BILD 3 Discussion Section Activity: Genetic Drift
\nYour name, student ID number, and section_____________________________
\nNames of the other students in your group: _____________________________
\nGenetic drift is a change in a population\u2019s gene pool that occurs as a result of chance events.
\nGenetic drift results from the fact that some individuals have more offspring than others by chance alone.
\nContrast this with natural selection, a mostly non-random process, in which some individuals have more
\noffspring than others because they have traits that allow them to survive or reproduce better in a particular
\nenvironment. Natural selection tends to lead to better adaptation (higher fitness), but genetic drift may help
\nor hurt the fitness of the population by chance.
\nWe often find that both natural selection and genetic drift affect how allele frequencies change over
\ntime. In this lab, you will study the effects of genetic drift on its own (no natural selection) so you can see
\nhow drift works.
\nSome mice living near a river have a mix of coat colors: some mice have black coats and some
\nhave white coats. Coat color in this species is controlled by 1 gene with two variants (alleles). Mice with
\nthe white-coat allele have white coats, and mice with the black-coat allele have black coats. (To keep this
\nexample simple, we are pretending that each mouse only has one copy of each gene. This assumption
\ndoesn\u2019t affect the final outcome of genetic drift.)
\nDuring a flood, a river shifts its course and cuts off a group of 6 mice on a small piece of land.
\nFrom then on, the small group breeds and lives in isolation. The land is only large enough to support 6
\nmice, so the population stays constant at 6 mice through the generations. Initially, 3 of the mice have black
\ncoats and 3 have white coats.
\nQuestion 1 (2 points):
\nBefore going on, make a prediction: After 1000 generations, how many of the 6 mice would you expect
\nto have black coats? How many would you expect to have white coats? Explain your reasoning.
\nNote: the accuracy of your prediction won\u2019t affect your grade on this assignment, so don\u2019t change your
\nprediction after completing the rest of the exercise!
\nCopyright 2022 by Sarah Stockwell. Do not distribute without permission. 2
\nIn this lab, you will test the effects of genetic drift on the population of island mice.
\nTurn the completed worksheet in to your IA at the end of this discussion section or at the beginning
\nof your section next week.
\nIf you aren\u2019t able to make it to section next week, you can turn it in to your IA in a lecture that meets before
\nyour section, or (last resort) scan\/photograph it and email it to your IA by the time your section starts. See
\nthe syllabus for the policy on late assignments.
\nBe sure to fill out the names at the top of the worksheet (above).
\nWork in a group of 3 to generate data and fill out the tables on the next page (2 points).
\nDiscuss the results with your group and the rest of the class, but answer the written questions in your
\nown (individual) words.
\nEach group will need:
\n\u2022 2 cups
\n\u2022 A bag of extra black and white beans
\nLabel one of your cups \u201cCurrent generation\u201d and the other cup \u201cNext generation.\u201d
\nPlace 3 black and 3 white beans in the \u201cCurrent generation\u201d cup. This is the starting population of mice,
\nwith 3 black-coat mice and 3 white-coat mice.
\nIn each generation, each mouse gets an equal, random chance to reproduce and contribute a baby to the
\nnext generation. To simulate breeding, each lab group should do the following:
\n1. Using the first table below, record the number of black and white mice (beans) that are in the
\ncurrent generation cup. For the first generation, this is already written in the table for you.
\n2. Close your eyes and choose a mouse (bean) randomly from the current population cup. This mouse
\ngets to reproduce and contribute a baby to the next generation. The parent will pass on its coatcolor allele to the baby.
\n3. Add the baby to the next generation: Take a bean from your bag of extra beans that has the same
\ncolor as the parent you chose randomly. Put that bean into next-generation cup.
\n4. Important: Put the parent back into the current-generation cup, so it has a chance to reproduce
\nagain this generation.
\n5. Repeat steps 2-4 until you have 6 baby mice (beans) in the next-generation cup.
\n6. The current generation dies: empty the current-generation cup into the bag of extra beans.
\n7. 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.
\nRepeat steps 1-7 20 times. Remember to record the number of black and white mice in the currentgeneration cup for each generation.
\nCopyright 2022 by Sarah Stockwell. Do not distribute without permission. 3
\nNumber of
\nblack mice
\nNumber of
\nwhite mice
\nNumber of
\nblack mice
\nNumber of
\nwhite mice
\nGeneration 1
\n3<\/p>\n
3
\nGeneration 11
\nGeneration 2 Generation 12
\nGeneration 3 Generation 13
\nGeneration 4 Generation 14
\nGeneration 5 Generation 15
\nGeneration 6 Generation 16
\nGeneration 7 Generation 17
\nGeneration 8 Generation 18
\nGeneration 9 Generation 19
\nGeneration 10 Generation 20
\nNow convert the allele counts to allele frequencies (proportions) by dividing the number of mice of each
\ncolor by the total number of mice. For example, 3\/6 = 0.5 (see table below). Do the calculation for each
\ngeneration and enter the allele frequencies in the table below. The first one has been calculated for you.
\nFrequency of
\nblack mice
\nFrequency of
\nwhite mice
\nFrequency of
\nblack mice
\nFrequency of
\nwhite mice
\nGeneration 1
\n0.5<\/p>\n
0.5
\nGeneration 11
\nGeneration 2 Generation 12
\nGeneration 3 Generation 13
\nGeneration 4 Generation 14
\nGeneration 5 Generation 15
\nGeneration 6 Generation 16
\nGeneration 7 Generation 17
\nGeneration 8 Generation 18
\nGeneration 9 Generation 19
\nGeneration
\n10
\nGeneration 20
\nCopyright 2022 by Sarah Stockwell. Do not distribute without permission. 4
\n1 point: Using the \u201cFrequencies\u201d data from the table above, plot a graph of Allele Frequency versus
\nGeneration Number. Use one symbol or color for the black allele and a different symbol or color for the
\nwhite allele. Scale your plot so that it takes up most of the grid below. Label the axes and give your
\ngraph a title.
\nTitle:
\n2 (1 point). What happened to the black and white alleles in your population of mice?
\n3 (1 point). Do you still think your prediction for 1000 generations is right? If not, what is your new
\nprediction, and what happened that made you change your mind?
\n4 (1 point). Talk to other groups to find out what happened in their mouse populations. Did they get the
\nsame result you did? How do your results compare with theirs?
\nCopyright 2022 by Sarah Stockwell. Do not distribute without permission. 5
\n5 (1 point). Will each population in the class go to all-black or all-white eventually? Why or why not?
\n6 (1 point). Was there natural selection pushing the population toward all-black or all-white coats?
\n7 (1 point). If you had started with 1 white and 5 black mice, would that have likely made a difference to
\nthe ultimate outcome? Explain your reasoning.
\n8 (1 point). If the island held 100 mice, do you think it would have taken longer to go to all-black\/all-white?
\nExplain your reasoning.
\n9 (1 point). Genetic variation (multiple alleles for each gene) is important for the long-term survival of
\nspecies. For example, if a new disease infects the population, more genetic variation in immune system
\ngenes makes it more likely that some members of the population will be able to resist the disease.
\nEndangered species, by definition, have small populations. Why is genetic drift a particular problem for
\nendangered species?
\nBefore you leave, please return the beans and cups to your IA.<\/p>\n","protected":false},"excerpt":{"rendered":"
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