Performance Test Standard 5, Objective 1
Title: Simulating Natural Selection
Description: In this investigation students will make a model using beans to
represent the moths (or other organisms) undergoing natural selection.
Materials: (per group)
Small paper bag, 400 mL beaker, 250 g pinto beans, 50 large lima beans, 50 red
kidney beans, stopwatch.
Time required: one to two class periods
Student Background: Students should be familiar with Darwin’s theory of
natural selection.
Procedures:
1. Form student groups and hand out student sheets.
2. Make sure students can recognize the two types of beans.
3. Explain procedures listed on student sheet.
4. Allow time for students to collect data. Work through the first round with
students, the math is tricky.
5. Summarize with class discussion.
Scoring Guide:
1. Students collect and record data………………………..25 pts
2. Students correctly answer questions……………………25 pts
Sample Answers:
1. Fifty percent lima, fifty percent kidney.
2. Yes, lima beans.
3. The frequency of the lima beans decreased over five generations,
kidney beans increased.
4. The total number of beans left increased. 5. The kidney beans
represent the beneficial genes in the population. The lima beans
represent the harmful genes.
6. Organisms with harmful genes are selected against and removed from
the population. More of the remaining population have beneficial genes.
7. Harmful genes tend to be selected against and removed from a
population over time.
Student Sheet
Name_______________________
Title: Simulating Natural Selection
Purpose: Prior to the Industrial Revolution in England, light colored peppered
moths rested safely on the bark of light colored trees, unable to be seen by their
predators. In the mid-nineteenth century, however, trees that had light colored
trunks became darkened by soot. Simultaneously, the number of light colored
moths decreased, whereas the number of dark moths increased. How did this
change occur? In this investigation you will make a model using beans to
represent the moths and investigate.
Materials: (per group)
Small paper bag, 400 mL beaker, 250 g pinto beans, 50 large lima beans, 50 red
kidney beans, stopwatch.
Procedures:
1. Examine and note the differences and similarities among the three types of
beans.
2. Fill the beaker about three-fourths full with the pinto beans. Then pour the
beans into the paper bag.
3. Add 50 lima beans and 50 kidney beans to the paper bag. The lima and
kidney beans represent organisms. The pinto beans represent the environment
in which the organisms are hiding.
4. During a three minute interval, remove one bean at a time from the paper bag.
Without looking, try to remove as many lima and kidney beans as you can. Use
your fingers to identify the shapes of the beans.
5. Record the number of lima beans and kidney beans that you removed in the
data table.
6. Determine the number of remaining lima and kidney beans, add these
numbers together and record.
7. To find the frequencies of lima beans and kidney beans that remain in the
paper bag, divide the numbers of each of the remaining beans by the total
number of remaining lima and kidney beans. Round these numbers off to the
nearest hundredths place. Record this information.
8. The frequencies of each of the beans that remain in the paper bag represent
the distribution of genes in the population. To determine the starting number of
lima beans and kidney beans that are present in the next generation, multiply
the frequency of each bean by 100. Record this information. Place the
calculated numbers of lima beans and kidney beans back in the bag. The
number of lima beans plus the number of kidney beans should equal 100.
9. Repeat steps 4 through 8 until the data table is complete for five generations.
Data Table
Generation
1
1. Total number of lima and kidney beans
100
2. Total number of lima beans
50
3. Total number of kidney beans
50
4. Number of lima beans removed
5. Number of lima beans remaining in bag
6. Number of kidney beans removed
7. Number of kidney beans remaining in bag
8.Total number of beans remaining in bag (add #5
and #6.)
9. Frequency of lima beans remaining in bag
(divide #5 by #8 then multiply by 100)
10. Frequency of kidney beans remaining in bag
(divide #7 by #8 then multiply by 100)
11. Number of lima beans for next round (add or
subtract the number of lima beans needed to equal
the frequency from #9) look at #5 to see how many
are already there.
12. Number of kidney beans for next round (add or
subtract the number of kidney beans needed to
equal the frequency from #9) look at #7 to see how
many are already there.
2
3
4
Questions:
1. What was the ratio of lima beans to kidney beans at first?
2. Based on your data, which type of bean was removed from the bag more
frequently than the other?
Why?
3. What happened to the frequencies of the lima beans in the population over
five generations?
The frequencies of the kidney beans?
4. What happened to the total number of beans remaining in the bag after each
generation?
5. Which beans represent the beneficial genes in a population? The harmful
genes?
6. Explain why the frequency of remaining genes changes in each generation.
7. What do you think happens to harmful genes over time?
5
8. Draw a bar graph to show the change in beans over time:
Conclusion: