Biology 11: Simulating Natural Selection



Simulating Natural Selection

Background Information

Prior to the Industrial Revolution in England, light colored 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 laboratory investigation, you will make a model using beans to represent the moths in similar situations.

Problem:

What happens to disadvantageous genes over time?

Materials

brown paper envelope/plastic bag 50 dried lima beans

plastic cup (or hand) 50 red kidney/navy beans

pinto beans stopwatch or clock with second hand

Procedure

1. After reading the lab procedures, set up your data table using the full width of the page.

2. Examine and note the differences and similarities among the three types of beans. Record your observations.

3. Fill the cup about three quarters full with pinto beans. (About 5 handfuls). Pour the pinto beans into the paper envelopeplastic bag. These represent the environment in which your prey will hide.

4. Add 50 lima beans and 50 kidney beans to the envelope. Mix. These represent your prey.

5. Have your partner time you for 1 minute. During this time, withoutyou will be the predator. Without looking, remove one bean at a time from the paper envelope.plastic bag. Try to remove as many beans as you can. If you remove pinto beans, put them back in the back immediately.

6. Record the number of lima and kidney beans that you removed in the appropriate place in the data table (lines #4 and #6).

7. To determine the number of lima and kidney beans that remain in the bag, subtract the number of lima beans removed from the starting number (#2). Do the same for the kidney beans (#3).

8. Add the numbers of remaining lima beans and kidney beans together (lines #5 + #7). Record this information in the appropriate place in the data table (line #8).

9. To find the frequencies of lima beans and kidney beans that remain in the paper envelope, divide the numbers of each of the remaining beans by the total number of remaining lima beans and kidney beans. Round these numbers off to the nearest hundredth (two decimal places). Calculate by dividing value of line #5 by line #8 and line #7 by line #8. Record this information in lines #9 or #10 of the data table.

10. The frequencies of each of the beans that remain in the envelope 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 into lines #2 or #3 in the next generation. Place this calculated number of lima beans and kidney beans in the envelope. Rows 2 and 3 in generation (column) 2, should match Rows 9 and 10 in generation (column) 1 respectively. (The number of lima beans plus beans must always equal 100)

11. Repeat steps 5 – 10 until the information in the data table is complete for five generations.

Observations

Generation |1 |2 |3 |4 |5 | |1. Total number of lima beans and beans |100 |100 |100 |100 |100 | |2. Total number of lima beans |50 | | | | | |3. Total number of beans |50 | | | | | |4. Number of lima beans removed | | | | | | |5. Number of lima beans remaining in envelope (2 – 4) | | | | | | |6. Number of (kidney/navy )beans removed | | | | | | |7. Number of (kidney/navy) beans remaining in envelope (3 – 6) | | | | | | |8. Total number of beans remaining in envelope (5+7) | | | | | | |9. Frequency of lima beans remaining in envelope (5/8) | | | | | | |10. Frequency of beans remaining in envelope (7/8) | | | | | | |

Data Questions

1. a. At the start of the lab, what was the ratio of lima beans to kidney beans?

b. At the end of the lab, what was the ratio of lima beans to kidney beans?

2. Based on your data, was one bean removed from the envelope more frequently than the other? If so, which type?

3. What happened to the frequencies of the kidney beans in the population over five generations? The frequencies of the lima beans?

4. What happened to the total number of beans remaining in the envelope after each generation?

Analysis and Application Questions

1. What waswere the pinto beans supposed to represent? What were the hands supposed to represent?

2. Explain why the frequency of genes changes over time.

3. Consider whether the same or different students “selected” the beans. Does this make a difference?

4. Construct a 3-line graph that shows the relationship between the total number of lima beans and the total number of kidney beans to the total number of beans (y-axis) in each generation (x-axis). What does this indicate about the effect of beneficial genes on population size? (Make sure your graph is appropriately labeled)

5. Which characteristic represented “fitness” in this population? How do you know?

6. What happens to harmful genes over time?

7. Does this model support the theory of Natural Selection, OR Lamarck’s idea that living organisms strive for complexity and pass on acquired characteristics? Explain.

8. Most harmful genes are recessive. Why do you think this is so?

9. What type of selection was represented by this lab?

10. Devise a simulation to test one of the other 2 types of selection not represented by this lab.

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