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Teaching About Evolution and the Nature of Science
Choose fabric patterns that simulate natural environments, such as floral, leaf, or fruit prints. The patterns should have several colors and be of intricate design; small prints work better than large blocky prints. Select two designs, each with a different predominant color. Label one design Fabric A and the other Fabric B. The use of two designs enables the students to demonstrate the evolution of different color types from the same starting population.
Use a paper punch to punch out quarter-inch paper dots from construction paper of six different colors. Select two light colors (including white) and two dark colors so that they will compete against each other. Include at least two colors that blend well with the fabrics. For each color, put 100 dots into each of 8 zip-type plastic sandwich bags. Put 20 dots of each color (for a total of 120 dots of 6 colors) into each of 24 additional bags. Label these bags "Beginning Population." Enlist student aides or ask for student volunteers to punch dots or stuff bags at home or after school. As an alternative to paper dots, you might try colored aquarium gravel or colored rice. Both are heavier than paper dots and are less likely to blow around the room. You could color the rice grains with food dyes according to the criteria specified above for the dots. You also might use gift-wrapping paper instead of the pieces of fabric.
Engage Begin by asking students what they know about the theory of natural selection. Ask them what predator-prey relationships have to do with biological evolution. Use the discussion as a means to have them explain how they think evolution occurs and the role of predator-prey relations in the process. At this point in the lesson, accept the variety of student responses, and determine any misconceptions the students express. You could present a historical example (see the discussion of fossils in chapter 3 of this volume) or an example from The Beak of the Finch by Jonathan Weiner or Darwin's Dreampond by Tijs Goldschmidt.
Because the instructional procedures are complex for this activity, you will have to be fairly explicit about the process. Tell the students they will work in teams of four. (If your class does not divide evenly, use teams of five). The activity calls for half of the teams to use Fabric A and half of the teams to use Fabric B. It will help if you go through a "trial run" before students begin the activity.
Explore Step 1. Tell the students to pick a "game warden" from each group of four. The other group members will be the predators.
Step 2. Examine the paper dots in the bags labeled "starting Population" and record the number of individuals (dots) of each color. All of the dots represent individuals of a particular species, and the individuals can be one of six colors.
Step 3. Make certain that half of the teams use Fabric A and half use Fabric B. The procedures remain the same for both groups.
Steps 4 and 5. Tell the predators to turn away from the habitats. The game warden then spreads one of the bags of "Beginning Population" across the fabric and tells the predators to turn around and gather prey—i.e., the dots. The predators must stop hunting (picking up dots) when the game warden says "Stop" in 20 seconds. If the predators have difficulty picking up the paper dots, provide forceps.
Step 6. After the hunting is stopped, the students should carefully collect all of the dots that remain on the fabric and sort them by color. The game wardens are responsible for recording these data on the graph paper using the colored pencils corresponding to the dot colors.
Step 7. To simulate reproduction among the paper dots, add three paper dots for each remaining dot of that color. These paper dots, obtained from the bags containing extra dots, represent offspring.
Step 8. Repeat the predation using the second generation of dots. Again record the number of remaining dots in the second generation.
Step 9. Explain to the students that they do not have to simulate reproduction as they did before, but rather should calculate the number of individuals that would be in the third-generation beginning population.
Step 10. The construction and analysis of bar graphs is a critical and time-consuming part of this activity. Place the color of survivors on the horizontal axis and the number of the beginning population (or second generation) on the vertical axis of this activity. If you have ready access to computers and spreadsheet programs, you could incorporate the use of spreadsheets during this step.