Name ____________________________________
Biology: Darwin and Natural Selection
Period _____
Natural Selection of the Rock Pocket Mouse
COLOR VARIATON OVER TIME IN ROCK POCKET MOUSE POPULATION
INTRODUCTION
A typical rock pocket mouse is about 170 millimeters (6 ½ inches)
long from nose to rump which is shorter than an average pencil. And at
just 15 grams, this tiny mouse weighs about as much as a handful of paper
clips. Rock pocket mice, however, have had an enormous impact on
science. What’s so special about them?
One can find populations of rock pocket mice all over the
Sonoran Desert in the southwestern United States. There are two
common varieties - a light-colored variety and a dark-colored variety. There are also two major colors of
substrate, or surface materials, that make up the desert floor. Most of the landscape consists of lightcolored sand and rock, but patches of dark volcanic rocks that formed from cooling lava flows are found,
separated by several kilometers of light-colored substrate.
PROCEDURE
Locate the illustrations (pgs. 5 -8) that represent snapshots of rock pocket mice populations. Each
illustration shows the color variation at two locations, A and B, at a particular moment in time.
Step 1: Place the illustrations in the order you think is correct from the oldest to the most recent. Indicate
the correct order using the numbers 1 – 4 at the top left of each illustration and record those numbers in
the chart provided below. (see # signs)
Step 2: Explain how you decided which illustration represents the most recent rock pocket mouse
population and why you positioned the others in the sequence as you did?
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Step 3: Watch the short film, The Making of the Fittest: Natural Selection and Adaptation. As you watch,
look for an explanation for the differences among the illustrations that will help you confirm that the
order in which you arranged the illustration is correct.
Answer the following as you watch the film:

Why are some mice light-colored and others dark?

How does fur color provide a selective advantage or selective disadvantage?

What role does the rock pocket mouse play in the food web?

What factor explains the differences in the illustrations?
Step 4: Once you are sure that you have the illustrations in the right order, count the number of light-
colored mice and dark-colored mice present at each location. Record your counts in the chart below:
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Step 5: Use colored pencils to create a bar graph based on the data that shows the distribution of the mice
at locations A and B through time.
Title: Color Distribution of Pocket Mice at Location A
Number of Mice
Key
Light fur
Dark fur
light
dark
light
First (oldest)
dark
light
Second
dark
Third
light
dark
Fourth (most recent)
Title: Color Distribution of Pocket Mice at Location B
Number of Mice
Key
Light fur
Dark fur
light
dark
First (oldest)
light
dark
Second
light
dark
Third
light
dark
Fourth (most recent)
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Step 6: Questions/ Analysis
1. Explain why a rock pocket mouse’s color influences its overall fitness? Remember that fitness is
defined by an organism’s ability to survive and produce offspring.
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2. Explain the presence of dark-colored mice at location A. Why didn’t this phenotype become more
common?
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3. Write a scientific summary that describes changes in the mouse population at location B. Your
summary should include:
 A description of how the population has changed over time.
 An explanation of what caused the changes to occur, and
 A prediction what the population will look like in 100 years in the future. Base your
prediction on trends you can see in the data you collected. You can assume that the
environmental conditions do not change in the next 100 years.
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4. Use the data and what you have learned about evolution to explain how mutation is a random
process, but natural selection is not random.
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