National Academies Northstar Institute for Undergraduate Education in Biology
Teachable Unit Framework
Title of Unit
Date and Approach
for Unit
Development
Unit Developers &
Contact Information
Context
Abstract
Learning Goals &
Outcomes/Objectives
Understanding the mechanisms of evolution: random genetic drift
July 8-12 2013, NANSI University of Minnesota-Twin Cities
Ned Dochtermann (ned.dochtermann@ndsu.edu), Erin Gillam (erin.gillam@ndsu.edu), Timothy
Greives (timothy.greives@ndsu.edu), Kristina Holder (kichler@ku.edu), Steve Travers
(steven.travers@ndsu.edu), Jennifer Weghorst (weghorst@ku.edu)
This unit would be part of a sophomore-level course in evolution for biological science majors. The
previous units covered natural selection and mutation. The following unit would cover gene flow.
This unit focuses on the evolutionary mechanism of random genetic drift. Students explore how
population size affects allele frequencies by engaging in a group activity that involves generating
and plotting data, interpreting graphs, and formulating hypotheses.
Goal(s): what students will know,
Desired Outcome(s)/Objectives(s): specific student
understand, and be able to do;
behaviors or performances that will indicate they have
includes content knowledge,
successfully accomplished the goal(s)
attitudes, & skills (i.e. “understand
natural selection;” “appreciate the
role of biology in society;” “think like
a scientist”
Understand the evolutionary
mechanism of random genetic drift
1. Students can recognize an example of random
genetic drift
2. Students can explain how drift differs from natural
selection
3. Students can use drift to explain why it is incorrect
to state that evolution leads to perfection
4. Students can explain how population size influences
the relative ability of drift to affect allele
frequencies
5. Students can graph data and formulate hypotheses
to explain their observations
6. Students can identify multiple biological scenarios
can result in drift (e.g. bottleneck, founder effect)
7. Students can explain the utility of model systems in
studying complex biological systems
8. Students can use a simulation program to evaluate
the mechanisms of natural selection, mutation, and
drift
National Academies Northstar Institute for Undergraduate Education in Biology
Teachable Unit Framework
Incorporation of Scientific Teaching Themes
Active Learning
How students will engage actively in
learning the concepts
Assessment
How teachers will measure learning; how
students will self-evaluate learning
Activities in class before tidbit:
 Pre-assessment clicker questions
that ask questions about
evolution, natural selection, and
mutation
Pre-assessments (Outcome 1,2):
 Responses to the clicker questions will
indicate if students grasp the major
concept from the previous lecture (natural
selection and mutation)


Discussion of Huntington’s
disease (HD) in the Dutch
population that migrated to
South Africa
Diversity
How the unit is designed to
include all participants
Groups develop hypotheses about the
high rates of HD in South African
immigrants. Responses will provide
evidence about their understanding of
evolutionary mechanisms
Activities during tidbit:
 Students sample from a
population of haploid, sexually
reproducing organisms, creating a
small “founder” population from
a larger “source” population
Tidbit assessments (Outcome 3,4,5,6,7):

Students plot their data

Students demonstrate whether they can
effectively derive frequencies of a
phenotype and accurately plot the data

Discussion 1: Did everyone get
50:50? Who got more orange?
More blue? Anybody have just
one color? If we extend this out
several generations, what do you
think will happen?

Group responses reflect that students
understand that founder populations are
not always the same, and are not
necessarily reflective of phenotype/allele
frequencies in the source population

Discussion 2a: Look at your
graph, what happened to the
frequency of orange over time?
Is this evolution?

Group responses reflect that students
understand that change over time in a
founder population is ultimately
constrained by the phenotype/allele
frequencies of the founder population

Discussion 2b: Look around the
room – how does your data differ
from those collected by other
groups? Why might this be?

Group responses reflect that students
understand that change over time in
different founder populations can be
highly variable due to different starting
conditions in the founder populations.
Diversity in the tidbit:
 We will provide verbal and
written instructions about the
tidbit activity

Students work in a group
setting. This includes
cooperating to complete the
activity, as well as talking
about the discussion questions
in a group as well as with the
whole class
National Academies Northstar Institute for Undergraduate Education in Biology
Teachable Unit Framework

Discussion 2c: Was there
selection? Focus on one graph
that goes to (or near) fixation

Group responses reflect that students
understand that fixation of a
phenotype/allele does not necessarily
mean selection is occurring, but rather
that it can be a product of
phenotype/allele loss when a small
founder population is established from a
larger source population.

Discussion 4: Return to Dutch
example, tie in results from
M&Ms activity, show full graph
with incidence of HD in native
South Africans. At this point, the
term “founder effect” is
introduced.

Students conclude that the incidence of
HD in Afrikaners is not adaptive, as there
is no increased incidence of this disease in
native South Africans. Instead, this is an
example of random genetic drift.
Activities after class
 Homework assignment using
simulation program

Next class: Discussion of
homework

Next class: Case study:
Conservation genetics of Cape
buffalo. The concept of a
bottleneck is introduced.
Post-tidbit assessment (Outcomes 3,4,5,6,7,8):
 Students can understand how adjusting
the parameters of drift (starting
population size) and selection (fitness
differences between alleles) impact the
evolution of population
 Students conclude that multiple biological
scenarios can result in small population
sizes that are strongly affected by drift.

Students are exposed to the
concept of drift through action
(collecting data in class,
manipulating variables during
simulations and writing
(summarizing their answers
when responding to the
homework questions).
Reading could also be
accommodated by
incorporating a post-unit
reading of the textbook or a
foundational paper, such as
Dobzhansky and Pavlovsky
(1957)
National Academies Northstar Institute for Undergraduate Education in Biology
Teachable Unit Framework
Sample Presentation Plan (detailed schedule with approximate timing for unit)
Session 1
Pre-class Previous unit should have covered mutation and natural
selection
10 minutes Start of class: clicker quiz reviewing concepts of natural
selection
- formative assessment
8 minutes Introduction of the pattern of the frequency of Huntington’s
disease worldwide vs. Afrikaners
Students discuss and formulate hypotheses
35 minutes M&M activity
1. introduction of activity, taking time to distinguish it from
the Huntington’s example
- give students time to read instructions
- outcome 7
2. students follow directions to create splinter populations,
begin recording data and graphing their results
- discussion of their results so far and why not everyone got a
frequency of 50%
- students make predictions about how frequencies will
change if the population size is constrained for multiple
generations
- formative assessment
- outcome 4, 5
3. students continue following directions to produce a total of
5 generations, recording the data and graphing their results
discussion of results highlighting:
- drift is a mechanism of evolution
- differences with natural selection
- formative assessment
- outcome 1, 2, 5
12 minutes Follow up on Huntington’s disease – discuss the example of
Huntington’s disease in light of the exercise on drift
Discussion should include
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-
how might students want to modify their initial
hypotheses
- additional information to distinguish drift and natural
selection
- explicit description of the Huntington’s example as a
founder effect
- formative assessment
- outcome 1, 2, 3, 6
9 minutes Introduction of simulation program, run simulation with
larger population size and have students discuss the
difference between the simulation results and their M&M
results.
- PopG can be downloaded at:
http://evolution.gs.washington.edu/popg/
- formative assessment
- outcome 4, 5, 8
1 minute Assign homework that will involve students using the
simulation program and explaining the differences in the
results due to the effects of selection and drift
- summative assessment
- outcome 2, 4, 5, 8
Add additional activities information as needed for the unit.
Session 2
Pre-class Homework assignment completed
In Class - Formative assessment (clicker questions) focused on
simulation results.
- Discussion of homework
- Could present a modification of a graph from the
Dobzhansky and Pavlovsky (1957) paper and ask students
to interpret the data and develop hypotheses. Modified
graph can be found at:
http://www.blackwellpublishing.com/ridley/az/Genetic_drift.asp
- Discuss conservation case study demonstrating a
bottleneck in cape buffalo
- Outcomes 1-8
Resources for Teaching the Unit
National Academies Northstar Institute for Undergraduate Education in Biology
Teachable Unit Framework
(other files and information needed/helpful to teach the unit, including files for papers from which original data
for class activities is taken, supporting information for the instructor, handouts, in class activities materials,
assessments with answer keys, homework assignments, etc.)
1.
2.
3.
4.
5.
Activity data sheet
Activity instruction sheet
PowerPoint presentation to accompany the activity and discussions
Tidbit agenda
Potential post-unit readings:
a. Dobzhansky T & Pavlovsky O. 1957. An experimental study of interaction between genetic drift
and natural selection. Evolution 11: 311–319.
b. Heller R, Okello JBA, Siegismund H. 2010. Can small wildlife conservancies maintain genetically
stable populations of large mammals? Evidence for increased genetic drift in geographically
restricted populations of Cape buffalo in East Africa. Molecular Ecology 19: 1324-1334.
Summary of Origin of the Idea
We decided to focus on this topic because students are usually able to understand the concept of natural
selection fairly easily, but they often struggle with the concept of drift and the concept that differences they
observe in nature can be driven by random processes. As a result, we felt it would be useful to develop an
activity that lays out the tenets of random genetic drift, addresses common misconceptions, and strengthens
critical scientific skills, such as developing hypotheses, collecting data, and interpreting results.
Summary of Feedback
We benefitted from helpful comments from colleagues. Constructive feedback included the following
comments/suggestions:


There were many positive comments about the M&M activity: participants found it fun, memorable,
and useful for conveying important concepts. Having small groups (2-3 students) would help ensure
that everyone fully participates, but larger groups still involve everyone by passing the cups from person
to person. It was suggested that giving students instructions for only the first phase of the activity,
discussing the initial results, and then giving them further instructions might allow a more natural flow
to the activity and would eliminate some groups’ having to wait for other groups. This strategy would
also reduce the amount of reading required at one time and therefore be more inclusive for students
who read slowly.
Several reviews remarked that the large-group discussion during and following the activity was very
helpful for processing the results and clarifying the conclusions students should reach. However, they
National Academies Northstar Institute for Undergraduate Education in Biology
Teachable Unit Framework

pointed out that large-group discussion doesn’t effectively assess whether all students have achieved
the learning goals. Additional formative assessment such as clicker questions would be useful to gauge
success and/or reveal persistent misconceptions. One possibility would be to ask a clicker question after
the introduction of the Huntington’s case study and then ask the same question at the end of the class
discussion.
The instructor should be prepared to discuss with students how social and political factors in colonial
South Africa affected interactions (and allele frequencies) between Afrikaners and non-Afrikaner South
Africans. If discussed appropriately, addressing apartheid and its ramifications could add a powerful
element to this activity.
Acknowledgements
Thanks to Catherine Kirkpatrick and Steven Ralph for being our team facilitators and providing critical feedback
throughout the entire process of developing this teachable unit.