NetLogo User Community Models
by Ed Hazzard (Submitted: 02/07/2005)
Here are are some explorations:
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.
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 doesnÕt explore how new features could arise that werenÕt present at all. New features arise from mutations in the genes Š 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?
6. Now explore these questions in the model. In this version, all of the sheep have standard teeth. There is a new feature Š 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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