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TOPIC: The Effect of Gene Flow and Genetic Drift on the Efficiency of
Natural Selection in Evolution
MODULE CONTENT: This module contains simple exercises for biology majors
to begin to explore the influence of gene flow (mediated by individuals moving
among populations) and genetic drift on the efficiency of the process of
adaptation by natural selection. The module is designed to follow a companion
module on the influence of natural selection on changes in allele frequency
(entitled Population Genetics I: Natural Selection and Allele Frequencies Breeding Bunnies). Because the same general procedures are used in this
module compared with the last one, no additional pre-laboratory exercises are
included.
TABLE OF CONTENTS
Alignment to HHMI Competencies for Entering Medical Students………………...1
Outline of concepts covered, module activities, and implementation……..……....2
Module: Worksheet for completion in class......................................................3 - 8
Suggested Questions for Assessment............................................................9 - 10
Guidelines for Implementation……………………………..............…....................11
Contact Information for Module Developers........................................................12
Alignment to HHMI Competencies for Entering Medical Students:
Competency
E1. Apply quantitative reasoning
and appropriate mathematics to
describe or explain phenomena
in the natural world.
E8. Demonstrate an
understanding of how the
organizing principle of
evolution by natural selection
explains the diversity of life on
earth.*****
Learning Objective
E1.1. Demonstrate quantitative numeracy
and facility with the language of
mathematics.
Activity
Procedures 38
E1.2. Interpret data sets and communicate
those interpretations using visual and other
appropriate tools.
E8.2 Explain how evolutionary mechanisms
contribute to change in gene
frequencies in populations
Discussion
1,2,3
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Discussion
1,2,3
Mathematical/Statistical Concepts covered:
- probability
- chi-square test
In class activities:
- calculating allele frequencies, Hardy-Weinberg Equilibrium
- graphing
- using chi-square test
Components of module:
- preparatory assignment to complete and turn in as homework before class
- in class worksheet
- suggested assessment questions
- guidelines for implementation
Estimated time to complete in class worksheet
- 60 minutes
Targeted students:
- first year-biology majors in introductory biology course covering evolution
Quantitative Skills Required:
- Basic arithmetic
- Logical reasoning
- Interpreting data from tables
- Graph/Data Interpretation
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WORKSHEET: The Effect of Gene Flow and Genetic Drift on the Efficiency
of Natural Selection in Evolution
Objective: In this activity, you will examine the influence of gene flow (simulated
by migration of individuals) and genetic drift on alleles that are under natural
selection. You will be asked to compare this week's results to those you got in
the Breeding Bunnies module in which populations of bunnies were only subject
to natural selection.
In this activity, the allele for normal fur is represented by F and is dominant. The
allele for no fur is recessive, and is represented by f. Bunnies that inherit two F
alleles or one F and one f allele have fur, while bunnies that inherit two f alleles
have no fur.
Background and overview:
 Students at each table should divide into two groups. Each group will
represent 1 of 2 populations of bunnies that live on different islands separated
by water. The water limits the movement of individuals between populations
(in this case the two student groups at a table).
 Islands have either a cold climate, where normal fur is favored, or a warm
climate where the furless phenotype is favored. Decide which student group
will study the cold climate/island and which will study the warm climate/island.
 Natural selection will proceed just as it did in the Breeding Bunnies module,
however in this case 50% of the individuals with the unfavorable phenotype(s)
are being selected against: FF and Ff will be unfavorable on the warmer
island and ff individuals are unfavorable on the cooler one. In other words, if
16 FF and Ff bunnies are on the warm island, then 8 of them will die off.
 After selection has occurred, and the appropriate number of individuals have
been removed from each island population, each group will then exchange
three of the individuals from each population (for example: if the population
size is 50, then 3 individuals (a total of 6 beans) should be taken at random
from one population and moved to the other, simulating the movement of 6
alleles from one island to the next via migration. Make sure that migrating
individuals are chosen at random among the available phenotypes after
selection.
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Procedure:
1. Before you begin this exercise decide which students at your table will
represent the “cold” island and who will represent the “warm” island.
2. The red beans represent the allele for fur (F), and the white beans represent
the allele for no fur (f). The bag represents the island habitat where the
rabbits live, and mate randomly.
3. As you did previously for Breeding Bunnies, use the beans (alleles) in your
bag (habitat) to count and record your starting allele frequencies. Record the
number of red beans (F) in the “Generation 0” row in the column labeled
"Number of F Alleles;" white beans in the column "Number of f Alleles." You
should have 50 F alleles and 50 f alleles (see your TA if you need extra
beans). Calculate the frequency of each allele by dividing the number of
each allele by the total number of alleles. Then, place all beans (alleles) back
in the bag and shake up (mate) the rabbits.
4. Breed the bunnies by removing two beans at random from the “habitat.”
Pairs of beans represent the offspring produced by the mating activity. Keep
each pair of beans ("diploid offspring rabbits") together on the table. Keep
breeding bunnies until all alleles in the bag (habitat) are used up.
5. Record the results for unfit genotype numbers in the appropriate generation
in the data table.
6. Now, depending on which island population you are in (the warm or cold
island) remove 50% of the individuals with phenotypes that are not favored in
that environment (for example: if selecting against FF or Ff, both of these
have the same phenotype, so chose randomly between these two genotypes
so that the total removed is 50% of the total number of individuals with the
regular fur phenotype). Set the beans that did not survive in a cup so they
are out of the way.
7. Now exchange 3 of the survivors with another group to simulate gene flow
(migration) (haphazardly choose the individual pairs of alleles that will move
to the other population).
8. Now count the F and f alleles (beans) that are in your population. Enter the
number of each allele and their frequencies in the right side of the table.
9. Place the alleles of the surviving and the newly migrated rabbits (which have
grown, survived and reached reproductive age) back into the bag and mate
them again to get the next generation.
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10. Repeat steps 3-8 to obtain data for generations 2 through 5. Keep these
steps in order by remembering 1) MATE, 2) KILL, 3) MIGRATE, 4) COUNT
Gen.
Parents
Number of
Unfit
Bunnies
Number of
Bunnies
Killed
Alleles contributing to next generation
Number
Number
Frequency Frequency
of F
of f
of F
of f
alleles
alleles
Number
Given/
Received
0
1
2
3
4
5
Always 3
bunnies
Always 3
bunnies
Always 3
bunnies
Always 3
bunnies
Always 3
bunnies
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Make sure dead bunnies are not included in the
above columns but the migrated bunnies are
included in the columns above.
11. We have included a graph of the results from the previous Breeding Bunnies
exercise (see graph on last page of this worksheet). Write your results on
this same graph. Use a solid line for F and a dashed line for f. Be sure to
include axes labels and a legend to indicate which line belongs to which
allele frequency. Note the starting frequencies of F and f (in Generation
0) for your group in the space below:
12. After Generation 5, disaster strikes the islands and half of the populations
are greatly reduced. Depending on your group’s instructions (ask TA), “mate”
either 2 of your bunnies, or all of your bunnies except 1, record the genotype
frequencies, remove the unfit individuals, and then record the allele
frequencies. Circle below which of the two options your group did.
ALL BUT ONE
ONLY TWO
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13. Include Generation 6 on your graph. Each table must replicate their graph
(with data for both warm and cold islands) on the classroom dry-erase board.
On this graph that you share with the class also write the change in allele
frequency for your group on the board from generations 5 and 6 (for each
group, one allele will increase in frequency by a certain amount and the other
will decrease in frequency by an equal and opposite amount).
Discussion Questions:
1. Referring to the graph you just completed, compare the data you plotted for
ONLY Generations 1-5 to the data from the Breeding Bunnies module in
which natural selection was the only evolutionary force. How does the rate of
evolution by natural selection with migration compare to that of evolution
when only natural selection is affecting the population? Provide two potential
explanations for why you may see a different rate (and outcome) of evolution
in this week's exercise.
2. If selection favored the same phenotype in both populations (the ones
exchanging migrants) do you think the rate of evolution would differ from
what you observed in the first Breeding Bunnies module (where the only
evolutionary force was natural selection)? If so, explain how and why.
3. Now, as a class, compare the final allele frequencies in Generation 6
between the groups that mated all of their bunnies except 1 with the groups
that only mated 2. To do this, calculate the average change in allele
frequency between the two groups and the standard deviation of the
average change (you will need to look at the white boards for other groups
to do this). How do the changes in allele frequencies and the variation in this
change (measured as standard deviations) differ between those who mated
all of their bunnies minus 1 versus those that only mated 2 and why do you
think it differs?
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MODULE FEEDBACK - Each year we work to improve the modules in the active
learning "discussion" sections. Please answer the following question with regard
to this module on this sheet and turn in your answer to the TA. You can do this
anonymously if you like by turning in this sheet separately from your module
answers.
How helpful was this module in helping you understand a fundamental
concept in population genetics?
A = Extremely helpful
B= Very helpful
C= Moderately helpful
D= A little bit helpful
E = Not helpful at all
Module Rating ____________
Thank you!
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Suggested Questions for Assessment
Competency
E1. Apply quantitative reasoning
and appropriate mathematics to
describe or explain phenomena
in the natural world.
E8. Demonstrate an
understanding of how the
organizing principle of
evolution by natural selection
explains the diversity of life on
earth.*****
Learning Objective
E1.2. Interpret data sets and communicate
those interpretations using visual and other
appropriate tools.
Activity
Discussion 1
E8.2 Explain how evolutionary mechanisms
contribute to change in gene
frequencies in populations
Discussion 1,3
Possible additional questions for assessment:
1. Consider two plant populations that exist in different types of habitats, one
relatively wet for most of the year, and the other one dry. Explain how you think
migration (gene flow) between these two populations might influence the
adaptation of individuals to the local environmental conditions.
2. Why are conservation biologists especially concerned about maintaining
genetic variation in endangered species?
3. In which situation below do you think evolution by genetic drift will be the most
effective evolutionary force?
a. two populations that exist in similar types of habitats separated by a
geographic barrier that allows exchange of migrants occasionally
b. a random mating population of 100 individuals in which alleles are sampled
for each generation at the same frequency as they were in the previous
generation
c. an island population that was reduced from 10,000 to 10 individuals due to a
volcanic eruption that killed the majority of the inhabitants
d. evolution by genetic drift is equally effective in all situations
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Guidelines for Implementation
***This module is designed to follow the Natural Selection Module;
Breeding Bunnies and would need to be significantly modified if used on
its own.
Before Class Preparation: TAs or instructors will need to count out beans to be
placed in bags, one bag per group of students. For this exercise each bag of
beans should contain 50 red beans and 50 white beans (or light and dark kidney
beans, or M&Ms).
In Class: Have students break up into groups, ideally of 3-4 students each, 2
groups per table.
TAs should provide a 5 minute review of what students will do in the class,
reminding them that they will compare this week's results with those of the
related module on natural selection that also used the furry and furless bunnies.
1. Provide each group with a bag of beans.
2. Have students work through the module. As the students work, circulate and
assist them (but without giving them answers). When all the groups finish
question 3, work should stop to allow each group to present their results to the
class. We have included instructions for writing the results on a white board but
having students prepare an overhead, make an excel graph for projection or
construct some other visual aid so that each group can present their results to
the class would be effective.
3. Discussion question 3 could actually be elaborated to formally test for
significant differences between groups in the change in mean allele frequencies
(for example, in a t-test). Because the variance will differ between groups either
the data may need to be transformed or a t-test for unequal variance could be
used.
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Module Developers:
Please contact us if you have comments/suggestions/corrections
Kathleen Hoffman
Department of Mathematics and Statistics
University of Maryland Baltimore County
[email protected]
Jeff Leips
Department of Biological Sciences
University of Maryland Baltimore County
[email protected]
Sarah Leupen
Department of Biological Sciences
University of Maryland Baltimore County
[email protected]
Acknowledgments:
This module was developed as part of the National Experiment in Undergraduate
Science Education (NEXUS) through Grant No. 52007126 to the University of
Maryland, Baltimore County (UMBC) from the Howard Hughes Medical Institute.
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