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Migration and genetic drift as mechanisms of evolution by Jon C. Herron, University of Washington
Typology: Exercises
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Part II: Mechanisms of Evolutionary Change Case Studies in Evolution MIGRATION AND GENETIC DRIFT AS MECHANISMS OF EVOLUTION by Jon C. Herron, University of Washington Introduction This case study will help you develop an intuition about how migration and genetic drift cause evolution. You will use a software simulation of an evolving population to analyze examples discussed in Chapter 7, and to answer a variety questions concerning changes in the frequencies of alleles. Once you are familiar with the simulation program, you can use it to answer questions of your own. For example, How often does a mildly deleterious allele drift to fixation in populations of different sizes? To complete the case study you will need the application program AlleleA1. You can download AlleleA from the Evolutionary Analysis website. Versions are provided that run under MacOS and Windows. AlleleA1 simulates evolution at a single locus in an ideal population. The locus has 2 alleles: A 1 and A 2. AlleleA1 allows you to enter parameters controlling selection, mutation, migration, drift, and inbreeding. The program then plots a graph showing the frequency of allele A 1 over time. Each generation's frequency is calculated from the previous generation's frequency, according to the equations described in Chapters 6 and 7. AlleleA1 is easy to use. Small boxes in the lower portion of the AlleleA1 window allow you to enter and change the parameters for the simulation. The tool palette has buttons that allow you to run the simulation, clear the graph, reset all parameters to their default values, print your graph, and quit. More details on using AlleleA1 can be found in the manual, available both as a separate PDF file and online, under the Help menu, while you are running AlleleA1. Exercises Hardy-Weinberg equilibrium
a) Click on the Reset button to restore all parameters to their default values. Predict what will happen when you set the fraction of migrants each generation to 0.01 and the frequency of A 1 in the source population to 0.8. Then set the parameters to these values and run the simulation. If your prediction was not correct, try to explain the difference between what you expected and what actually happened. G eneration Fr eq u en cy of
lle le
Prediction: G eneration Fr eq u en cy of
lle le
What actually happened: Explanation: b) Leave the frequency of A 1 in the source population at 0.8, and try setting the fraction of migrants each generation to 0.05, then 0.1. c) Try several different values for the both the fraction of migrants and the frequency of allele A 1 in the source population. d) Based on your experiences in parts a, b, and c, summarize what migration from the mainland does to the frequency of A 1 on the island. How long does it take for migration to exert its influence? How effective is migration as a mechanism of evolution? Migration and selection
Final frequency of allele A 1 0 0. 1 0. 2 0. 3 0. 4 0. 5 0. 6 0. 7 0. 8 0. 9 1. 0
u m b er o f^ ru ns 5 10 15 20 25 b) Repeat the experiment you performed in part a, but use a starting frequency for A 1 that is substantially different from the one you used before. Record your results in the grid below. Starting frequency of allele A 1 : Percentage of runs in which allele A 1 drifted to fixation: Final frequencies for 100 runs: c) Based on your experiments in parts a and b, can you use the starting frequency of an allele to predict the probability that the allele will eventually drift to fixation? Compare your conclusion to the analysis in Box 7.3 on page 248.
a) Set the graph line color to black, and run the simulation once with the population size set to infinite. How long does it take for allele A 1 to be created by mutation and carried by natural selection to fixation? b) Now set the graph line mode to multiple, the graph line color to red, and the population size to 10. Run the simulation several times. What typically happens? c) Set the graph line color to orange and the population size to 100. Run the simulation several times. What typically happens? d) Set the graph line color to green and the population size to 1000. Run the simulation several times. What typically happens? e) Finally, set the graph line color to blue and the population size to 10000. Run the simulation two or three times. What typically happens? f) How large does a population have to be before a mildly advangateous allele will become fixed as rapidly as it would in a population of infinite size? How strongly does the answer depend on the strength of selection and the mutation rate? Drift, selection, migration, and genetic diversity
Literature Cited Buri, P. 1956. Gene frequency in small populations of mutant Drosophila. Evolution 10:367–402. King, R.B., and R. Lawson. 1995. Color-pattern variation in Lake Erie water snakes: The role of gene flow. Evolution 49:885–896.