Studying Living Organisms
Beans and Birds: A Natural Selection Simulation
Woodrow Wilson Biology Institute
1995
Target age or Grade 9-10 biology.
ability group:
Class time
required:
Approximately four 45 minute periods.
Materials and 100 pinto bean seeds of each of the following colors: white, brown,
orange, red, and blue four meter sticks and graph paper a habitat
equipment:
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:
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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
Begin Eaten Survived Begin Eaten Survived Begin Eaten Survived
Bean
Survival
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?
Beans and Birds: A Natural Selection Simulation
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)
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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.
HYPOTHESIS
PROCEDURE
DATA TO COLLECT
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.
HYPOTHESIS
PROCEDURE
DATA TABLE
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