TEEMSS 2
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All Units>Unit 13 - Adaptation>Investigation 2 - Adaptation>Trial 2

Trial 2 - Conflicting selection pressures

What happens when there are opposing selection pressures?

[model: selection1]

  1. There is another option in this model. Each year, the rancher can remove a certain number of sheep. The sheep with better teeth are chosen, because they have the most market value. What do you think will happen?


  2. Test your idea. Set SELECTION? = OFF. Run the model several times, using different values of REMOVE-NUMBER. Watch the three graphs and notice what happens to the proportions of better, standard, and worse teeth. Also record how long it takes for TEETH AVERAGE to reach 0.9 or less.

    REMOVE-NUMBER Years for teeth average to reach 0.9

  3. Combine your results with other teams. What can you conclude about the effect of removing sheep with better teeth?


  4. Now set SELECTION? = ON. Natural selection will encourage better teeth, but the farmer's action will encourage worse teeth. This is common in nature: a trait will be favorable for one reason, but not favorable for another. Run the model several times, using different values of REMOVE-NUMBER. 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. See if you can find a balance point that will keep the average of TEETH close to 1.0. What is the value of REMOVE-NUMBER that seems to be a balance point?


  5. Combine your results with other teams. What can you conclude about situations with conflicting selection pressures?


  6. Moose may have evolved to be large because they could better protect themselves from predators. What other pressures might have caused this?


  7. If these pressures existed, why didn't moose just keep getting bigger and bigger? Think of some possible reasons.



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