This process of long term genetic change in a population by means of natural selection is a critical principle of life. This process spans generations of individuals and therefore cannot be studied with living organisms in a 3 hour exercise. Therefore we rely upon a computer to assist us in time compression. Lichens, a symbiosis between fungi and algae, are a light mottled color compared to the bare bark. Lichens are very sensitive to atmospheric pollution and so are abundant in low-pollution areas.
Lichens are sparse in high-pollution areas.
The moths are prey for English birds. In the mid-1800s naturalists noted that the mottled morph far outnumbered the dark melanic morph and hypothesized that this was due to their camouflage among the abundant lichens. By 1898 the industrial revolution was in full swing in England. Soot belching from coal-fired factories and power plants had poisoned the lichens and darkened the bark of the tree trunks. Naturalists noted that melanic morphs then outnumbered the mottled morphs.