The Case of the Peppered Moth
Darwin's Theory Applied
Variation within a Species
There are three main colors of peppered moths: white wings with peppered specks of black, black wings with peppered specks of white, and black wings. Before the Industrial Revolution, the black-winged peppered moth was quite rare, a mutation that showed up from time-to-time, with the speckled variations being much more common.
Overproduction of Offspring
The peppered moth like most insects produces far more eggs than will ever reach reproductive age. The peppered moth is prey for birds and relies on being able to blend into the tree trunks upon which they rest during the day for protection.
Struggle for Survival
Before the Industrial Revolution, silvery lichen covered the tree trunks of trees across England. Moths resting on such trees were protected by camouflage from predators during the day. Black-winged moths were less protected than the speckled moths. They were more likely to be eaten by birds and less likely to survive to reproduce.
Natural Selection
The speckled moths carried both black and white pattern genes. The individuals who receive the mixed characteristics were more likely to survive to reproduce. It was also more likely that offspring would receive a mix of colors from parents. The black-winged individuals continued to appear since it was possible to receive black genes from both parents. However, these represented a rare and small percentage of the total population.
The Experiment
Background Information
During the Industrial Revolution, coal-burning factories spewed tons of soot into the air around industrial centers such as Birmingham. The soot coated the trees killing the silvery lichen. The soot caused the tree bark to darken until it became almost black. Dr. Kettlewell noticed that the heretofore-rare black-winged peppered moth was becoming the dominant color form in industrial areas. The speckled moths were found only in distant forests far from industrial centers. This change in balance of the variations in the population intrigued biologists. Dr. Kettlewell suspected that the black-winged moths had gained a selective advantage since they were now better camouflaged from predators than the light speckled moths.
The Hypothesis
The change in color of the peppered moth is caused by predators being able to see light colored moths on dark tree trunks and dark colored moths on light colored tree trunks.
Materials
Two sets of moths with an equal number of dark and light moths
Trees in an industrial region without silver lichen
Trees with silvery lichen
Procedure
Release a set of moths in each area.
Collect the moths and count the survivors.
Control: The trees with silvery lichen
Variable: Soot covered trees without silvery lichen
The Results
In industrial areas where the tree trunks were dark, black-winged moths were recovered. In natural areas with silvery lichen covered trees, speckled light colored moths were recovered. In the natural areas, Dr. Kettlewell recovered two light moths to every one dark moth. He had the reverse recovery rate in industrial areas. He also observed birds eating dark moths on light tree trunks and light moths on dark tree trunks.
Repetition
Dr. Kettlewell repeated the experiment using dead moths so that they would stay in place and he could find them for an accurate count. Again, birds ate the moths that stood out against their background. Birds found 60% of the light moths against dark backgrounds. Dr. Kettlewell also tested moth preferences for backgrounds by placing moths in a cage with black and white stripes. He found that black moths rested on black stripes and light moths rested on white stripes.
Conclusion
The moths best able to blend into the environment were more likely to survive to reproduce.
Why a New Species Did Not Develop
For a new species to develop, the individuals must be isolated from other populations. Black-winged moths interbreed with light speckled moths. Because they are able to interbreed and produce viable offspring, they are considered one species. If one or the other were to be isolated, then it might be possible for a new species to develop.
What Happened After the Industrial Revolution?
As tree trunks once again became habitat for silvery lichen, the coloration of the peppered moth shifted so that more individuals were light colored than dark. This confirmed that the moth color better camouflaged from predators is more likely to survive.
