Natural Selection: Life as a Peppered Moth

There are two types of peppered moths in England: one has light-colored wings and the other, dark-colored wings. Prior to the Industrial Revolution in the 1850s, the light-colored moth was more prevalent. By 1900, sooty pollutants had changed the bark of trees from light gray to dark gray. The dark tree bark provided camouflage for dark moths and enabled them to hide from predators. The number of dark moths rose dramatically as the number of light-colored moths declined. This increase in dark moths is an example of natural selection, a concept introduced by Charles Darwin that explains how organisms best suited to the environment survive and reproduce. In this activity you will see how a change in the color of tree bark affected the populations of moths.

Materials

Sheet of tan or gray construction paper

Sheet of black construction paper

Thirty tan or gray beans

Thirty black beans

Watch with a second hand

Activity

1. Place the sheet of gray or tan construction paper on a table. Spread all sixty beans around the paper.
2. Close your eyes and count to ten. Open your eyes and pick up as many beans as you can in twenty seconds. You can only pick up one bean at a time. Record the number of each color of bean that you picked up.
3. Repeat this process, but this time use the black paper as your background.

Follow-Up Questions

1. How many of each color bean did you pick up on light-colored paper? How many of each color bean did you pick up on dark-colored paper?
2. If you imagine the beans were moths and the paper was tree bark, explain how these results help illustrate natural selection.

Answers

1. Answers will vary, but students usually pick up more dark beans on light paper and more light beans on dark paper.
2. When the tree was light gray in color, the dark moths were easily seen and eaten by predators. Once the bark color changed to dark gray, the dark moths were harder to see. They survived and began to reproduce in greater numbers than the light-colored moths, who now were clearly seen and eaten by the predators.

Extension

Eventually antipollution laws were enacted in England. Do you think this changed the number of each type of moth there? Do some research and see what actually happened and which species is more prevalent today.