Genetic Drift

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.)

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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 : 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!

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 coat- color 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 current-

generation 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 current- generation cup for each generation.


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