Trial 1 - Better teeth
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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.
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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
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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.
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If SELECTION? = ON, what do you
think will happen to the average value of
TEETH over time?
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If SELECTION? = OFF, what do you
think will happen to the average value of
TEETH over time?
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Open the model. See
Technical Hints to run and save the evolution
model. [model:
sheep-selection1]
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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 |
|
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Combine your results with other
teams. What can you conclude about evolution
of teeth in the herd?
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Why do you think it's not the same
every time?
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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.
Run # |
Value of TEETH after 50 years | 1 |
|
2 |
|
3 |
|
4 |
|
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Combine your results with other
teams. What can you conclude about the
evolution of teeth in the herd?
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Here's 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 that's
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. |
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Explain your answer.
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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 |
|
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Combine your results with other
teams. What can you conclude about the
effect of a scarcity of grass on evolution
of teeth?
Copyright 2005 The Concord Consortium, All
rights reserved.
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