Trial 2 - Conflicting selection pressures
What happens when there are
opposing selection pressures?
[model: selection1]
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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?
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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 |
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Combine your results with other
teams. What can you conclude about the
effect of removing sheep with better teeth?
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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?
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Combine your results with other
teams. What can you conclude about
situations with conflicting selection pressures?
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Moose may have evolved to be large
because they could better protect themselves
from predators. What other pressures might
have caused this?
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If these pressures existed, why
didn't moose just keep getting bigger and
bigger? Think of some possible reasons.
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