In Chapter 6 we saw that both natural selection and genetic drift can lead to evolution by altering the frequency of alleles in populations. The difference between natural selection and genetic drift is that selection acts in a deterministic manner, while genetic drift is a random (sometimes called stochastic) process. Both genetic drift and natural selection can result in an allele being the only allele at that genetic locus in the population. Geneticists refer to this outcome by saying that the allele has been fixed in the population. Of course, if one allele is fixed, that implies that all other alleles have been lost from the population. Thus, genetic drift and natural selection can both lead to the loss of alleles—and hence, genetic variability—in populations.
Because natural populations are not infinitely large, genetic drift is always operating in them. The strength of genetic drift, however, varies with population size: In small populations, genetic drift is more powerful than in large populations. In small populations, genetic drift can sometimes overcome the force of selection.
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In the following set of questions, we consider what happens when genetic drift is the only force in the population. We will examine the simple case of just one genetic locus with two alleles in the population: A and a. To further simplify the exercise, each individual will have just one allele (either A or a). Such organisms are called haploid. Humans and most animals and plants have two alleles (one from their mother and one from their father) at each genetic locus; such organisms are called diploid. Nearly all bacteria and some yeasts and protists are haploid.
Question 1
Set s to 0. This signifies that neither allele has a selection advantage over the other. Let the population size equal 10 by setting N to 10. Start the population with five copies of individuals with the A allele (the remaining five copies will have the a allele) by having X = 5. Run the simulation 40 times, recording how many times A is fixed in the population, how many times a is fixed, and how many times both alleles persist in the population after 100 generations. Describe your observations.
Question 2
Repeat the simulations under the same conditions, but start the populations with two copies of the A allele (the remaining eight will have the a allele) by having X = 2. Run the simulation 40 times, recording how many times A is fixed in the population, how many times a is fixed, and how many times both alleles persist in the population after 100 generations. Describe your observations.
Question 3
Repeat the simulations, but this time set the population size to 50 by letting N = 50. Start the population with 25 copies of individuals with the A allele (the remaining 25 copies will have the a allele) by having X = 25. Run the simulation 40 times, recording how many times A is fixed in the population, how many times a is fixed, and how many times both alleles persist in the population after 100 generations. Describe your observations.
Question 4
Now set N to 200. Start the population with 100 copies of individuals with the A allele (100 should have the a allele) by having X = 100. Run the simulation 40 times, recording how many times A is fixed in the population, how many times a is fixed, and how many times both alleles persist in the population after 100 generations. Describe your observations.
In the next set of questions, we add natural selection as a force that can alter the frequency of alleles in populations. Remember that even in the presence of selection, populations are always subject to genetic drift.
Question 5
First, we will consider a case of strong selection in a population that is not very small. By convention, we set the fitness of individuals with the A allele to be 1. In this exercise, the fitness of individuals with the a allele will be 90% of that of individuals with the A allele, or 0.9. The selection coefficient (s) indicates the difference in fitness between individuals with the A allele and those with the a allele. In this case, s is 1–0.9, or 0.1. Thus set s to 0.1.
Set N to 50. Start the population with 25 copies of individuals with the A allele (25 should have the a allele) by having X = 25. Run the simulation 40 times, recording how many times A is fixed in the population, how many times a is fixed, and how many times both alleles persist in the population after 100 generations. Describe your observations.
Question 6
In the next scenario, we consider weaker selection in a small population. Here, the population size (N) is 10 and the fitness of individuals with the a allele is 0.98. The selection coefficient is thus 1–0.98, or 0.02. Set s to 0.02.
Start the population with five copies of individuals with the A allele (the remaining five copies will have the a allele) by having X = 5. Run the simulation 40 times, recording how many times A is fixed in the population, how many times a is fixed, and how many times both alleles persist in the population after 100 generations. Describe your observations.
Question 7
Finally, we consider the case where there is only a single individual with the advantageous allele in the population. Thus, X, the starting number of individuals with the A allele, is 1. Let the population size (N) equal 100 and the fitness of the a allele is 0.9 (set s to 0.1). Run the simulation 40 times, recording how many times A is fixed in the population, how many times a is fixed, and how many times both alleles persist in the population after 100 generations. Describe your observations.
