Natural Selection Lab

Name/Date/Partners: _________________________________________________________

AP BIOLOGY NATURAL SELECTION LAB

INTRODUCTION

Throughout the oceans, small volcanic islands form when undersea volcanoes erupt and send materials above the surface of the water. When these islands first form, they are barren rock. Yet many of these islands become inhabited by a rich variety of plant and animal species.

How does such a transformation come about? Species from a mainland or other islands travel to the new island and colonize the new habitat. Bacteria, fungi, algae, mosses, and lichens are examples of early inhabitants that can thrive in harsh, barren environments. Some organisms, such as lichens and mosses, are transported to a new habitat as spores in the wind. Green plants usually arrive as seeds and fruits carried great distances by the wind and waves. Birds may have landed during stopovers on migration routes, while other land animals may simply have been carried to the shores by accident.

Some species are not as well suited to an island environment as to their original homes. To have survived on these islands, organisms must have adapted to different conditions. This provides a new environment for evolution—a place where humans can learn a great deal about the processes of natural selection.

Imagine that you are a naturalist studying a population of plants on an isolated volcanic island in the Pacific. A simple physical characteristic, such as seed color, could affect a plant’s chances of survival in a particular habitat. Plants with a favorable genetic variation tend to survive and pass that trait to their offspring, while those with unfavorable variations could become extinct.

In this lab, you will investigate the inheritance of a single trait, and explore how natural selection changes the frequency of this trait in the surviving plant population.

OBJECTIVES

Hypothesize how color variations in a population are affected by environmental factors.

Graph and compute the percentages of seed colors in three generations of offspring.

Explain how natural selection affects the inheritance of traits in succeeding generations.

MATERIALS

containers, such as empty margarine tubs (6)

graph paper

imaginary island habitat (on hallway floor)

bean seeds of different colors (500)

PROCEDURE

1. On the mainland, where the pinto bean plants originated, the original seed colors were brown and white. Mutations may have produced additional seed color variations such as the red, blue, and green seeds found on your island. It is reasonable to conclude that these colors are advantageous to the bean plants’ survival. Write a hypothesis to explain how you think such color variations might affect the survival of a population.

2. Select a 1-meter square area to simulate the island habitat of your population. Your teacher will decide whether you should work outdoors or indoors. If working outdoors, find and mark off a small area with soil or sand and some vegetation, such as green grass. If working indoors, find and mark off a small area of hallway floor. Write a brief description of your island habitat below.

3. Start with 100 seeds of each color in five plastic containers. Mix 20 seeds of each color together in the sixth container, and shake well. Have one team member scatter the seeds randomly over the ENTIRE 1-meter square island habitat. This person will act as the reporter.

4. Have the other three team members act as “birds” feeding on the seeds. Each person quickly picks up 25 of the scattered seeds and brings the results of the “hunt” to the reporter, who counts, sorts, and returns them to the plastic containers by color. Record data on the number and type of each color of seed “eaten.” Calculate and record the number of surviving seeds of each color left in the habitat for the first generation in Table 1.

5. Assume that each surviving seed produces three offspring with the same seed color and continues to live throughout the season. For each surviving seed, scatter three seeds of the same color randomly in the habitat. There should now be 100 seeds in your island habitat that represent the surviving parent generation and their offspring.

TABLE 1: SEED COLOR CHANGE OVER GENERATIONS

6. The reporter records the number of seeds of each color at the start of Generation 2 in the chart. The “birds” again collect and “eat” 25 seeds each and bring them to the reporter for tallying as before. Record data for the number of each color “eaten” and compute and record the number of each color surviving in Generation 2.

7. Add three more seeds of the same color to the habitat for each survivor. Repeat the procedure and record your data for Generation 3 in Table 1. Collect all the seeds and return them to the plastic containers. If you are working outdoors, return to the classroom to analyze your data.

ANALYSIS

1. Calculate the percentage of each surviving bean color in the three generations. Divide the number of survivors of each color by the total number of survivors and multiply this number by 100 to get your percentages. Record the your results in Table 2.

TABLE 2: SEED SURVIVAL

2. On a separate piece of graph paper, make a bar graph or histogram to illustrate your results. Plot the surviving seeds in each of the three generations on the horizontal, or x-axis, and indicate the number of each color of surviving seeds on the vertical, or y-axis.

3. Study your survivor populations for each generation. Describe the changes that occurred in the frequencies of colors among the three generations?

4. Compare the original and the survivor populations. Are any seed color(s) from the original population not represented in the survivor population? Are any seed color(s) from the original population more or less represented?

5. Given the above, which colors were the most fit? Least?

6. How do the colors/appearance of the survivors seem to relate to their habitat?

CONCLUSIONS

7. Describe, in your own words, using terms and concepts from class, what specifically happened to the seed populations in terms of fitness and their environment.

8. What do you predict would happen to the characteristics of the plant population if the seeds continued to be collected by the bird population for several more generations?

9. Explain, in general, how natural selection changes the frequency of genes over many generations.

POST-LAB

10. Separate from the specific environment used in this lab, consider the following "thought experiments" in natural selection – Explain the outcome you might expect under the following conditions…

a. If the color differences were less distinct (ex. all seeds were only shades of browns), would you expect similar results? Explain the outcome you might encounter and why.

b. What if you had a population with all 5 colors again, but the seeds that stood out most made the predator very ill; would you expect similar results? Explain the outcome you might encounter and why.

c. What assumptions must you make about the predator’s abilities for your prediction to come about in the question above (10b)?

d. What if the seeds that stood out most made the predator very ill and it learned to stay away from them, and there also was a new group of seeds very similar in color. What would happen to those seeds? Explain your answer.

e. Over the long term, what trait (ability) could be strongly selected for in the seed-eater population in the situation of similar color variants proposed above (10d)?

f. In (10e) you identified a trait (ability) that would strongly benefit the seed-eater population. Does that mean the population will evolve that trait, since it is a “need” they have. Why or why not?

11. Consider the results in this lab. Did any of the seeds survive because they chose to be the more fit color? Did any supernatural power design the surviving seeds to be more fit? What did you learn about how evolution works from this lab?