Download Selection, Drift, Mutation, and Gene Flow Use the Allele A1 software

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Transcript
Selection, Drift, Mutation, and Gene Flow
Use the Allele A1 software for this exercise. You can get it from the same website you got Evodots: Jon
C Herron Software.
You’ll need to change various parameters for each exercise. Change only those listed in the directions.
When you start a new exercise, return all the variables to their original settings (this is usually 0).
Drift: Does its impact on allele frequencies depend on population size?
Case 1: Let’s begin with “Population size” = 100, and a “starting frequency of allele A1” = 0.5. Run 10
simulations and keep track of whether allele A1 becomes fixed in the population (Final frequency = 1)
or lost (Final frequency = 0), as well as how many generations it took to fix or disappear.
1. What percentage of the trials resulted in fixation of allele A1?
2. On average, how many generations did it take to fix or lose allele A1? Is this number close to
twice the population size (2N)?
Case 2: Repeat the above experiment, this time with a smaller population (Population size = 10), and
answer the same two questions.
Mutations: How likely are they to spread?
Case 1: In a large population (size=1000), what is the likely fate of a rare (Mutation rate from A2 to
A1 = 0.001), recessive mutation with a fitness advantage of only 5% (i.e. fitness of A1A1= 1.0, fitness
of A1A2 = 0.95, etc.)?
1. Repeat the simulation 10 times to find out.
2. Once it was introduced, via mutation, did A1 ever disappear?
Case 2: What is the likely fate of a rare (Mutation rate from A2 to A1 = 0.001), dominant allele with a
fitness advantage of 5% (i.e. fitness of A1A2 = 1.0)?
1. Repeat the simulation 10 times to find out.
2. Once it was introduced, via mutation, did A1 ever disappear?
3. In general, in which case does it take longer for the mutant allele to fix or be lost?
Drift + Mutation
To see what effect drift can have (even when mutations have a selective advantage!) repeat the above
experiment, but this time set population size = 10. Now what is the likely fate of the mutant allele
under Cases 1 and 2 (same parameters as above, except for population size)?
1. Repeat this simulation 10 times for each case, and keep track of how many times A1 becomes
fixed and how many times it is lost.
2. What can you say about the relative impact of genetic drift on allele frequency changes in small
(N=10) versus large (N=1000) populations?
Gene Flow: Does it alter time to fixation or loss?
1. In a population of size 1000 that is receiving a few migrants per generation (Fraction migrants
each generation = 0.005; that’s only 5 migrants per generation), how many generations does it
take (on average) for an allele (Starting freq. = 0.10) to either fix or be lost?
2. Do allele frequencies tend to diverge much in populations or do they stay relatively stable over
time?
3. How does this compare to an identical population that is receiving no migrants (Fraction
migrants each generation = 0.0)? If the results differ between cases, explain why.