Evo-Ed: Integrative Cases in Evolution Education

Cases for Evolution Education

Ecology

The beach mouse, (Peromyscus polionotus) is a small rodent found in southeastern United States. Different sub-species of beach mouse tend to live on different beaches in Florida's panhandle region. One of the major differences among these subspecies is that they have different fur colors ranging from light to very dark.

The distribution of fur colors is not random. The darkest colored subspecies tends to live inland where soils are dark brown while the lighter subspecies live on beaches where the sand is light brown.

illustrations of mice distributed by darkness over flordia.  The darkest mouse is inland.  The ligher mice are on the coast.

Soil Color and Fur color

Graph correlating average fur brightness versus soil brightness.  Ancestral populations of beach mouse 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 differentpopulations (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 and Fur Color

Simple food web including mice and predators.  All predators are sight and sound hunters except snakes.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.

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.

Owl hunting over light soil

Owl catches dark mouse first 64% of the time

Owl hunting over dark soil

Owl catches dark mouse first 37% of the time

 

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)