gene in skin color was instead first suggested by a group of cancer researchers who
were studying a mutant variety of zebrafish called “golden” zebrafish (Figure 1a, 1b) to find genes
involved in cancer (Lamason et al., 2005). Fish with the golden phenotype have delayed and
reduced development of melanin pigmentation. Detailed studies show that the golden zebrafish
have a reduced number, size, and density of melanosomes.
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a.
b.
c.
d.
e.
f.
Figure 3. The photos show a. the wildtype phenotype and b. the golden zebrafish
phenotype. The insets in a. and b. show the
cells that contain melanin in the fish (called
melanophores in fish). Scale bars in a. and b.
represent 5 millimeters (mm), in the insets,
0.5 mm. The transmission electron micrographs in c. and e. show the melanin-containing cells from the wild-type fish, and the
transmission electron micrographs in d. and
f. show the cells from the golden fish. Source:
Image adapted from Lamason et al., 2005.
Analysis Question
2.3.3. Compare the images of the cells in the zebrafish that contain melanin with the cells in
people with different skin colors (see Master 1.2, Figure 3). What similarities and differences do you notice?
Data Set 4: Scientists Learned about the Function of SLC24A5 by Studying Zebrafish, Part 2
Researchers determined the DNA sequence for the gene that is mutated in the golden zebrafish
(Lamason et al., 2005). They then sequenced the same gene in a large number of other vertebrates,
including humans. The researchers quickly recognized that the sequenced gene was very similar
to the SLC24A5 gene in humans. The amino acids that make up a portion of the gene are shown
in Figure 4. The one amino acid that differs between people with the A allele and the G allele is
highlighted.
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Figure 4. Much of this small section of the protein encoded by the gene SLC24A5 is the same across several species
(shown in black). Each of the 20 amino acids found in humans is assigned its own one-letter code. For example, A
stands for alanine. The one amino acid that differs between people with the A allele and the G allele is shown in red.
The mutation that causes the golden phenotype in zebrafish is further upstream from these amino acids. Source: Image
adapted from Lamason et al., 2005.
Analysis Questions
2.3.4. There is a high similarity of the sequences from different species. What can you deduce
from this about the importance of the function performed by the protein encoded by the
SLC24A5 gene?
2.3.5. What is the simplest explanation for the reason that the gene and protein from SLC24A5
are so similar in these species?
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Researchers constructed a messenger RNA (mRNA) fragment that was transcribed from the human
DNA sequence for the SLC24A5 G allele and injected it into golden zebrafish embryos, where it
was translated (Lamason et al., 2005). They then compared the embryo to wild-type zebrafish and
golden zebrafish. The results are shown in Figure 5.
Figure 5. Zebrafish embryos showing the amount
of melanin in different fish. The messenger RNA
(mRNA) transcribed from the human SLC24A5 gene
was injected into the fish on the left. Source: Image
adapted from Lamason et al., 2005.
Analysis Questions
2.3.6. 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 each species?
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Figure 6. A phylogeny for some of the major groups of vertebrates. Branch lengths are proportional to time. Note:
“mya” means “millions of years ago.”
2.3.7. Figure 6 shows a phylogeny of some of the major groups of vertebrates based on fossils
and sequences from other regions of DNA. Use information on the diagram to estimate
the time when the SLC24A5 gene evolved.
Reference
Lamason, R. L., Mohideen, M.-A. P. K., Mest, J. R., Wong, A. C., Norton, H. L., Aros, M. C., Jurynec, M. J., Mao,
X., Humphreville, V. R., Humbert, J. E., Sinha, S., Moore, J. L., Jagadeeswaran, P., Zhao, W., Ning, G., Makalowska, I., McKeigue, P. M., O’donnell, D., Kittles, R., Parra, E. J., Mangini, N. J., Grunwald, D. J., Shriver,
M. D., Canfield, V. A., & Cheng, K. C. (2005, December 16). SLC24A5, a putative cation exchanger, affects
pigmentation in zebrafish and humans. Science, 310(5755), 1782–1786.
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Part 2: Changes in Color in Populations over Time
1. Write your best answer to the following question: “How would biologists explain how
the mice on the lava flow evolved black fur?” Include all the elements you think are
needed for a full explanation.
2. Would biologists say that the mice changed because they wanted or needed to change?
Why?
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Master 2.5, Explanation Based on Natural Selection
Task
2.5.1. Use Table 1 to help you summarize the evidence needed for a full explanation based on
natural selection. You can use this table to help you analyze any specific case.
Table 1. Natural selection explanation.
Principle
Definition
Variation
Individuals in a population or group differ for some
trait of interest.
Inheritance
The variation for the trait of interest is at least partially inherited (passed from parents to offspring). The
origin of the variation stems from mutations (broadly
speaking) and the recombination that accompanies
sexual reproduction. The genetic variation may have
arisen many generations in the past.
Selection
Because of biotic potential, more offspring will be
born than can survive. The outcome of this fact is
competition among individuals. As a result, some individuals with a trait survive and leave relatively more
offspring compared to individuals that do not have the
trait. Selection depends on the specific context of a
species. Traits that are beneficial in one environment
may cause problems in another environment.
Adaptation
The frequency of the trait that improves fitness will
increase in the population over time, as will the
alleles that affect the trait. This process can take
many generations and extend over very long periods
of time.
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Evidence
Master 2.6, Mice, Melanin, and Natural Selection
So far, you have started to investigate the genetic basis for differences in skin color among different
people. You should recognize that melanin plays a critical role in skin color. Melanin is a group of
molecules found in a broad diversity of organisms, and it has a broad range of functions. It causes
the black color of both bird feathers and the ink shot out by some squid and octopuses as a defense
mechanism. Melanin plays a role in protecting microorganisms against damage from high temperatures, heavy metals, oxidizing agents, and biochemical threats. These molecules are also involved in
the immune system response of some invertebrates.
In mice, melanin can play a critical role in determining coat color. The light brown or tan color
of several species of mice is caused by hairs having a banded black and yellow pattern, which makes
the hair appear tan. In black mice, also called melanic mice, the hair is not banded but is solid
black.
The American Southwest is a fantastic place to study mice with different coat colors. Lava flows
about 1.5 million years ago (mya) created patches of dark rock among the surrounding light-colored sand. Researchers noticed that black rock pocket mice seemed to be more common on the
dark lava flows, whereas the tan rock pocket mice were more common on the light-colored sand.
The researchers wondered if this pattern could be explained by natural selection. Use the following
data sets to better understand how the researchers used evidence to build an argument for natural
selection. Fill in your “Natural Selection Explanation” table as you analyze each data set.
Data Set 1
Biologists knew from museum specimens that rock pocket mice of different colors had lived in
southern Arizona for a long time. Figure 1 shows mice with different fur colors against both light
and dark backgrounds.
Figure 1. Two main classes of fur color are seen in rock pocket
mice in southern Arizona. In this image, the two color forms are
in light and dark environments. Source: Nachman, Hoekstra,
& D’Agostino, 2003.
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The researchers collected rock pocket mice in traps from a number of different sites in southern
Arizona, some from dark environments and some from light environments (Hoekstra, Drumm, &
Nachman, 2004). The frequency of light and dark colors in different sites is shown in Figure 2.
Figure 2. The x-axis shows six different sites where rock
pocket mice were collected. The color bar indicates whether
the soil is dark (black) or light (white). The y-axis shows
the frequency of black mice. Source: Hoekstra, Drumm, &
Nachman, 2004; reproduced with permission from H. E.
Hoekstra.
Data Set 2
Researchers wanted to quantify fur color instead of just using color categories. They used a spectrophotometer to measure the reflectance of dark and light rock pocket mice across six sites (Hoekstra,
Drumm, & Nachman, 2004). A lower value for reflectance means darker fur.
The researchers knew that the determination of fur color in mice is influenced by the action
of many genes, but they had evidence that alleles for one particular gene accounted for a large
proportion of the differences between dark and light mice in these populations. The gene, Mc1r,
codes for the melanocortin 1 receptor. The melanocortin 1 receptor controls which type of melanin
is produced by melanocytes. When the receptor is activated, it triggers a series of chemical reactions inside melanocytes that stimulate these cells to make eumelanin, the pigment associated with
darkening. If the receptor is not activated or is blocked, melanocytes make pheomelanin instead of
eumelanin.
Within these populations of rock pocket mice, there were two forms of the Mc1r gene: the D
or the d allele. The Mc1r D allele differs from the d allele by four amino acids. Figure 3 shows the
relationship between reflectance and Mc1r genotype (Hoekstra, Drumm, & Nachman, 2004).
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Figure 3. The genotype of rock pocket mice for
the Mc1r gene affects fur color. Source: Hoekstra,
Drumm, & Nachman, 2004; reproduced with
permission from H. E. Hoekstra.
Data Set 3
Researchers next measured the frequency of the two alleles for the Mc1r gene from rock pocket
mice in light- and dark-colored backgrounds (Hoekstra, Drumm, & Nachman, 2004). The results
are shown in Figure 4.
Figure 4. The genotype of rock pocket mice for the Mc1r gene is different
when the color of the underlying substrate changes. Source: Hoekstra,
Drumm, & Nachman, 2004; reproduced with permission from H. E.
Hoekstra.
Though researchers did not measure predation rates in this experiment, previous experiments in
deer mice suggested that dark-colored mice had a lower risk of predation from owls when they were
on a dark background.
References
Hoekstra, H. E., Drumm, K. E., & Nachman, M. W. (2004). Ecological genetics of adaptive color polymorphism in
pocket mice: Geographic variation in selected and neutral genes. Evolution, 58(6), 1329–1341.
Nachman, M. W., Hoekstra, H. E., & D’Agostino, S. L. (2003, April 18). The genetic basis of adaptive melanism in
pocket mice. Proceedings of the National Academy of Sciences of the United States of America, 100(9), 5268–5273.
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3. Use Table B to fill in the evidence you have compiled so far to make a case for the evolution of skin color in humans and what evidence you still need.
Table B. Natural selection explanation.
Principle
Definition
Variation
Individuals in a population or group differ for some trait of
interest.
Inheritance
The variation for the trait of interest is at least partially
inherited (passed from parents to offspring). The origin of
the variation stems from mutations (broadly speaking) and
the recombination that accompanies sexual reproduction.
The genetic variation may have arisen many generations in
the past.
Selection
Because of biotic potential, more offspring will be born than
can survive. The outcome of this fact is competition among
individuals. As a result, some individuals with a trait survive
and leave relatively more offspring compared to individuals
that do not have the trait. Selection depends on the specific
context of a species. Traits that are beneficial in one environment may cause problems in another environment.
Adaptation
The frequency of the trait that improves fitness will
increase in the population over time, as will the alleles that
affect the trait. This process can take many generations
and extend over very long periods of time.
Evidence
4. Scientists studied the evolution of fur color. Other scientists are interested in the evolution of human skin color. What additional challenges do you imagine in collecting data
or designing experiments to explore skin color evolution in humans? Develop a list of at
least three additional challenges.
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L ess o n 3
Developing a Fuller
Explanation for Skin Color
Evolution in Humans
Part 1: UV-Melanin Relationship Linked to Geography
Master 3.1, Two Hypotheses about Skin Color
and Evolutionary Fitness
Identifying the factors that lead to selective advantages and disadvantages of different skin colors in
different environments is a fascinating challenge and an area of active scientific inquiry. Scientists
are currently exploring and debating a range of hypotheses. It is important to bear in mind the
role of the environment and a population’s particular circumstances. Fitness is often a compromise
balancing the many traits and physiological needs of an organism.
Below are two leading hypotheses for factors that may have played a prominent role in shaping
human skin colors. The first hypothesis involves the B vitamin folate, and it explains why darker
skin was favored in some environments. The second hypothesis involves vitamin D and explains
why lighter skin was favored in certain environments. Keep in mind that modern lifestyles and
cultural innovations have radically changed many selective environments, so the same issues may be
much less important today.
Task
3.1.1. Your task is to read through the two hypotheses and then create a diagram, such as a
flowchart, to summarize how the factor could have led to a fitness advantage for people
with certain skin colors.
Hypothesis 1: Selection for Heavier Melanization (Darker Skin)
A leading hypothesis for a selective advantage for darker skin involves the biochemical folate and its
interaction with ultraviolet (UV) radiation.
What Is Folate?
Folate is a water-soluble B vitamin that occurs naturally in food. Folic acid is the synthetic form of
folate that is found in supplements and added to fortified foods.
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How Is Folate Affected by UV Radiation?
High amounts of UV-A and UV-B can directly or indirectly cause the breakdown of folate (also
known as photolysis).
What Biologically Active Role Does It Play?
Folate functions as a coenzyme or a cosubstrate in enzyme-catalyzed reactions. The reactions it
helps catalyze play important roles in
• DNA synthesis,
• DNA repair,
• DNA methylation (important for gene regulation),
• amino acid metabolism, and
• melanin production.
What Problems Are Associated with Low Levels of Folate?
• Increased risk for pregnant women giving birth to infants with neural tube defects
• Low infant birth weight, preterm delivery, and fetal growth retardation
• Impaired DNA repair mechanisms, which increase the risk of cellular problems
• Problems in rapidly dividing cells such as those found in embryos and seminiferous
tubules (where new sperm are formed)
• Increased risk of cancer
What Is the Reasoning for Folate Being a Factor That Affects Fitness for People with Different Skin Colors?
Folate is critical to many cellular processes, especially processes that affect the development of
embryos and the development of sperm. Individuals who have mechanisms to maintain adequate
folate levels are expected to have a fitness advantage over those who cannot maintain adequate
folate levels. In areas with high amounts of UV radiation, people with more pigmentation (melanin)
would have maintained higher levels of folate because there would be less of a breakdown of folate.
Hypothesis 2: Selection for Lighter Melanization (Lighter Skin)
A leading hypothesis for a selective advantage for lighter skin involves vitamin D and the fact that
vitamin D can be synthesized in the skin when it interacts with UV-B radiation.
What Is Vitamin D?
• Vitamin D is a steroid, fat-soluble vitamin that encourages the absorption and
metabolism of calcium and phosphorus.
• Vitamin D for humans is obtained through sun exposure, food, and supplements.
People who are exposed to sufficient quantities of sunlight do not need vitamin D
supplements because sunlight promotes sufficient vitamin D synthesis in the skin.
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How Is Vitamin D Affected by UV Radiation?
• UV-B radiation penetrates the skin, and the absorbed wavelengths of light help
form vitamin D in the epidermis and dermis.
• The amount of synthesis of vitamin D in the skin depends upon the angle at which
the light strikes Earth, which changes with season, latitude, and time of day.
What Biologically Active Role Does It Play?
Vitamin D is an important regulator of many biological processes, including
• bone metabolism (vitamin D is critical for the absorption and metabolism of
calcium and phosphorus);
• the innate immune system;
• cell proliferation and differentiation; and
• normal functioning of the pancreas, brain, and heart.
What Problems Are Associated with Low Levels of Vitamin D?
• Increased probability of the bone disease rickets, which reduces fertility
• Other sources of reduced fertility
• Increased risk of infections
• Increased risk of autoimmune diseases
What Is the Reasoning for Vitamin D Being a Factor That Affects Fitness for People
with Different Skin Colors?
Low amounts of vitamin D can have a negative impact on fertility and survival. Individuals who
have mechanisms to maintain adequate vitamin D levels are expected to have a fitness advantage
over those who cannot maintain adequate levels. In areas with low and varying amounts of UV-B
radiation, people with less pigmentation (melanin) would capture more UV-B photons and make
more vitamin D.
Primary Reference
Jablonski, N. G., & Chaplin, G. (2010). Human skin pigmentation as an adaptation to UV radiation. Proceedings of
the National Academy of Sciences of the United States of America, 107, 8962–8968.
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Diagram Summarizing How Folate Could Have Led to a Fitness Advantage for People with
Certain Skin Colors
Diagram Summarizing How Vitamin D Could Have Led to a Fitness Advantage for People
with Certain Skin Colors
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Master 3.2, Patterns in UV Radiation
In Lesson 1, you learned about melanin and its role in skin color. You may also have investigated
the interaction of melanin and ultraviolet (UV) radiation using UV-sensitive beads. Now you will
explore global patterns in UV radiation. This information will further your ability to develop an
explanation for the evolution of skin color. For review, recall the following information about two
different types of UV radiation.
• UV-A (315–400 nanometers, or nm) is lower energy, and at sea level, 99 percent of
UV is in this lower-energy form.
• UV-B (280–315 nm) is higher energy, causes damage to DNA, and causes sunburns. It is needed for the synthesis of vitamin D.
To help you make sense of the images, complete the following tasks.
Tasks
3.2.1. For Figure 1, compare the average amount of UV-A at the equator, in the tropics, and in
areas outside the tropics. What generalizations can you make?
Figure 1. Annual mean intensity of UV-A (380 nanometers, or nm). Lighter colors indicate a higher intensity
of UV-A light over the year (oceans are partially grayed
out). Source: Jablonski & Chaplin, 2010. Map created by
George Chaplin. Used with permission.
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Figure 2. Annual mean intensity of UV-B (305 nm).
Lighter colors indicate a higher intensity of UV-B light
over the year (oceans are partially grayed out). Source:
Jablonski & Chaplin, 2010. Map created by George Chaplin. Used with permission.
3.2.2. For Figure 2, compare the average amount of UV-B at the equator, in the tropics, and in
areas outside the tropics. What generalizations can you make?
Figure 3. Annual amount of variation in UV-B (305
nm). Lighter colors indicate a higher amount of variation. A higher amount of variation means that at some
points in the year there are relatively large amounts
of UV-B and at other points in the year there are low
amounts of UV-B. Source: Jablonski & Chaplin, 2010.
Map created by George Chaplin. Used with permission.
3.2.3. For Figure 3, describe the areas of the globe that show the largest variability for the
amount of UV-B received over a year. Briefly describe what the data mean.
3.2.4. Use the data and your prior knowledge to make a prediction for the distribution of skin
colors across the globe. Record your prediction on Master 3.3.
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Master 3.3, Global Skin Color Prediction
Task
3.3.1. Devise your own skin color scale and include a legend. Predict the distribution of human
skin colors around the globe. Then write an explanation of your prediction and use it as
the legend for Figure 1.
Figure 1.
Legend: This image shows ____________________________________________________________
___________________________________________________________________________________
___________________________________________________________________________________
___________________________________________________________________________________
___________________________________________________________________________________
___________________________________________________________________________________
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Master 3.5, Practicing with Explanations
Task
3.5.1. Read the following three explanations. Each is designed to help you make sense of ultraviolet (UV) radiation as a selective agent in the evolution of human skin color. Use
margin comments to note errors in facts or logic. Also, as you read, color code the text
according to the following.
• Green: Facts, data, evidence, history, reports, observation, detail
• Blue: Claim, assertion, statement of cause and effect, prediction
• Pink: Scientific principle, overarching concept, logical reasoning
• Red: Conjecture, opinion, or other statement that does not belong in a scientific explanation
Explanation 1
Sunlight shines on all humans. It is essential to life on this planet. In the past, people
with all different skin colors needed the sun, too. That’s how they got some really
important vitamins like vitamin D and vitamin B. When humans got the right
amount of sun, they lived healthier lives. Generally, the healthier people were, the
better parents they were. Unhealthy people had a hard time having kids. Skin color
doesn’t determine how good a parent someone can be. As long as people got the
right amount of sunlight, they had a better chance of having healthy children and
grandchildren.
Explanation 2
UV radiation is the main environmental factor that affects fitness in the evolution
of skin color in humans. UV radiation affects human skin color in two ways: (1)
UV radiation damages folate in skin. Folate is essential in cell division. Without
cell division, a person will die. Dark skin protects against folate damage. Thus, dark
skin was an adaptation to high exposure to UV radiation. (2) UV radiation helps
people produce vitamin D in their skin. Vitamin D helps bones develop, especially
in infants. Low melanin content (light-colored skin) may be an adaptation in areas
with low or inconsistent UV because people with lighter skin will produce more
vitamin D. Differences in UV radiation levels at different latitudes affected reproductive success for people in the past.
Explanation 3
There is always a need for diversity. That’s the main thing in evolution. The more
kinds of different people there are, the more evolution there is. So when humans in
the past got too much or too little sun, they needed to adapt. In the past, when people got too much sun, they just turned dark. People who got too little sun did not
need to be dark to protect themselves from UV, so they changed to a lighter color so
they could make more vitamin D. Now they can go inside, put on sunscreen, or, if
they can afford it, move to a better climate instead of needing to change.
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1. After completing Task 1, rank the three explanations in quality by
• developing evaluation criteria and
• applying the criteria to the explanations.
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Part 2: Skin Color and Human Dispersal
Master 3.6, One Species, Living Worldwide
Analysis Questions
3.6.1. Chimpanzees, gorillas, and orangutans generally have light skin under their fur. Evidence
suggests that the common ancestor of humans and apes was covered in long hair, similar
to apes. Use this information to make an inference about the skin color of the common
ancestor of humans and apes.
3.6.2. Using what you know from Part 1, what is the most likely skin color of the most recent
common ancestors of modern humans, who evolved in Africa? Explain, using evidence
on the relationship between ultraviolet (UV) radiation and skin color.
3.6.3. How did different routes of dispersal for some groups of people expose ancestral humans
to different amounts or variability in UV radiation?
3.6.4. Some groups of humans migrated to other parts of the world away from Africa; what
predictions would you make about changes in skin color?
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A Biology Challenge!
1. Construct an argument supporting the hypothesis that the evolution of light skin color
in humans is an example of convergent evolution. You will use what you have already
learned, plus data on Master 3.7.
Rules: Your argument
• must satisfy the definition of “convergent evolution”;
• must include phenotypic and genotypic evidence;
• must communicate how time and geographic scales factor into the logic
of the answer; and
• must consist mostly of graphic information with supplementary written
portions, all placed onto a large sheet of chart paper.
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Master 3.7, Human Dispersal and the Frequency
of SLC24A5 Alleles
Figure 1. This map shows proposed paths of human dispersal and the time frame for each dispersal.
Figure 2. This map shows the frequency of the G allele for SLC24A5 as the dark portion of a circle and the A allele
as the gray portion of the circle. Each circle represents samples from distinct groups of people. The location of the
circle corresponds to the ancestral origin of the population. High frequencies of the G allele are shown with increasing
amounts of black. Source: Norton et al., 2007. Reproduced with permission from H. Norton.
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L ess o n 5
Explaining Human
Skin Color Evolution
Master 5.1, Relating to Skin Color
You have learned a great deal about human skin color, including some of the most recent scientific
discoveries on the topic. But the experiences we remember best are those that are personal, including those of our friends or ourselves. The following tasks are designed to help you reflect on your
own experiences. To complete the tasks, either use yourself as an example, or choose a friend or a
celebrity.
Tasks
5.1.1. Identify the regions of the world in which your person’s parents, grandparents, and more
distant ancestors lived. Include an explanation of how you obtained this information and
an honest assessment of the limits of its accuracy.
5.1.2. Locate the geographic regions for some of the ancestors on a world map and make a
prediction of the skin color for your person. Keep in mind that a person’s skin color is a
product of the mixture of the various ancestors’ genes. Evaluate whether your person’s actual skin color matches the skin color that would be predicted for the person based on the
ancestors’ geography. If there is a discrepancy, comment on a possible explanation for it.
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5.1.3. Explain how a person’s skin color reflects a biological compromise that was made thousands of years ago.
5.1.4. Imagine that you were offered a scholarship to attend a university in a region of the world
where people have a much different skin color from yours. Select a specific country to
study in and explain what health risks you potentially face by moving to that part of the
world. Describe what actions you could take to minimize these health risks.
End-of-Unit Project!
1. Develop a final summary of an argument for evolution by natural selection for baseline
skin color and propose an explanation and research plan to investigate the evolution of
tanning.
You need to do the following.
• Summarize all the evidence for the evolution of baseline skin color in
humans and how each piece of evidence illustrates a key principle of
natural selection (see Master 2.5).
• Propose an explanation for facultative skin color that fits into the
knowledge about the evolution of baseline (constitutive) skin color.
• Suggest a research plan of concrete steps that would support your explanation of tanning. The plan should include hypothetical evidence that
would falsify your explanation.
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Credits
Cover: Source: “All for one! Hands stacked in unity and support” @iStock/RapidEye.
Lesson 1: Can Everybody Tan?
Master 1.2. Figure 1. Source: “Human Skin Diagram” by “Daniel de Souza Telles” at http://en.wikipedia.org/wiki/File:Human
SkinDiagram.jpg is licensed under the Creative Commons Attribution-Share Alike 3.0 Unported, 2.5 Generic, 2.0 Generic, and
1.0 Generic license and GNU Free Documentation License, Version 1.2 license (CC BY-SA 3.0, 2.5, 2.0, 1.0; GFDL). Modified
and used with permission; Figure 3. Redrawn from Barsh, G. S. Source: Barsh, G. S. (2003, October). What controls variation
in human skin color? [published correction appears in PLOS Biology, (2003, December 22), 1(3), e91]. PLOS Biology, 1(1), e27.
Retrieved from http://www.ncbi.nlm.nih.gov/pmc/articles/PMC212702/figure/pbio-0000027-g001/; Figure 4. “All for one!
Hands stacked in unity and support” @iStock/RapidEye. Figure 5. Source: “Illu skin02” by “Arcadian” at http://commons.wikimedia.org/wiki/File:Illu_skin02.jpg is a part of the public domain and is free from any copyright restrictions. Modified and used with
permission.
Lesson 2: The Melanin Connection
Master 2.1. Figure 1. Source: Same as Master 1.2, Figure 4; Figure 2. Source: Relethford, J. H. (2009, February 18). Race and
global patterns of phenotypic variation. American Journal of Physical Anthropology, 139(1), 16–22. This material is reproduced with
permission of John Wiley & Sons, Inc.
Master 2.3. Figure 1. Source: Image adapted from Genetics Home Reference. (2013). SLC24A5. Retrieved from http://ghr.nlm.
nih.gov/gene/SLC24A5; Figures 2, 3, 4, and 5. Redrawn with permission from K. C. Cheng. Source: Lamason, R. L., Mohideen,
M.-A. P. K., Mest, J. R., Wong, A. C., Norton, H. L., Aros, M. C., Jurynec, M. J., Mao, X., Humphreville, V. R., Humbert,
J. E., Sinha, S., Moore, J. L., Jagadeeswaran, P., Zhao, W., Ning, G., Makalowska, I., McKeigue, P. M., O’donnell, D., Kittles, R.,
Parra, E. J., Mangini, N. J., Grunwald, D. J., Shriver, M. D., Canfield, V. A., & Cheng, K. C. (2005, December 16). SLC24A5, a
putative cation exchanger, affects pigmentation in zebrafish and humans. Science, 310(5755), 1782–1786. Used with permission.
Master 2.6. Figure 1. Source: Nachman, M. W., Hoekstra, H. E., & D’Agostino, S. L. (2003, April 18). The genetic basis of adaptive melanism in pocket mice. Proceedings of the National Academy of Sciences of the United States of America, 100(9), 5268–5273.
Reproduced with permission from H. E. Hoekstra; Figures 2, 3, and 4. Source: Hoekstra, H. E., Drumm, K. E., & Nachman,
M. W. (2004). Ecological genetics of adaptive color polymorphism in pocket mice: Geographic variation in selected and neutral
genes. Evolution, 58(6), 1329–1341. Reproduced with permission from H. E. Hoekstra.
Lesson 3: Developing a Fuller Explanation for Skin Color Evolution in Humans
Master 3.2. Figures 1, 2, and 3. Maps created by George Chaplin based on NASA remotely sensed UVR data. Source: Jablonski,
N. G., & Chaplin, G. (2010). Human skin pigmentation as an adaptation to UV radiation. Proceedings of the National Academy of
Sciences of the United States of America, 107 (Supplement 2), 8962–8968. Used with permission.
Master 3.3. Figure 1. Source: “World Mercator Projection Map with Country Outlines” by “Bruce Jones Design Inc.” Retrieved
from http://www.freeusandworldmaps.com/html/World_Projections/WorldPrint.html. Cropped and used with permission.
Master 3.7. Figure 1. “Human migration out of Africa” by “Ephert” at http://en.wikipedia.org/wiki/File:Human_migration_out_
of_Africa.png is licensed under the Creative Commons Attribution-Share Alike 3.0 Unported license (CC BY-SA 3.0). Used with
permission; Figure 2. Source: Norton, H. L., Kittles, R. A., Parra, E., McKeigue, P., Mao, X., Cheng, K., Canfield, V. A., Bradley,
D. G., McEvoy, B., & Shriver, M. D. (2007). Genetic evidence for the convergent evolution of light skin in Europeans and East
Asians. Molecular Biology and Evolution, 24(3), 710–722. Reproduced with permission from H. Norton.
Lesson 4: Deleted in condensed version.
Lesson 5: Explaining Human Skin Color Evolution
Not applicable.
Evolution of Human Skin Color
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