Biological Principles II
Darwinian Snails (EvoDots)
Name _______________________________________
INTRODUCTION
The flat periwinkle is a small (~1 cm long) marine snail that lives
in the intertidal zone in New England. Among this snail’s
predators is the European green crab, an invasive species that
reached the East Coast from Europe early in the 19th century.
Prior to 1900, the green crab’s range was south of Cape Cod,
Massachusetts. After the turn of the century, however, the crab
expanded its range northward, and is now found as far north as
Nova Scotia. The crab’s range expansion exposed periwinkle
populations north of Cape Cod to a new agent of natural
selection.
Robin Hadlock-Seeley
Biologist Robin Hadlock-Seeley investigated whether flat
periwinkle populations evolved in response to predation by green
crabs. Hadlock-Seeley found museum specimens of pre-1900 periwinkle shells collected at
Appledore Island, off the coast of Maine. She compared these old shells to shells she collected
herself at the same place. Hadlock-Seeley measured the spire height and thickness of each shell.
Hadlock-Seeley’s data are depicted in the graphs below. As the graphs—and the photos—show,
the crab population on Appledore Island in the early 1980s was, indeed, dramatically different
than it was in 1871. The snails had, on average, shells that were both thicker and flatter than
those of their ancestors.
How did this change—this descent with modification—happen?
The mechanism of evolution is the subject of this tutorial. We will do experiments on a model
population to explore how evolution works.
PART 1: A MODEL OF EVOLUTION BY NATURAL SELECTION
A. Getting started
To complete this tutorial, you will need the software application EvoDots. EvoDots lets you
explore evolution by simulating natural selection in a population of dots.
1. The EvoDots window contains three white areas, three buttons, and three check boxes.
Look to make sure that all three checkboxes are checked.
2. Under the File menu, select Options. Select size as the characteristic in which the dots
vary, then click Okay.
3. Now click on the New Population button. This creates a new population of 50 dots,
scattered at random across the white area on the left. Note also that the white area on the
upper right now contains a graph called Starting Population, showing how many dots of
each color (and size) there are in your population.
4. Click on the little triangle on the top right side of the window. A “History” panel will
open that shows you the number of each type of dot (1-7) for each generation (g). Your
starting population is “generation 0.”
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B. Time to eat some dots!
1. In the EvoDots simulation, you will be a predator on the dots. You will eat the dots by
chasing them and clicking on them with the mouse. Click on the Run button to watch
the dots run around.
2. It will be your job to catch and eat 25 dots as fast as possible simply by clicking on them.
Before proceeding, predict how you think the population of dots will change in response
to predation, and explain your reasoning.
PREDICTION:
REASONING:
3. To play your role correctly, you must act like a hungry predator. Don’t just wait for the
dots to come to you. Go after them! When you click on a dot successfully, it first turns
red, then disappears.
4. Eat 25 dots as fast as you can (note the display that tells you how many dots are left),
then click on the Stop button.
5. When you click the Stop button, the dots stop moving and the white area on the lower
right displays a histogram showing the distribution of colors among the survivors.
Compare the survivors to the staring population. Has the distribution of colors changed? In
what way?
6. Now click on the Reproduce button. Each of the survivor dots splits into two daughter
dots. Note that each mother dot splits to become two daughter dots that are identical in
color and size to each other and to their mother (who no longer exists). This is analogous
to the asexual reproduction of single-celled organisms like bacteria and Paramecium.
7. Click on the Run button again, and eat 25 more dots as fast as you can.
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Again, compare the survivors to the starting population. Has the distribution of colors
changed again? How?
Was the prediction you made above correct? Why or why not?
8. Complete one more round of reproduction and predation. When you’re done, fill in the
table below by copying over the numbers from your History panel (the one you opened
with the little triangle).
● ● ● ● ● ● ●
Generation 0 (before predation)
Generation 0 (after predation)
Generation 1 (before predation)
Generation 1 (after predation)
Generation 2 (before predation)
Generation 2 (after predation)
How many generations would you predict it would take for your population of dots to reach a
point at which it would no longer evolve?
As a population of prey evolves, their predators must evolve as well. Did your strategy for
catching the dots change as the simulation proceeded? How?
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PART 2: REQUIREMENTS FOR EVOLUTION BY NATURAL SELECTION
Now that you have a feel for how EvoDots works, we will start modifying the biology of our
model population to explore the requirements for evolution by natural selection.
Requirement 1: VARIATION
1. Note that each new population of dots you create contains considerable variation in size
(and color, which is coded to indicate size). Do you think the population of dots would
evolve if there were no variation in the starting population? Write down a prediction, and
explain your reasoning.
PREDICTION:
REASONING:
2. Now test your hypothesis. Uncheck the box labeled Variable and create a new population.
All the dots are the same size (and color).
3. Go through two rounds of predation and reproduction.
Does the population evolve? Was your prediction correct?
If, in the absence of variation, the population does not change from generation to generation,
then why is variation necessary for evolution to occur?
4. Before proceeding, click on the Variable check box to give the dots back their variable
sizes.
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Requirement 2: INHERITANCE
As we noted above, when the dots reproduce, each mother dot produces two daughters identical
in size to each other and to their mother. In other words, size is a heritable trait; it is passed from
parents to offspring.
If the size of the “children” dots were assigned randomly and was not handed down from
their parents, do you think the population of dots would evolve? Why or why not?
1. Test your hypothesis. Uncheck the box labeled Heritable and create a new population. All
the dots are the same size (and color).
2. Click Run and “eat” 25 dots as before.
3. Now, when you click on the Reproduce button, watch what happens closely. Each
mother dot produces two daughter dots whose size is chosen at random. They may or may
not be identical to each other or their mother. Go through two rounds of predation and
reproduction.
Does the population of dots evolve? If so, does it evolve the same way it does when size is
heritable? Was your prediction correct?
4. Before proceeding, click on the Heritable checkbox to make size a heritable trait again.
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Requirement 3: SELECTION
Until now, when you have eaten dots you have done so selectively. Because smaller dots are
harder to catch, the smaller dots are much more likely to survive than the larger dots. If the dots
were eaten at random, instead of selectively, do you think the population would still evolve? Write
down a prediction and explain your reasoning.
PREDICTION:
REASONING:
1. To test your hypothesis, uncheck the checkbox labeled Selective, and create a new
population. Make sure the other two checkboxes (Variable and Heritable) are checked.
2. Click Run and eat 25 dots. Notice that when you click the mouse button, you kill not the
dot you are pointing at, but a dot selected at random. (In fact, clicking anywhere inside
the EvoDots window will kill a randomly-selected dot.)
3. Go through a few rounds of random predation and reproduction. When you’re finished,
compare your History panel from this simulation with your data table from p. 4.
Does the population of dots evolve? If so, does it evolve in the same way it does when survival
is selective? Was your prediction correct?
In the boxes below, summarize why each of the following is a requirement for a population to
adaptively change:
VARIATION
HERITABILITY
SELECTION
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