Studying Living Organisms

Beans and Birds: A Natural Selection Simulation

Target age or ability group:	Grade 9-10 biology.
Class time required:	Approximately four 45 minute periods.
Materials and equipment:	100 pinto bean seeds of each of the following colors: white, brown, orange, red, and blue four meter sticks and graph paper a habitat such as the school lawn or a field NOTE: Pinto bean seeds can be purchased at local food stores for less than $1 per pound. Purchase four pounds of white and one pound of brown. Using fast drying enamel spray paint ($3 per can at the hardware store), color one pound each of the white pinto bean seeds to obtain blue, red and orange seeds (or whatever colors you select) One pound of the white and the brown seeds should not be painted. Place about 100 seeds of each color in a separate nonbreakable container such as a discarded eight-ounce plastic butter tub. Each team should have its own set of colored seeds.
Summary of activity:	In this natural selection simulation students solve the following problem concerning the evolution of seed color in pinto bean plants: "How does natural selection change the frequency of genes or traits over many generations?" Students use the constructivist approach to learning as they work in teams to design and conduct an experiment that solves this natural selection "problem." Since this is not a cookbook investigation, students must write an appropriate hypothesis, develop procedures using materials, and decide what data to collect and how to present it in table and graphic form. Students then analyze their data by answering a series of questions about scientific investigation and the process of natural selection. Begin the investigation by discussing the main features of the process of natural selection. List these ideas on the blackboard. Responses may include: a population tends to overproduce offspring population size tends to remain in dynamic equilibrium over many generations; therefore, there is competition for resources there is genetic variation in a population variations may be advantageous or disadvantageous to an organism adaptive variations tend to be transmitted to future generations; therefore, gene frequencies change over generations

Teacher Instructions

Have students carefully read the directions for the investigation. It may be helpful to write the "problem" on the blackboard since it is the focal point of the entire investigation. Give students about 20 minutes to develop their team designs. A lot of coaching is necessary. A spokesperson from each team should share the team's design with the entire class. During a teacher led discussion develop a single class design . The class design should be based on student input and is expected to vary from class to class. The following is a sample class design :

Hypothesis:
If seeds of various colors are scattered on the lawn and a student models the feeding behavior of a bird, then seeds with a color similar to the color of the lawn will survive better than seeds with contrasting colors.

Procedure:
Scatter 100 seeds (20 seeds of each of the five color variations) over one square meter of lawn surface using the four meter sticks to define the boundaries of the plot. One team member acting as a feeding bird quickly picks up 75 of the 100 seeds. Record data on the number and type of each color of seed "eaten" and calculate and record the number of seeds left on the lawn. Now assume that each surviving seed produces three offspring of like color and continues to live throughout the season. For each survivor, add three seeds of like color to the plot scattering them as before. There will now be 100 seeds on the plot representing the surviving parent generation and their offspring. Now the "bird" should "eat" 75 more seeds repeating the same procedure as was done with the original 100 seeds. Again, record data on the number of each color "eaten" and calculate and record the number of each color surviving. Repeat the procedure to obtain data for another generation.

Generation

1st Generation

2nd Generation

Pinto Bean Survival	Begin	Eaten	Survived	Begin	Eaten	Survived	Begin	Eaten	Survived
white	20
blue	20
brown	20
orange	20
red	20
Total	100	75	25	100	75	25	100	75	25

Answers to Analysis Questions:

1. Students should make a histogram to illustrate their data.
   

2. Answers vary.
   

3. Answers vary. They may all eliminate red, orange, blue, and white seeds.
   

4. Surviving seeds tend to have colors similar to the habitat.
   

5. Any surviving red, blue, orange, and white seeds might be eliminated leaving only brown seeds.
   

6. Mutations could introduce new color variations that would be more advantageous to the plants than existing colors. Selection would then be expected to increase the frequency of the new variation within the population. Mutations that introduce conspicuous colors would tend to be eliminated.
   

7. Gene frequencies for a particular color may go up or down.
   

8. If the habitat changed in color, then a different seed color may be selected. Also, if a change occurred in birds (such as most birds becoming color blind), then seed color selection is expected to change.
   

9. Answers vary.
   

10. .Students should relate the main ideas of natural selection to this simulation.
    

11. .Student responses should explore the control of variables, sources of error, and alternative ways of designing the investigation.

Seed Colors:
Use other or additional colors. Use green because grass is green. Make sure all seeds have the same luster. The painted seeds are shiny while unpainted seeds are dull. The brown seeds have a speckled appearance while the others have a uniform color.

Herbivores:
Model a mammal that eats seeds. Use real birds. Place the seeds outside on flat bird feeders using seeds colored with nontoxic food dyes. Do birds have color vision? Was the student who modeled the bird color blind?

Repetitions:
Repeat the experiment several times compiling data from the entire class.

Habitat:
Repeat the experiment on a different habitat such as the floor of a forest or bare ground.

Season:
Seeds may be present on the ground only during certain seasons such as autumn or winter when the ground is brown or snow covered. Even during late summer when the seeds are produced, the seeds are likely to fall on brown soil. The school lawn does not accurately model the natural habitat of bean plants.

Other:
Since pinto bean plants are cultivated, seed color may be the result of artificial selection. Discuss the white seed variation. Would it have selective value in natural populations?

Introduction

Natural selection is the chief mechanism of evolution. It is the process that gives rise to populations that are adapted to their environments. Organisms with favorable variations tend to survive and pass their variations to offspring while those with unfavorable variations are eliminated. In this investigation your team, working with other teams in the class, will design and conduct a simulation experiment to answer a question concerning the evolution of seed coloration in pinto bean seeds.

The Problem

It is important to a population of bean plants that its seeds survive and grow into a new generation of plants. Mutations may have produced many seed color variations such as red, blue, brown, orange, and white. Since the seed colors that actually exist in pinto bean plants are brown and white, it seems reasonable to conclude that these colors are an advantage to the bean plants' survival and were selected over many generations. The problem you will investigate using pinto bean seeds is: "How does natural selection change the frequency of genes or traits in a population over many generations?"

Materials (per team)

• One container each of red, blue, brown, white, and orange pinto bean seeds
• Four meter sticks
• Habitat such as the school lawn
• Graph paper

Team Design

Using the materials on the above list, design an experiment that answers the question posed by the problem:"How does natural selection change the frequency of genes or traits over many generations?" In designing your investigation, state a hypothesis, describe a procedure, and determine what data to collect. Place your answer in your notebook or on another piece of paper.

Class Design

Share your team design with other teams in the class and select a common class hypothesis, procedure, and data table. Place your answer in your notebook or on another piece of paper.

Analysis:

1. Make a graph to illustrate your data. Decide whether to make a line graph or a histogram and determine what data to plot on each axis.
   

2. Study your survivor populations for each generation. What changes occurred in the frequencies of colors between each generation?
   

3. Compare the original and survivor populations. Is there any seed color or colors from the original population that are not represented in the survivor population?
   

4. How do the colors of the survivors relate to their habitat?
   

5. What do you predict would happen to the frequencies of colors if you continued the simulation activity for several more generations?
   

6. How might new mutations for additional seed colors affect the frequencies of genes for color in the population?
   

7. How might the population change if pinto bean plants migrated into or out of the population?
   

8. How might a change in the habitat or in the animals (herbivores) eating the seeds affect the frequencies of seed colors?
   

9. Have you confirmed your hypothesis? Explain.
   

10. Explain how natural selection changes the frequency of genes over many generations.
    

11. How would you improve this experiment? Comment on seed color, habitat, seed eating herbivores, number of repetitions, season of the year, etc.

On to Estimation of Population Growth by Counting Offspring: Seed Multiplication / Judith K. Wood
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