Question

The rock pocket mouse, Chaetodipus intermedius, is a small, nocturnal animal found in the deserts of the southwestern United States. Most rock pocket mice have sandy, light-colored coats; they can blend in with the light color of the desert rocks and sand that they live on. The populations of primarily dark-colored rock pocket mice have been found living in areas where the ground is covered in a dark rock called basalt, which was caused by geologic lava flows thousands of years ago. Scientists have collected data from a population of mainly dark-colored mice living in an area of basalt in Arizona’s Pinacate lava flow and from a nearby light-colored population. Researchers analyzed the data from these two populations to search for the genetic mutation responsible for the dark coat color. Through their analyses, they discovered a mutation in the Mc1r gene that is involved in coat-color determination.

What has this mutation caused in the two populations of rock pocket mice?

A decrease in genetic variation.

An increase in genetic variation.

No change in genetic variation.

The death of mice in both populations.

Answer 1

A decrease in genetic variation.

Explanation: The advantageous Mc1r mutation in the basalt population rose to high frequency by positive (directional) selection, producing a selective sweep that reduced genetic variation at and near that locus compared with the light-colored population.