Introduction to the Peppered Moth
The case of the peppered moth (Biston betularia) is a classic example of evolution through directional selection (selection favoring extreme phenotypes).
Prior to the industrial revolution in England (pre-1740), the peppered moth was found almost entirely
in its light form (light body colored with black spots). The moths would spend daylight hours on trees
covered by a light colored lichen, their light colors giving them almost perfect camouflage against
predatory birds. There were a few dark individuals in the population, but their occurrence was very rare.
By 1819, the proportion of dark moths in the population had increased significantly. Researchers found
that the light-colored lichens covering the trees were being killed by sulfur dioxide emissions from
the new coal burning mills and factories built during the industrial revolution. Without the light
background of the trees, the light moths were more visible to vision-oriented predators (birds). MVHS
offers two activities that explore the effects of pollution and natural selection on the peppered moth
In the first activity, the students build a computer model in a systems programming language. The purpose of the peppered moth activity is to model the effects of natural selection on the appearance and genetic make-up of a natural population. By adjusting the amount of pollution in the environment, the student is able to see the differences in the frequencies of light and dark moths in the population. The student may also investigate the survival differences between dominant and recessive genes when one phenotype has a selective advantage over the other.
The second activity is a web-based simulation derived from the systems model. In this activity, the student simulates how directional selection changes the proportion of dark and light moths in a peppered moth population over a 200 year time span. By varying the pollution parameter and analyzing the graphs of phenotype and allele frequencies, the student may see the impact of pollution on the population.