Birds on an Island

In this investigation you and your classmates will simulate the fitness of birds of a fictional species called Saccharae utensilus. This bird species has three possible variations in beak phenotype. Each bird’s ability to acquire food will determine whether it dies or whether it survives and reproduces. The number of offspring produced depends on the amount of food each bird acquires, which can vary greatly under changing environmental conditions. After simulating changes in the bird population for six generations, you will analyze data to discover how the frequency of each beak phenotype in the population changed over the generations.

Materials & Equipment
•  plastic spoon, forks, or chopsticks
•  resealable plastic sandwich bag
•  food pieces (for example, candies, unshelled nuts, beans)
•  plastic container for nest (optional)

Question

Can natural selection change the frequency of traits in a population in only a few generations?

Rules of the Island

• You may not use your hands except to hold the plastic utensil (your “beak”) and open the plastic bag (the “nest”).

• You may not push other “birds,” deliberately knock the food out of the other “birds’ beaks,” or steal food from the other “birds’ nests.”

• You must put your nest in the same general area as the other birds’ nests.

• When your teacher says that time is up, stop where you are. If you have food held securely in your beak, you may bring it to your nest.

Procedure

(NOTE: Not every student will participate in the simulation for each round. If you are not participating in the simulation for a round, your responsibility is to collect the data and share it with those who did participate.)

1. Record the initial population for each beak variation in Table C7.1.

Table C7.1

After Round 1 / After Round 2 / After Round 3
Beak Variation / Initial Population Size / Frequency (percent) / Population Size / Frequency (percent) / Population Size / Frequency (percent) / Population Size / Frequency
(percent)
Spoon
Fork
Chopsticks
Total

2. Holding your “beak” in your hand, gather food from the food-source area designated by your teacher. Return to your “nest” to deposit it. Return to the food source to get more food as many times as possible until your teacher tells you to stop. Remember to follow the rules of the island.

3. When the round is over, consult Table C7.2 to determine whether you collected enough food to survive to take part in the next round and possibly reproduce.

Table C7.2

Food Pieces Collected / Outcome
Fewer than 6 / Does not survive
6–11 / Survives but does not reproduce
12–17 / Survives and produces 1 offspring
18–23 / Survives and produces 2 offspring
24–29 / Survives and produces 3 offspring

4. In Table C7.1, record the population size for each beak variation at the end of the round (after reproducing) as well as the total population size. (You will need to collect data from your classmates to record these numbers.) Next, use the following formula to calculate the frequency of each variation as a percentage. Enter your results in Table C7.1.

5. Repeat steps 2 to 4 for two more rounds. Complete the rest of
Table C7.1 after rounds 2 and 3. As the number of offspring increase, different members of your team can gather food with the “offspring” beaks (utensils). You cannot co-operate with team members to gather food items.

6. Now suppose that your island is experiencing a drought. The type of food available for the island’s birds to eat has changed. Your teacher will alter the food source. Perform rounds 4–6 in the same way you performed rounds 1–3. Record the results in Table C7.3.

Table C7.3

After Round 4 / After Round 5 / After Round 6
Beak Variation / Population Size / Frequency
(percent) / Population Size / Frequency
(percent) / Population Size / Frequency
(percent)
Spoon
Fork
Chopstick
Total

Analyzing and Interpreting

7. Was there one beak phenotype that was more successful than the others in rounds 1–3? If so, which one?

8. On the same x- and y-axes, plot three line graphs that show the frequencies of each beak variation throughout the six rounds. Plot rounds 1–6 on the x-axis. Plot the percent frequency of each variation
on the y-axis. Be sure to label the axes and the three graph lines and give your graph a title.

9. Describe the pattern of change for each beak type as displayed in your graph. Identify the most successful beak type or types, and suggest reasons for the success.

10. Did the frequency of the different beak variations change when the food supply changed?

11. How do you think your results might have been different if the beak variations were not genetically based traits (were not passed on from generation to generation)?

12. Competition and variation are two factors that play key roles in natural selection. Describe how these two factors resulted in natural selection in the population during the drought.

13. In what ways did the model in this activity simulate natural conditions and in what ways did the model differ from natural conditions? Suggest one change to the model that could control for an additional variable or more closely simulate the natural world.

Skill Practice

14. Why is it important to graph the frequencies of the beaks rather than the actual number of beaks after each round?

Forming Conclusions

15.  Answer the question at the beginning of this inquiry and use your data to support your answer.