Understanding the mechanisms of evolution: random genetic drift

Created by Ned Dochtermann, Erin Gillam, Timothy Greives, Kristina Holder, Steve Travers, and Jennifer Weghorst, this lesson 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.

During the lesson, students use M&M's to represent sampling from a population of haploid, sexually reproducing organisms, creating a small “founder” population from a larger “source” population.  They then plot their data and carry out the following discussions:

  • 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?
  • Discussion 2a:  Look at your graph, what happened to the frequency of orange over time?  Is this evolution?
  • Discussion 2b:  Look around the room – how does your data differ from those collected by other groups?  Why might this be?
  • Discussion 2c:  Was there selection?  Focus on one graph that goes to (or near) fixation.
  • 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.

(Text from the Yale Center for Scientific Teaching's Teachable Tidbits).

Below is a more detailed breakdown of the activity:

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

 

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

 

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

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

- 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

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

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

 

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/a-z/Genetic_drift.asp

-     Discuss conservation case study demonstrating a bottleneck in cape buffalo

The corresponding powerpoint, lesson plan, and handout are available for download below.
Additional materials are available at the Center for Scientific Teaching at Yale.


This activity was contributed by Yale University.
understanding_the_mechanisms_of_evolution_genetic_drift_midwest_2013.pptx4.34 MB
understanding_the_mechanisms_of_evolution_genetic_drift_agenda.docx14 KB
understanding_the_mechanisms_of_evolution_genetic_drift_instructions.docx12 KB