Color Variation Over Time in
Rock Pocket Mouse Populations
A typical rock pocket mouse is about 170 millimeters long from its nose to the end of its tail, 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?
You can find populations of rock pocket mice all over the Sonoran Desert in the southwestern United States.
There are two common varieties of these mice — 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 desert is
covered in light-colored sand and rock. However, there are also patches of dark volcanic rocks that formed from
cooling lava flows. These patches of dark-colored substrate are often separated by several kilometers of light-
• Rock Pocket Mouse Illustrations (provided by your teacher)
• The Making of the Fittest: Natural Selection and Adaptation
• Supplies for creating bar graphs (e.g. computer graphing software or graph paper and colored pencils)
1. The four illustrations provided by your teacher represent snapshots of rock pocket mouse populations. Each
illustration shows the color variation at two different locations, A and B, at a particular moment in time. The
illustrations may be out of order. Count the number of light-colored and dark-colored mice present at each
location at each moment in time. Record your counts in the table below.
2. Place the illustrations in what you think is the correct order from oldest to most recent. In the space below,
write the numbers of the illustrations in the order you decided.