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sheep-selection

by Ed Hazzard (Submitted: 02/07/2005)

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Evolution: Sheep-selection
This is a model of a flock of sheep whose survival depends on eating grass. The babies inherit traits from their parents according to Mendelian genetics. One can study the effect on the population of a trait that changes the energy they get from eating grass.

Here are are some explorations:
1. Picture yourself as a farmer with a large field of sheep. You start with an equal number of males (horns) and females (no horns). They live for six years. The sheep move around the field and eat grass, which grows back at a certain rate. The patch is green if there is grass there, and brown if there is no grass. In the model, the sheep move and eat during each year. They use up energy as they move, and they gain energy from eating grass. If their energy goes to zero, they die.

2. The sheep have one variable trait Š the quality of their teeth. The ones with better teeth get more energy from the grass they eat, so they are less likely to lose energy and die as they wander around the field. This trait is turned on if SELECTION? = ON. It has no effect if SELECTION? = OFF.
There are three levels of teeth:
TEETH = 1.2 better
TEETH = 1.0 standard
TEETH = 0.8 worse

3. Once a year, from age 3 to age 6, each female mates with a randomly chosen male and gives birth to a baby. The baby inherits its TEETH trait from both its parents.

4. If SELECTION? = ON, what do you think will happen to the average value of TEETH over time?

5. if SELECTION? = OFF, what do you think will happen to the average value of TEETH over time?

6. Run the model several times with the initial settings. Make sure SELECTION? = OFF. Note that the average value of TEETH, which is shown in the monitor above the graph, starts at 1.0.Watch the three graphs and notice what happens to the proportions of better, standard, and worse teeth. Also record the average value of TEETH after 50 years.

Run # Value of TEETH after 50 years
1
2
3
4

7. What can you conclude about evolution of teeth in the herd?

8. Why do you think itÕs not the same every time?

9. Set SELECTION? = ON. Now sheep with better teeth will get more food from grass and have less chance of starving to death. Run the model several times. Watch the three graphs and notice what happens to the proportions of better, standard, and worse teeth. Also record the average value of TEETH after 50 years.

10. What can you conclude about the evolution of teeth in the herd?

11. HereÕs a challenge, designed to test our theory that the sheep population evolves better teeth because the ones with worse teeth starve to death more often. If thatÕs true, think about the effect of the value of GRASS-REGROWTH-RATE. As this value decreases (grass grows back more slowly), what would happen to how fast this adaptation becomes predominant in the population?
a. [ ] Better teeth would evolve more quickly.
b. [ ] Better teeth would evolve more slowly.
c. [ ] The evolution rate would not change.
d. [ ] The result would be unpredictable.

12. Explain your answer.

13. Now test this idea by running the model. Try changing GRASS-REGROWTH-RATE and record the average value of TEETH after 50 years. Keep all of the other variables constant. (INITIAL-NUMBER=100, BIRTHRATE-%=60, GAIN-FROM-FOOD=2)

GRASS-REGROWTH-
RATE Value of TEETH after 50 years
85
70
55
40

14. Combine your results with other teams. What can you conclude about the effect of a scarcity of grass on evolution of teeth?

To watch a mutation in the sheep population, go on to sheep-mutation.

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