BILD 3 Discussion Section Activity: Genetic Drift
Your name, student ID number, and section_____________________________
Names of the other students in your group: _____________________________
Genetic drift is a change in a population’s gene pool that occurs as a result of chance events.
Genetic drift results from the fact that some individuals have more offspring than others by chance alone.
Contrast this with natural selection, a mostly non-random process, in which some individuals have more
offspring than others because they have traits that allow them to survive or reproduce better in a particular
environment. Natural selection tends to lead to better adaptation (higher fitness), but genetic drift may help
or hurt the fitness of the population by chance.
We often find that both natural selection and genetic drift affect how allele frequencies change over
time. In this lab, you will study the effects of genetic drift on its own (no natural selection) so you can see
how drift works.
Some mice living near a river have a mix of coat colors: some mice have black coats and some
have white coats. Coat color in this species is controlled by 1 gene with two variants (alleles). Mice with
the white-coat allele have white coats, and mice with the black-coat allele have black coats. (To keep this
example simple, we are pretending that each mouse only has one copy of each gene. This assumption
doesn’t affect the final outcome of genetic drift.)
During a flood, a river shifts its course and cuts off a group of 6 mice on a small piece of land.
From then on, the small group breeds and lives in isolation. The land is only large enough to support 6
mice, so the population stays constant at 6 mice through the generations. Initially, 3 of the mice have black
coats and 3 have white coats.
Question 1 (2 points):
Before going on, make a prediction: After 1000 generations, how many of the 6 mice would you expect
to have black coats? How many would you expect to have white coats? Explain your reasoning.
Note: the accuracy of your prediction won’t affect your grade on this assignment, so don’t change your
prediction after completing the rest of the exercise!
Copyright 2022 by Sarah Stockwell. Do not distribute without permission. 2
In this lab, you will test the effects of genetic drift on the population of island mice.
Turn the completed worksheet in to your IA at the end of this discussion section or at the beginning
of your section next week.
If you aren’t able to make it to section next week, you can turn it in to your IA in a lecture that meets before
your section, or (last resort) scan/photograph it and email it to your IA by the time your section starts. See
the syllabus for the policy on late assignments.
Be sure to fill out the names at the top of the worksheet (above).
Work in a group of 3 to generate data and fill out the tables on the next page (2 points).
Discuss the results with your group and the rest of the class, but answer the written questions in your
own (individual) words.
Each group will need:
• 2 cups
• A bag of extra black and white beans
Label one of your cups “Current generation” and the other cup “Next generation.”
Place 3 black and 3 white beans in the “Current generation” cup. This is the starting population of mice,
with 3 black-coat mice and 3 white-coat mice.
In each generation, each mouse gets an equal, random chance to reproduce and contribute a baby to the
next generation. To simulate breeding, each lab group should do the following:
1. Using the first table below, record the number of black and white mice (beans) that are in the
current generation cup. For the first generation, this is already written in the table for you.
2. Close your eyes and choose a mouse (bean) randomly from the current population cup. This mouse
gets to reproduce and contribute a baby to the next generation. The parent will pass on its coatcolor allele to the baby.
3. Add the baby to the next generation: Take a bean from your bag of extra beans that has the same
color as the parent you chose randomly. Put that bean into next-generation cup.
4. Important: Put the parent back into the current-generation cup, so it has a chance to reproduce
again this generation.
5. Repeat steps 2-4 until you have 6 baby mice (beans) in the next-generation cup.
6. The current generation dies: empty the current-generation cup into the bag of extra beans.
7. The next generation takes over: pour the beans in the next-generation cup into the currentgeneration cup. This is the new breeding population of mice.
Repeat steps 1-7 20 times. Remember to record the number of black and white mice in the currentgeneration cup for each generation.
Copyright 2022 by Sarah Stockwell. Do not distribute without permission. 3
Number of
black mice
Number of
white mice
Number of
black mice
Number of
white mice
Generation 1
3
3
Generation 11
Generation 2 Generation 12
Generation 3 Generation 13
Generation 4 Generation 14
Generation 5 Generation 15
Generation 6 Generation 16
Generation 7 Generation 17
Generation 8 Generation 18
Generation 9 Generation 19
Generation 10 Generation 20
Now convert the allele counts to allele frequencies (proportions) by dividing the number of mice of each
color by the total number of mice. For example, 3/6 = 0.5 (see table below). Do the calculation for each
generation and enter the allele frequencies in the table below. The first one has been calculated for you.
Frequency of
black mice
Frequency of
white mice
Frequency of
black mice
Frequency of
white mice
Generation 1
0.5
0.5
Generation 11
Generation 2 Generation 12
Generation 3 Generation 13
Generation 4 Generation 14
Generation 5 Generation 15
Generation 6 Generation 16
Generation 7 Generation 17
Generation 8 Generation 18
Generation 9 Generation 19
Generation
10
Generation 20
Copyright 2022 by Sarah Stockwell. Do not distribute without permission. 4
1 point: Using the “Frequencies” data from the table above, plot a graph of Allele Frequency versus
Generation Number. Use one symbol or color for the black allele and a different symbol or color for the
white allele. Scale your plot so that it takes up most of the grid below. Label the axes and give your
graph a title.
Title:
2 (1 point). What happened to the black and white alleles in your population of mice?
3 (1 point). Do you still think your prediction for 1000 generations is right? If not, what is your new
prediction, and what happened that made you change your mind?
4 (1 point). Talk to other groups to find out what happened in their mouse populations. Did they get the
same result you did? How do your results compare with theirs?
Copyright 2022 by Sarah Stockwell. Do not distribute without permission. 5
5 (1 point). Will each population in the class go to all-black or all-white eventually? Why or why not?
6 (1 point). Was there natural selection pushing the population toward all-black or all-white coats?
7 (1 point). If you had started with 1 white and 5 black mice, would that have likely made a difference to
the ultimate outcome? Explain your reasoning.
8 (1 point). If the island held 100 mice, do you think it would have taken longer to go to all-black/all-white?
Explain your reasoning.
9 (1 point). Genetic variation (multiple alleles for each gene) is important for the long-term survival of
species. For example, if a new disease infects the population, more genetic variation in immune system
genes makes it more likely that some members of the population will be able to resist the disease.
Endangered species, by definition, have small populations. Why is genetic drift a particular problem for
endangered species?
Before you leave, please return the beans and cups to your IA.
