The rock pocket mouse is a small desert rodent that provides one of the clearest examples of natural selection in action. This species exhibits variation in coat color, ranging from a light, sandy hue to a dark, nearly black one. By studying the environmental pressures and genetic changes, researchers have pinpointed the molecular steps that drive evolutionary change in this wild population. The connection between fur color and survival makes the rock pocket mouse a model for understanding adaptation.
The Mice and Their Environment
The rock pocket mouse, Chaetodipus intermedius, inhabits the rocky desert landscapes of the southwestern United States and Mexico. The typical environment consists of light-colored granite or sand, which historically favored mice with a light, sandy coat. This landscape is punctuated by geological formations of dark volcanic basalt, created by lava flows thousands of years ago. These dark rock areas are isolated, creating distinct patches of contrasting habitat color. Mice on light substrate are almost exclusively light-colored, while those on dark basalt flows are predominantly dark-colored. This association between color and background sets the stage for selection.
Predation Pressure and Survival
The color variation is driven by the threat of visual predators who rely on sight to locate their prey. On the light desert floor, a light-colored mouse is well-camouflaged, making it difficult for predators to spot them. Conversely, a dark mouse on this light background stands out and is an easy target. This dynamic flips when the mice venture onto the dark volcanic rocks. A light-colored mouse on the black basalt is visually mismatched and quickly removed from the population. Dark-colored mice blend seamlessly with the lava rock and are far more likely to survive and successfully reproduce. This differential survival, where only the best-camouflaged individuals escape predation, is the core mechanism of natural selection shaping the population’s characteristics.
The Genetic Basis of Color Change
The variation in fur color ultimately traces back to random changes in the mice’s DNA, known as mutations. The specific trait of dark fur is largely controlled by a gene involved in pigment production, the melanocortin-1 receptor gene, or Mc1r. This gene provides instructions for making a protein receptor found on the surface of pigment-producing cells, called melanocytes. The functional form of the Mc1r protein typically regulates the production of a lighter, yellowish pigment known as pheomelanin, resulting in light fur.
In the dark-colored mice from some lava flows, specific mutations in the Mc1r gene cause the receptor to be constantly activated. This continuous activation shifts the melanocytes to produce high levels of the dark pigment, eumelanin, resulting in a black coat. Natural selection then acted on this existing variation, favoring individuals with the dark coat and causing the mutated gene to rapidly increase in frequency within the isolated lava flow populations.
Evidence of Rapid Evolution
The study of rock pocket mice confirms that natural selection can act quickly and repeatedly. Researchers found that the dark coat color evolved independently on different, geographically separated lava flows, a phenomenon known as convergent evolution. In one lava flow population, the dark fur was caused by the Mc1r gene mutations, but in another population, the dark coloration was caused by mutations in a completely different gene. This demonstrates the selective pressure imposed by visual predation, as the environment consistently selected for the dark fur phenotype regardless of the specific genetic change. Population genetic models suggest that the dark fur trait can become dominant in a population in as little as 1,000 years, illustrating how rapidly selective forces mold a population.