The rock pocket mouse, a small nocturnal rodent inhabiting the deserts of the southwestern United States and Mexico, provides a clear illustration of natural selection. Its survival is closely linked to its fur color, which typically matches its environment. This species, Chaetodipus intermedius, demonstrates how environmental pressures shape physical traits over generations. Distinct populations of these mice, with varying fur colors, serve as an example of evolution.
Environments and Predators
Rock pocket mice live in two primary habitats with contrasting appearances. Many populations reside on light-colored desert rocks and sandy terrain, where their sandy-colored fur provides excellent concealment. Other populations inhabit dark volcanic rock formations, such as ancient basalt lava flows. In these dark environments, mice with darker fur are observed.
A selective pressure on these mice comes from visual predators, including owls and hawks. These predators hunt by sight, making camouflage a defense mechanism. A mouse that blends seamlessly into its background is less likely to be detected and captured, increasing its chances of survival and reproduction. This color-matching adaptation offers a survival advantage in both the light and dark habitats.
The Genetics of Camouflage
The fur color of rock pocket mice is a heritable trait, meaning it is passed down from parents to offspring through genes. A primary gene involved in controlling pigment production for fur color is the Melanocortin-1 receptor, or Mc1r gene. This gene encodes a protein, the MC1R receptor, which plays a role in regulating the type of pigment produced in the mouse’s fur.
When the normal Mc1r gene is present, melanocytes, the cells responsible for pigment, tend to produce more light-colored pheomelanin and less dark-colored eumelanin, resulting in a sandy fur color. However, specific mutations within the Mc1r gene can alter the function of the MC1R protein. These mutations lead to an increase in the production of dark eumelanin, resulting in a dark coat color phenotype. These genetic mutations arise randomly and are not a direct response to the environment or a conscious choice by the mouse.
How Natural Selection Shapes Mouse Populations
Natural selection acts on these random variations in fur color based on the environment, leading to changes in mouse populations over time. On dark basaltic lava flows, dark-colored mice are well-camouflaged against the black rock. This camouflage means they are less visible to predators like owls and hawks, increasing their likelihood of surviving to reproductive age. Consequently, these dark mice produce more offspring, passing on the alleles for dark fur to the next generation. Over many generations, the frequency of the allele responsible for dark fur increases within that specific population.
Conversely, in areas with light-colored sand and granite, light-colored mice possess the survival advantage. Their sandy fur provides camouflage, making them less susceptible to predation. Light-colored mice survive longer and have more opportunities to reproduce, ensuring that the alleles for light fur remain prevalent in these populations. This process illustrates that natural selection leads to changes in the genetic makeup of a population across generations, not a change within an individual mouse’s lifetime.