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Wfb 224 Conservation Biology Genetic Drift


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WFB 224 Conservation Biology
Genetic Drift
1. (modified from question 2 in Chapter 25).

How do the initial frequencies in the population affect the probability of extinction or of fixation? Change your initial allele frequencies to p = 0.8, q = 0.2. Set cell K3 to 0.8, and cell L3 to 0.2. Open Tools | Macro | Macros, then edit your Trials macro. You should see the Visual Basic for Applications Code that Excel “wrote” as you recorded your macro. Modify the values from 0.5 to 0.8 and 0.2 (see page 334). Close out of the edit box and return to your spreadsheet. Clear the results of your 100 trials, then run your 100 trials again. Repeat the question for p = 0.6, q = 0.4. Report the number of trials in which one allele was lost.


For the next questions, you will use a genetic drift model at http://darwin.eeb.uconn.edu/simulations/drift.html. You will need to write down the results of each simulation, i.e., is an allele lost at the end of x generations? The program will graph the results of 7 trials, then clear the graph to start again.

2. Starting with an allele frequency of 0.5, and number of generations set to 50, do 10 trials at each of the following population sizes, and record the proportion of trials in which the allele was lost. What is the effect of decreasing population size on the probability that an allele will be lost?


Population sizes: 10, 25, 50, 100, 250


  1. Repeat question 2, with the number of generations set to 250. How does this change the probability that an allele will be lost?




  1. What is a ‘safe’ population size that will keep the probability of losing genetic diversity (at any allelic frequency that you’ve modeled) to a minimum over 250 generations? Why would the target for conservation ever be lower than this?


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