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Unit 13 Investigation 2 Adaptation
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Adaptation
Contents
• Introduction
• Thinking About the Question
• Materials
• Safety
• Trial I: Better teeth
• Trial II: Conflicting selection pressures
• Trail III: Mutation
• Technical Hints
• Analysis
• Further Investigations
Introduction
cartoonbeaks.jpg
Discovery question: How do populations adjust to changing environments?
In this investigation, you will use a computer model to observe how heredity and
natural selection allow a population to adapt to a changing environment by
making favorable traits or mutations more common and unfavorable traits less
common.
Thinking About the Question
How do populations adjust to changing environments?
In the natural world, the environment is constantly changing. The weather in a
region may get wetter or drier. A plant species that some animal depended on for
food may be displaced by another plant species. A new predator or competing
species may spread into the area.
Suppose a region gradually has less and less rainfall over many years. It has a
mixture of plants, some of which are more tolerant of drought than others.
Describe what you think will happen to the plant life in the region.
[
]
Unit 13 Investigation 2 Adaptation
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Now suppose that the seeds of a certain kind of cactus are not all the same.
Some of them sprout even if there is little rain. Others produce plants whose
leaves conserve internal moisture better when it gets dry. Others have slightly
deeper root systems. Describe how you think that cactus species might change
over many generations.
[
]
An individual cactus can’t “learn” to have a deeper root system. Yet over many
generations, this species may develop a deeper root system if it helps with
survival in times of drought. How do you think this is possible?
[
]
This final question – how plant and animal populations change in response to
their environment – is at the center of evolution. This model of sheep and grass
will help you explore it.
Materials
• Evolution model
Safety
There are no special safety concerns in this investigation.
Trial 1
Better teeth
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
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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. If SELECTION? = ON, what do you think will happen to the average value
of TEETH over time?
[It will increase. ]
5. if SELECTION? = OFF, what do you think will happen to the average
value of TEETH over time?
[ It will stay near the middle value. ]
6. Open the model. See the Technical hint Running and saving the
evolution model.
[model: sheep-selection1]
7. 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
[There will be quite a bit of statistical variation in the numbers, but no
pattern.]
8. Combine your results with other teams. What can you conclude about
evolution of teeth in the herd?
[ It will not evolve in one direction or the other in a predicable way.]
9. Why do you think it’s not the same every time?
[There is statistical variation in the model. ]
10. 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
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after 50 years
1
1.2
2
1.18
3
1.12
4
1.2
[These are actual experimental values.]
11. Combine your results with other teams. What can you conclude about the
evolution of teeth in the herd?
[It tends to increase toward the maximum value of 1.2.]
12. 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 GRASSREGROWTH-RATE. As this value decreases (grass grows back more
slowly), what would happen to how fast this adaptation becomes
predominant in the population?
a. [ x] 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.
13. Explain your answer.
[The selection pressure for better teeth is greater is there’s less food
available.]
14. Now test this idea by running the model. Try changing GRASSREGROWTH-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)
GRASSREGROWTHRATE
85
70
55
40
Value of TEETH
after 50 years
1.13
1.16
1.2
1.2
15. Combine your results with other teams. What can you conclude about the
effect of a scarcity of grass on evolution of teeth?
[ Greater scarcity means faster adaptation. ]
Trial 2
Conflicting selection pressures
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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?
[That will create a selection pressure toward worse teeth, so the average will
go down.]
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- Years for teeth
NUMBER average to reach 0.9
1
14.24,40,16
2
30,19,15,20
4
16,7,20,12
6
6,17,11,12
[This is actual experimental data.]
3. Combine your results with other teams. What can you conclude about the
effect of removing sheep with better teeth?
[It decreases the average value of 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 REMOVENUMBER. 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 REMOVENUMBER that seems to be a balance point?
[ 2-3
]
5. Combine your results with other teams. What can you conclude about
situations with conflicting selection pressures?
[There may be a balance, and it may or may not be very stable. ]
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6. Moose may have evolved to be large because they could better protect
themselves from predators. What other pressures might have caused
this?
[Maybe they could wade through deeper swamps, or handle snow better.]
7. If these pressures existed, why didn’t moose just keep getting bigger and
bigger? Think of some possible reasons.
[If they’re bigger, they need more food and may not survive as well. Perhaps
the larger babies are more vulnerable. ]
Trial 3
Mutations
[model: mutation]
1. The change in teeth 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.
2. Suppose a mutation occurs in an individual animal. What do you think
would happen if the mutation had these different effects on the animal?
Effect of
mutation
Has a greater
number of
offspring
More resistant to
some deadly
disease
Less attractive to
potential mates
No change in the
animal
Mutation will:
Become disappear
common
x
Why?
Remain at
a low level
[More babies carry that
mutation.]
x
[More survive.]
x
x
[Fewer reproduce.]
x
[Random fluctuations may
remove it.]
3. 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.
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4. 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.
5. Open the model. 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?
[ They usually disappear again.]
6. Keep running the model and add more blue sheep. Does the mutation
become common?
[ It may take over, or it may remain at a modest level.]
7. What can you conclude about a mutation that is neither favorable nor
unfavorable?
[ It fluctuates and doesn’t become common.
]
8. 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.
9.
Run the model and add a mutation. If the mutation dies out, add a few
more. What happens?
[Above a certain level, the mutation takes over quickly.]
10. How does selection help a mutation to become more common?
[The sheep with the mutation are much more likely to survive and reproduce.]
11. What if the mutation were not favorable? Would it become more common?
[No.]
12. Here’s a variation on the model. Suppose that blue sheep had much better
teeth, but females refused to mate with blue males. What do you think
would happen?
[The two effects would balance, and the flock would only be partly blue.]
13. Open the “fussy females” model. [sheep-fussyfemales model] Try the
model with SELECTION? on or off, and FUSSYFEMALES? on or off. Run
the model, add at least 15 mutants, and fill in the following table.
Condition
SELECTION? = OFF
FUSSYFEMALES? = ON
What happens
[Mutation disappears]
SELECTION? = ON
[Mutation becomes more common]
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FUSSYFEMALES? = OFF
SELECTION? = ON
FUSSYFEMALES? = ON
[Mixture of white and blue sheep
remains]
Technical Hints
• Running and saving the evolution model
Analysis
1. Think of some other examples of mutations, besides getting more energy
from grass, which might be favored to become more common in this
sheep population.
Mutation
Why it might become more common
2. Suppose the environment changes:
a. The rainfall decreases.
b. A competing species (goats) moves in that eats the same kind of
grass.
c. Small coyotes arrive with a fondness for baby sheep.
Think of some small mutations that might help the sheep population
survive in these new circumstances.
Mutation
Why it might help
3. Male peacocks have very large tails, and they can hardly fly because their
tails are so big! How could you explain the evolution of such large tails?
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[They use the large tails to attract females, so the ones with larger tails are
more likely to reproduce and pass that trait on to the next generation.]
4. One way that two different species develop from one is that two
populations become geographically separated. Mutations occur in each
group, and they gradually become distinct – especially if environmental
pressures are different.
Invent a story of deer that become separated by a large desert that they
don’t like to cross. They live separately in two somewhat different habitats.
One is mainly forest and the other is mainly grasslands. How might the
two groups be different after 10,000 years?
[ They would learn to eat different foods. One would have grass, and the
other would have buds and twigs. Maybe the predators would be different,
and the deer would have different ways of escaping or defending themselves.
Maybe their coloring would become different, so the deer and their babies
would blend into the grasslands or the forest.
]
5. Instead of a rancher removing some sheep each year, imagine natural
predators, such as wolves. How would this have the same or a different
effect on the genetic features of the sheep, compared to selection by a
rancher? (Hint: what are the features of a sheep that a rancher or a wolf is
most likely to remove?)
[The wolf picks the old or sick sheep. Hence selection pressure from the wolf
tends to keep the flock healthy. The rancher picks the largest and healthiest
sheep. So the rancher must carefully control the breeding process to keep up a
healthy flock.]
6. Think about these two theories about how giraffes acquired long necks.
Theory 1: As they grew up, giraffes stretched out their necks to reach higher in
trees for leaves. The babies inherited their parents’ longer necks. This was
repeated every generation until all giraffes had longer necks.
Theory 2: There were different neck lengths in the giraffe population, due to
natural variations and mutations. The giraffes with longer necks were more
successful getting food because they could reach higher into the trees. They
were more likely to reproduce, and their babies inherited their longer neck
feature. Gradually there were more giraffes with long necks than short necks.
Over many generations, the average neck length of giraffes increased.
Which theory do you think is more correct, and why?
[The first theory isn’t right, because babies inherit their parents’ genes, which
don’t change during the parents’ lifetime. The second theory describes the
process of natural selection, which has been observed in a great variety of
species and also fits well with the fossil record.]
Unit 13 Investigation 2 Adaptation
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Further Investigations


Draft horses (which are heavy, slow, patient, and strong) and racehorses
(which are lighter, very fast, and skittish) come from the same original
stock thousands of years ago. How do you think they came to be so
different?
Read about how bacteria can evolve to be resistant to antibiotic drugs.
Explain how this process works, and what can be done slow it down.
Here are some sample references.
http://www.freep.com/news/health/nstaph12_20021112.htm
http://www.fda.gov/fdac/features/795_antibio.html

Suppose you wanted to breed a special kind of dog that would be happy
and easy to care for in a city. You start with about 20 dogs of different
breeds, and you have 50 years to develop your new “urban hound”. What
characteristics would you look for? How would you proceed with your
project?

MAYBE!! Write down some things you would like to try that involve making
changes in the rules of this model. Send them to the Concord Consortium.
If the changes aren’t too complicated, someone will send you a new model
following your suggestions.