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TOPIC: The Effect of Gene Flow and Genetic Drift on the Efficiency of Natural Selection in Evolution MODULE CONTENT: This module contains simple exercises for biology majors to begin to explore the influence of gene flow (mediated by individuals moving among populations) and genetic drift on the efficiency of the process of adaptation by natural selection. The module is designed to follow a companion module on the influence of natural selection on changes in allele frequency (entitled Population Genetics I: Natural Selection and Allele Frequencies Breeding Bunnies). Because the same general procedures are used in this module compared with the last one, no additional pre-laboratory exercises are included. TABLE OF CONTENTS Alignment to HHMI Competencies for Entering Medical Students………………...1 Outline of concepts covered, module activities, and implementation……..……....2 Module: Worksheet for completion in class......................................................3 - 8 Suggested Questions for Assessment............................................................9 - 10 Guidelines for Implementation……………………………..............…....................11 Contact Information for Module Developers........................................................12 Alignment to HHMI Competencies for Entering Medical Students: Competency E1. Apply quantitative reasoning and appropriate mathematics to describe or explain phenomena in the natural world. E8. Demonstrate an understanding of how the organizing principle of evolution by natural selection explains the diversity of life on earth.***** Learning Objective E1.1. Demonstrate quantitative numeracy and facility with the language of mathematics. Activity Procedures 38 E1.2. Interpret data sets and communicate those interpretations using visual and other appropriate tools. E8.2 Explain how evolutionary mechanisms contribute to change in gene frequencies in populations Discussion 1,2,3 1 Discussion 1,2,3 Mathematical/Statistical Concepts covered: - probability - chi-square test In class activities: - calculating allele frequencies, Hardy-Weinberg Equilibrium - graphing - using chi-square test Components of module: - preparatory assignment to complete and turn in as homework before class - in class worksheet - suggested assessment questions - guidelines for implementation Estimated time to complete in class worksheet - 60 minutes Targeted students: - first year-biology majors in introductory biology course covering evolution Quantitative Skills Required: - Basic arithmetic - Logical reasoning - Interpreting data from tables - Graph/Data Interpretation 2 WORKSHEET: The Effect of Gene Flow and Genetic Drift on the Efficiency of Natural Selection in Evolution Objective: In this activity, you will examine the influence of gene flow (simulated by migration of individuals) and genetic drift on alleles that are under natural selection. You will be asked to compare this week's results to those you got in the Breeding Bunnies module in which populations of bunnies were only subject to natural selection. In this activity, the allele for normal fur is represented by F and is dominant. The allele for no fur is recessive, and is represented by f. Bunnies that inherit two F alleles or one F and one f allele have fur, while bunnies that inherit two f alleles have no fur. Background and overview: Students at each table should divide into two groups. Each group will represent 1 of 2 populations of bunnies that live on different islands separated by water. The water limits the movement of individuals between populations (in this case the two student groups at a table). Islands have either a cold climate, where normal fur is favored, or a warm climate where the furless phenotype is favored. Decide which student group will study the cold climate/island and which will study the warm climate/island. Natural selection will proceed just as it did in the Breeding Bunnies module, however in this case 50% of the individuals with the unfavorable phenotype(s) are being selected against: FF and Ff will be unfavorable on the warmer island and ff individuals are unfavorable on the cooler one. In other words, if 16 FF and Ff bunnies are on the warm island, then 8 of them will die off. After selection has occurred, and the appropriate number of individuals have been removed from each island population, each group will then exchange three of the individuals from each population (for example: if the population size is 50, then 3 individuals (a total of 6 beans) should be taken at random from one population and moved to the other, simulating the movement of 6 alleles from one island to the next via migration. Make sure that migrating individuals are chosen at random among the available phenotypes after selection. 3 Procedure: 1. Before you begin this exercise decide which students at your table will represent the “cold” island and who will represent the “warm” island. 2. The red beans represent the allele for fur (F), and the white beans represent the allele for no fur (f). The bag represents the island habitat where the rabbits live, and mate randomly. 3. As you did previously for Breeding Bunnies, use the beans (alleles) in your bag (habitat) to count and record your starting allele frequencies. Record the number of red beans (F) in the “Generation 0” row in the column labeled "Number of F Alleles;" white beans in the column "Number of f Alleles." You should have 50 F alleles and 50 f alleles (see your TA if you need extra beans). Calculate the frequency of each allele by dividing the number of each allele by the total number of alleles. Then, place all beans (alleles) back in the bag and shake up (mate) the rabbits. 4. Breed the bunnies by removing two beans at random from the “habitat.” Pairs of beans represent the offspring produced by the mating activity. Keep each pair of beans ("diploid offspring rabbits") together on the table. Keep breeding bunnies until all alleles in the bag (habitat) are used up. 5. Record the results for unfit genotype numbers in the appropriate generation in the data table. 6. Now, depending on which island population you are in (the warm or cold island) remove 50% of the individuals with phenotypes that are not favored in that environment (for example: if selecting against FF or Ff, both of these have the same phenotype, so chose randomly between these two genotypes so that the total removed is 50% of the total number of individuals with the regular fur phenotype). Set the beans that did not survive in a cup so they are out of the way. 7. Now exchange 3 of the survivors with another group to simulate gene flow (migration) (haphazardly choose the individual pairs of alleles that will move to the other population). 8. Now count the F and f alleles (beans) that are in your population. Enter the number of each allele and their frequencies in the right side of the table. 9. Place the alleles of the surviving and the newly migrated rabbits (which have grown, survived and reached reproductive age) back into the bag and mate them again to get the next generation. 4 10. Repeat steps 3-8 to obtain data for generations 2 through 5. Keep these steps in order by remembering 1) MATE, 2) KILL, 3) MIGRATE, 4) COUNT Gen. Parents Number of Unfit Bunnies Number of Bunnies Killed Alleles contributing to next generation Number Number Frequency Frequency of F of f of F of f alleles alleles Number Given/ Received 0 1 2 3 4 5 Always 3 bunnies Always 3 bunnies Always 3 bunnies Always 3 bunnies Always 3 bunnies 6 Make sure dead bunnies are not included in the above columns but the migrated bunnies are included in the columns above. 11. We have included a graph of the results from the previous Breeding Bunnies exercise (see graph on last page of this worksheet). Write your results on this same graph. Use a solid line for F and a dashed line for f. Be sure to include axes labels and a legend to indicate which line belongs to which allele frequency. Note the starting frequencies of F and f (in Generation 0) for your group in the space below: 12. After Generation 5, disaster strikes the islands and half of the populations are greatly reduced. Depending on your group’s instructions (ask TA), “mate” either 2 of your bunnies, or all of your bunnies except 1, record the genotype frequencies, remove the unfit individuals, and then record the allele frequencies. Circle below which of the two options your group did. ALL BUT ONE ONLY TWO 5 13. Include Generation 6 on your graph. Each table must replicate their graph (with data for both warm and cold islands) on the classroom dry-erase board. On this graph that you share with the class also write the change in allele frequency for your group on the board from generations 5 and 6 (for each group, one allele will increase in frequency by a certain amount and the other will decrease in frequency by an equal and opposite amount). Discussion Questions: 1. Referring to the graph you just completed, compare the data you plotted for ONLY Generations 1-5 to the data from the Breeding Bunnies module in which natural selection was the only evolutionary force. How does the rate of evolution by natural selection with migration compare to that of evolution when only natural selection is affecting the population? Provide two potential explanations for why you may see a different rate (and outcome) of evolution in this week's exercise. 2. If selection favored the same phenotype in both populations (the ones exchanging migrants) do you think the rate of evolution would differ from what you observed in the first Breeding Bunnies module (where the only evolutionary force was natural selection)? If so, explain how and why. 3. Now, as a class, compare the final allele frequencies in Generation 6 between the groups that mated all of their bunnies except 1 with the groups that only mated 2. To do this, calculate the average change in allele frequency between the two groups and the standard deviation of the average change (you will need to look at the white boards for other groups to do this). How do the changes in allele frequencies and the variation in this change (measured as standard deviations) differ between those who mated all of their bunnies minus 1 versus those that only mated 2 and why do you think it differs? 6 MODULE FEEDBACK - Each year we work to improve the modules in the active learning "discussion" sections. Please answer the following question with regard to this module on this sheet and turn in your answer to the TA. You can do this anonymously if you like by turning in this sheet separately from your module answers. How helpful was this module in helping you understand a fundamental concept in population genetics? A = Extremely helpful B= Very helpful C= Moderately helpful D= A little bit helpful E = Not helpful at all Module Rating ____________ Thank you! 7 8 Suggested Questions for Assessment Competency E1. Apply quantitative reasoning and appropriate mathematics to describe or explain phenomena in the natural world. E8. Demonstrate an understanding of how the organizing principle of evolution by natural selection explains the diversity of life on earth.***** Learning Objective E1.2. Interpret data sets and communicate those interpretations using visual and other appropriate tools. Activity Discussion 1 E8.2 Explain how evolutionary mechanisms contribute to change in gene frequencies in populations Discussion 1,3 Possible additional questions for assessment: 1. Consider two plant populations that exist in different types of habitats, one relatively wet for most of the year, and the other one dry. Explain how you think migration (gene flow) between these two populations might influence the adaptation of individuals to the local environmental conditions. 2. Why are conservation biologists especially concerned about maintaining genetic variation in endangered species? 3. In which situation below do you think evolution by genetic drift will be the most effective evolutionary force? a. two populations that exist in similar types of habitats separated by a geographic barrier that allows exchange of migrants occasionally b. a random mating population of 100 individuals in which alleles are sampled for each generation at the same frequency as they were in the previous generation c. an island population that was reduced from 10,000 to 10 individuals due to a volcanic eruption that killed the majority of the inhabitants d. evolution by genetic drift is equally effective in all situations 9 Guidelines for Implementation ***This module is designed to follow the Natural Selection Module; Breeding Bunnies and would need to be significantly modified if used on its own. Before Class Preparation: TAs or instructors will need to count out beans to be placed in bags, one bag per group of students. For this exercise each bag of beans should contain 50 red beans and 50 white beans (or light and dark kidney beans, or M&Ms). In Class: Have students break up into groups, ideally of 3-4 students each, 2 groups per table. TAs should provide a 5 minute review of what students will do in the class, reminding them that they will compare this week's results with those of the related module on natural selection that also used the furry and furless bunnies. 1. Provide each group with a bag of beans. 2. Have students work through the module. As the students work, circulate and assist them (but without giving them answers). When all the groups finish question 3, work should stop to allow each group to present their results to the class. We have included instructions for writing the results on a white board but having students prepare an overhead, make an excel graph for projection or construct some other visual aid so that each group can present their results to the class would be effective. 3. Discussion question 3 could actually be elaborated to formally test for significant differences between groups in the change in mean allele frequencies (for example, in a t-test). Because the variance will differ between groups either the data may need to be transformed or a t-test for unequal variance could be used. 10 Module Developers: Please contact us if you have comments/suggestions/corrections Kathleen Hoffman Department of Mathematics and Statistics University of Maryland Baltimore County [email protected] Jeff Leips Department of Biological Sciences University of Maryland Baltimore County [email protected] Sarah Leupen Department of Biological Sciences University of Maryland Baltimore County [email protected] Acknowledgments: This module was developed as part of the National Experiment in Undergraduate Science Education (NEXUS) through Grant No. 52007126 to the University of Maryland, Baltimore County (UMBC) from the Howard Hughes Medical Institute. 11