Rock Pocket Mouse Evolution Essay Question

Spanish versions of these resources are also available for download (en Español). 

Appropriate for: middle school life science, high school biology (all levels)

This activity serves as an extension to the HHMI short film The Making of the Fittest: Natural Selection and Adaptation and a means of reinforcing the concepts of variation and natural selection.

The tiny rock pocket mouse weighs just 15 grams, about as much as a handful of paperclips. A typical rock pocket mouse is just about 170 millimeters long from nose to rump, shorter than an average pencil. Their impact on science, however, has been enormous. What’s so special about these little mice?

Populations of rock pocket mice are found all over the Sonoran Desert in the southwestern United States. There are two common varieties—a light-colored variety and a dark-colored variety. Similarly, there are two major colors of substrate, or surface materials, that make up the desert floor. Most of the landscape consists of light-colored sand and rock. Here and there, however, separated by several kilometers of light-colored substrate, are patches of dark volcanic rocks that formed from cooling lava flows.

Rock pocket mice live in the deserts of the American southwest. Ancestral pocket mice had light-colored coats that blended in with the region's rocks and sandy soil, keeping the mice hidden from their owl predators. Starting about 1.7 million years ago, a series of volcanic eruptions spewed out wide trails of black lava that wove right through the middle of pocket-mouse territory.

Today there are two forms of pocket mice: light-colored mice that live on sandy soil, and dark-colored mice that live on black lava rock. The dark mice came about through the process of evolution. Naturally occurring mutations to coat-color genes produced mice with dark fur. On black rocks, dark mice had an advantage over light mice: they were better-hidden from predators. They survived and reproduced, passing their dark-fur genes to their offspring, which still survive today.

Once a favorable variation occurs, it can quickly become the major form in a population. Each year, mice produce more offspring than will reach adulthood. Thanks to natural selection, the offspring with favorable characteristics are more likely to survive and reproduce.


Recessive alleles are weak and they will eventually disappear.


Natural selection maintains favorable alleles regardless of whether they are dominant or recessive. Even when they convey no selective advantage or disadvantage, recessive alleles are maintained in a population.

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