Adaptation and Natural Selection in Peppered Moths

I. Introduction

Charles Darwin collected many facts to support his theory of evolution by natural selection. Although the evidence for evolution in the fossil record is very compelling, at the time of Darwin no one had observed evolution actually taking place. Darwin pictured the process of evolution as requiring vast amounts of time, far greater than a person's life span. Although Darwin was unaware of it, remarkable examples of evolution were going on around him in the countryside of his native England. One example of this evolution was happening to a species of moth known as the peppered moth, Biston betularia. The Industrial Revolution began in England in the middle of the eighteenth century. Since then, tons of factory soot has been deposited in the countryside around industrial areas in England. The soot has discolored and generally darkened the trees, rocks, and other natural features of the landscape. Before the Industrial Revolution, the wild type peppered moth was mostly light colored. A very rare form of the moth that was covered with gray spots was generally dark in appearance. In contrast, today the situation has reversed. Today in some areas over 90% of the peppered moths are dark in color. More than 70% of other moth species in England have also changed from a light to darker phenotype. Similar observations have been made in other industrial nations including the United States. How has this striking change come about? In this investigation, you will perform an experiment and use your data to offer and support an explanation.

II. Procedure

In this investigation, you will simulate a predator/prey relationship. You will determine whether color contrast has an effect on the ability of a predator to rapidly locate prey. You will observe the impact on predation in a population of peppered moths living just outside an industrial city. Birds are the moth's natural predators.
  1. You will use one-inch squares of white and newsprint paper to represent the light and dark colored moths. Each team should cut out thirty pieces of white paper and newspaper about one inch square. It is important to make sure all the squares are approximately the same size.
  2. Place a piece of white butcher paper on your lab bench to represent the white-bark trees.
  3. Roll a pair of dice to see how many generations that the tree bark remains white.
  4. A person with a watch with a second hand should be designated as the timekeeper.
  5. While teammates are looking away, the timekeeper scatters the thirty white and thirty dark squares of paper moths all around the white paper tree.
  6. When your partner is ready with the forceps, which will represent the bird's beak, let the hunt begin! Start timing 15 seconds. During this time, the predator bird picks up moths one at a time using only one hand and the forceps, and places the moth in a nest (aside). The object is to collect as many moths as you possibly can.
  7. When the 15 seconds are up, count the number of light and dark moths captured and enter the numbers into the data table. Each surviving moth not captured gets to pass on their genes by breeding once, so place a new dark or light square for each moth left on the newspaper tree.
  8. Repeat this process for the number of generations, represented by the roll of the dice.
  9. Then replace the white butcher paper with newspaper. The newspaper will represent the soot-covered trees.
  10. A repeat step 5 to 7 for another 4 to 5 turns or until everyone in your group has had a chance to hunt during this second round.

III. Data Analysis and Questions

Each person in your group will answer at least one of the following questions, but all of the questions must be answered. The group will share their answers with the group's members. But each person must turn in his or her own paper.
  1. Make a bar graph that shows the changes in size of the two populations of moths over at least a five-generation period. See example on the board.
  2. How did the appearance of the light colored peppered moth compare to that of the tree trunks before the Industrial Revolution?
  3. How did the appearance of the dark colored moths compare to that of the tree trunks as the Industrial Revolution progressed?
  4. Based on the data you graphed, how did the population of moths change over five generations?
  5. What caused the change in the moth population?
  6. How would Lamarck account for the changes that took place in the moth population over five generations?
  7. How would Darwin account for these changes?