Natural Selection

Throughout the course of the previous modules, the correlation between the nucleotide change at position 199 on the MC1R gene and a phenotypic change in fur color have been discussed, as well as the causation between the MC1R transmembrane protein shape transformation along with its inability to react and ultimately produce eumelanin. To summarize, the mutation present within Peromyscus polionotus has been described. However, that isn’t all that is necessary to explain the different fur colors present in isolated populations. Sure, the mutation occurs. But why does it remain, and even increase in frequency? This is because of natural selection present within the environment.

Soil & Fur Color

Ancestral populations of beach mice likely had dark fur coats, indicating that light fur color is a recent evolutionary event. Researchers have observed a very strong correlation between soil brightness and coat color among different populations (r = 0.92 p = 0.0004). This could indicate that there may be some kind of selection pressure that initially established and currently maintains the differing coat colors among populations. Although the researchers who uncovered the relationship between fur color and soil brightness are skeptical about predation-related selection pressure (read more about their research here: Belk and Smith, 1996), other research indicates this may be occurring.

Selective Predation & Fur Color

Most animals that hunt beach mice are sight and sound predators. When the mice are visually hunted, blending into the substrate can provide a significant advantage to help them avoid detection. When mice avoid predation they have a better opportunity to find a mate and pass their fur color alleles along to their offspring. Predation could thus be a driving force that changes fur color allele frequencies within mice populations to match soil or sand color.

An Owl Test

To test this hypothesis, Kaufman (1974) set up two enclosures, one with dark soil and one with light. In each he tested the hunting behaviors of barn owls and screech owls. When both light and dark mice were released into enclosures, owls tended to catch the dark mouse first in light soil enclosures and the light mouse first in dark soil enclosures.

The owl catches the dark mouse first 64% of the time against a light soil background.

The owl catches the dark mouse first only 37% of the time against a dark soil background.

Frequency of Attacks

In a more recent study, Vignieri et al (2010) made realistic clay models of light and dark mice and set them out in light and dark soil habitats to be attacked by predators. They found that light colored clay mice were frequently attacked in dark soil environments and dark colored clay mice were frequently attacked in light soil environments.  Light colored clay mice in light soil environments and dark colored clay mice in dark soil environments were comparatively unharmed. (Picture taken from the Vignieri et al 2010; study found at http://www.ncbi.nlm.nih.gov/pubmed/20163447)