Intro to Ecology, ECOL 100
AGENTS OF EVOLUTION (Adapted after Vliet, 1993)
As a way of understanding the mechanisms that cause traits to change over time
(i.e., evolve), we will examine the frequencies of color traits under different
conditions.
Two mechanisms can lead to evolution or a change in allele frequencies over
time:
(1) Natural selection is the differential success in reproduction of different
phenotypes resulting from interaction of organisms with their environment.
(2) Genetic drift is a change in the gene pool of a small population due to chance.
The founder effect is a cause of genetic drift attributable to colonization by a
limited number of individuals from a parent population. The bottleneck effect is a
cause of genetic drift resulting from random reduction of a population such that
the surviving population is no longer genetically representative of the original
population.
Each group will perform all of the simulations listed below and then we will
comment with the rest of the class on the results and significance of everybody’s
findings.
EXERCISE 1: THE EFFECTS OF NATURAL SELECTION
Using white and black beans on the lab counters, a 'predator' preys on beans for
several generations.
Procedure:
Agents of Evolution - 1
1. Each group selects one person from the group to be a ‘predator’ and sends
them out of the room.
2. Scatter 10 white beans and 10 black beans on your portion of the lab counter.
3. Turn off the lights in the lab.
4. Give the predator 10 seconds to forage for beans in dim light. This predator
likes to eat either color equally well but can only use two fingers of one hand to
pick up one bean at a time while foraging.
5. When the predator has finished, record the color ratio of remaining (i.e.,
"surviving") beans.
6. Collect all remaining survivors and assume that each survivor has two new
offspring that are the same color as the parent bean (this is a reproductive event
in which both colors are equally successful at reproducing.)
7. Assemble this new generation and scatter the beans randomly on the table.
Repeat the above procedures of steps 3-7 for five generations. Record
percentages after each selection event in table below.
GENERATION
%
BLACK
%
WHITE
start
50%
50%
1
2
3
4
5
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EXERCISE 2: THE EFFECTS OF GENETIC DRIFT ON SMALL
POPULATIONS
Even without selection against a particular color, changes in color frequencies
may occur in populations over time due to random death and reproduction. As
these effects are entirely random, one might expect significant changes to be
more likely in a small population due to sampling error.
Procedure:
1. Start with five beans of each color (ten beans total) in a cup and mix.
2. Without looking, remove five beans (this represents random death.)
3. Record the color ratio of the surviving beans.
4. Collect all of the remaining survivors and assume that each survivor has two
new offspring that are the same color as the parent bean (this is a reproductive
event in which both colors are equally successful at reproducing.)
5. Repeat steps 2-4 five times, recording color ratios in table below after each
generation.
GENERATION
%
BLACK
%
WHITE
start
50%
50%
1
2
3
4
5
Agents of Evolution - 3
EXERCISE 3: THE EFFECTS OF GENETIC DRIFT ON LARGE
POPULATIONS
Genetic drift, shifts in frequencies of a particular color due to random events, may
be less obvious in a larger population than a smaller one.
Procedure:
1. Start with ten beans of each color (twenty beans total).
2. Without looking, remove ten beans (this represents random death.)
3. Record the color ratio of the surviving beans.
4. Collect all of the remaining survivors and assume that each survivor has two
new offspring that are the same color as the parent bean (this is a reproductive
event in which both colors are equally successful at reproducing.)
5. Repeat steps 2-4 five times, recording color ratios in table below after each
generation.
GENERATION
%
BLACK
%
WHITE
start
50%
50%
1
2
3
4
5
Agents of Evolution - 4
EXERCISE 4: THE FOUNDER OR BOTTLENECK EFFECT
Genotypic ratios can be changed by random sampling alone. This is often
essentially a matter of sampling error. If a few individuals from a large population
form a new small population (that is, if they become founders), by chance alone,
they may differ in genetic proportions from the parent population as a whole. The
new population would thus be different genetically from the parent one.
Procedure
1. Start with a very large population with an equal number of both bean colors in
a jar.
2. Without looking, withdraw a sample population of five individuals.
3. Record this 'founder' population's color ratio in the table below.
4. Replace the first sample population and repeat steps 1-4 five times.
5. Repeat above procedure but now remove larger founder populations of 30
individuals.
6. Record all color ratios in the table below.
Original large population
% BLACK
% WHITE
50%
50%
50%
50%
50%
50%
50%
50%
50%
50%
Small founder
population
% BLACK
% WHITE
Large founder population
% BLACK
% WHITE
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Some final summary questions to consider when writing your final report about
your findings.
1. Did evolution occur in our model populations?
2. What agent of evolution, drift or natural selection, caused the most dramatic
change in the model populations? Justify your answer.
3. Can sampling error during migration or natural catastrophe lead to evolution?
4. What do we assume when ‘reproduction’ takes place in our experiments in
addition to no variation in reproductive success of different colors?
5. How could you determine if a trait that you observe in an organism evolved by
natural selection or by genetic drift?
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