The Evolution of the Peppered Moth:

A Natural Selection Simulation

Background Information:

The Theory of Natural Selection, as put forth by Charles Darwin, suggests that individuals that have the best physical and behavioral characteristics are more likely to survive than those which are not so well endowed. The organisms that survive will produce offspring with the same advantages for survival, thus increasing the populations of the organism.

What are some traits that may be considered an advantage to have?

One example of such an advantage is camouflage. There are many examples in nature where organisms rely on camouflage for protection against predators. Chameleons are known for their ability to alter their skin color so as to blend in with the environment.

Describe another example of an instance where camouflage is an advantage for an organism.

The Story of the Peppered Moth:

The case of the peppered moth in Manchester, England is a well documented study demonstrating the value of camouflage in natural selection and the ongoing process of evolution. This case involves two forms of the same organism, a white-colored moth and a black-colored moth. Until approximately 1850 the black moths were considered rare, and were sought after by collectors. The white moths, which were much more numerous, were protected by their ability to blend in with the light-colored lichens which lived on the bark of the trees. The black moths were very visible against the light background and therefore more susceptible to being eaten by birds. Consequently, the black variety did not have as great a chance to reproduce and increase their numbers.

During the later part of the 19th century, smoke particles produced by developing industry began to gradually blacken the trunks of the trees on which the moths rested. This change in the environment caused the white moths to become more visible to their bird predators, while the black form became better camouflaged. This situation led to a decrease in the population of white moths while the black moths were better camouflaged. The black moths were now able to breed and therefore increase theirpopulation.

Vocabulary:

Camouflage

Natural Selection

Evolution

Lichens

Industry

Predators

Materials:

Light-colored tree bark

Dark-colored tree bark

25 light-colored moths

25 dark-colored moths

Student Jobs Assignments:

Recorder – writes down data in the chart

Moth Handler – adds and subtracts moths from the backgrounds

Bird – Hunts for and “eats” the moths

Procedure:

Part A

  1. Using the light colored background, the Moth Handler places 15 white moths and 15 black moths in random location on the background. The Bird should look away during the set-up.
  1. When ready the Bird will turn to the moths and pick as many as it can with in four seconds. The Bird must pick up the moths one at a time. When the four seconds is up the Bird must look away again.
  2. The Moth Handler then replaces the removed moths with one of the opposite color. For example, any white moths that were removed will be replaced with black moths and vice versa. The total number of moths on the background should always be thirty.
  1. The Recorder must write the number of moths present on the background after this first trial.
  1. Repeat steps 2-4 for a total of five trials.
  1. The Recorder must calculate the percentages for each trial.
  1. As a group, complete the graph using the information recorded in Table 1.

Part B

Using the dark background the procedure will be the same as it was for Part A.

Data and Observations:

Add titles to each table and write 1-2 sentences explaining the data below each table.

Table 1. ______

Trial / # Black Moths Remaining / # White Moths Remaining / % Black Moths / % White Moths
1
2
3
4
5

______

Table 2. ______

Trial / # Black Moths Remaining / # White Moths Remaining / % Black Moths / % White Moths
1
2
3
4
5

______

Light-Colored Background / Dark-Colored Background

Analysis Questions:

The following questions must be answered in complete sentences on a separate sheet of paper for full credit. You may use the bottom of this page for notes and scrap paper.

  1. Using your own words, explain how natural selection relates to the “Survival of the Fittest” concept.
  1. Why did the simulation start with the light-colored background?
  1. Based upon you studies, what could have originally caused the existence of two forms of the same species of moth?
  1. Based upon your observation, what would happen to the moth populations if the pollution created by the Industries were to be reversed?
  1. This simulation of the peppered moth is clearly an artificial situation. Describe how you think the actual situation differed from this simulation.