MIGRATION AND NATURAL SELECTION AS COMPETING MECHANISMS OF EVOLUTION
Violations of Hardy-Weinberg Equilibrium: Migration
In Allele A1, the parameter called Fraction of migrants each generation determines the number of individuals that move between two populations, e.g. between a mainland and an island, every generation, as a fraction of the receiving population. For example, setting the parameter to 0.1 means that each generation, ten percent of the individuals in the island population are new arrivals from the mainland. The parameter called Frequency of A1 in the source population determines the frequency of allele A1 on the mainland (and thus among each generation's migrants).
1. Click on the Reset button to restore all parameters to their default values (which means that the starting frequency of A1 in the island population will be 0.5). Predict what will happen when you set the fraction of migrants each generation to 0.01 and the frequency of A1 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.
2. Leave the frequency of A1 in the source population at 0.8, and try setting the fraction of migrants each generation to 0.05, then 0.1. What happened? Why?
.05- the generation gradually decreased.
.1- the generation
3. Try several different values for the both the fraction of migrants and the frequency of allele A1 in the source population. How do these parameters change the result?
4. Based on your experiences in parts 1, 2, and 3, summarize what migration from the mainland does to the frequency of A1 on an island. What determines how long it takes for migration to exert its influence? How effective is migration as a mechanism of evolution?
Migration goes from a large population to a smaller population, carrying over the best genes for the new environment.
Migration and selection
In most real populations, there are many factors acting simultaneously to remove the population from Hardy-Weinberg equilibrium. In these examples, we will look at a theoretical and a real example of natural selection and migration operating on a population at the same time
5. Imagine that allele A1 is deleterious for individuals living on the island, such that the fitnesses of genotypes A1A1, A1A2, and A2A2 are 0.9, 0.95, and 1.
a) If there is no migration, what is the frequency of A1 after 500 generations? Why?
0, because the homozygous recessive and heterozygous have a better fit than the dominant.
b) In accord with what you saw in part a, set the starting frequency of A1 to 0. Remember that this is the frequency in the island population. Now imagine that although it is deleterious on the island, allele A1 is beneficial on the mainland, such that A1 is fixed in the source population (that is, its frequency is equal to 1). What is the island...