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

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Evolution: Sheep-mutation
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 watch the spread of a mutation and how it depends on selection pressure.

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 – 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. The change in teeth in the model sheep-selection demonstrates how an existing trait could become more or less common in a population, in response to selection pressure from the environment. But it doesn’t explore how new features could arise that weren’t present at all. New features arise from mutations in the genes – chance alterations in genetic structure that change the organism.

5. Suppose a mutation occurs in an individual animal. What do you think would happen if the mutation had these different effects on the animal?
Has a greater number of offspring
More resistant to some deadly disease
Less attractive to potential mates
No change in the animal

6. Now explore these questions in the model. In this version, all of the sheep have standard teeth. There is a new feature – an ADD MUTANTS button. When you hit this button, some blue sheep are added to the herd. Sheep pass their color gene on to their offspring.

7. If SELECTION? = ON, a blue sheep gets twice as much energy from grass as a regular sheep. If SELECTION? = OFF, the blue color has no effect on its eating or how likely it is to survive and have offspring.

8. Set SELECTION? = OFF. Run the model and let the population settle down. Hit the ADD MUTANT button. Watch the BLUE SHEEP monitor. What happens to the number of blue sheep?

9. Keep running the model and add more blue sheep. Does the mutation become common?

10. What can you conclude about a mutation that is neither favorable nor unfavorable?

11. Now set SELECTION? = ON. This changes the model so that blue sheep also have much better teeth. This linkage between color and teeth is completely arbitrary! It was done so that you could see whether the mutation becomes more common as time goes by.

12. Run the model and add a mutation. If the mutation dies out, add a few more. What happens?

13. How does selection help a mutation to become more common?

14. What if the mutation were not favorable? Would it become more common?

For a variation on this model, go to sheep-fussyfemales.

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