HW Equilibrium - Simulation
The five evolutionary forces have the following effect:
1. Selection typically changes both allelic and genotypic frequencies. In this simulation selection is acting against both the yellow (aa) and the green (Aa) individuals. Thus, the blue (AA) individuals are favored.
2. Recurrent Mutation typically changes both allelic and genotypic frequencies, but is usually a much weaker force than the others. In this simulation, recurrent mutation acts to change the a allele to the a allele, thereby opposing selection.
3. Migration also opposes selection in this simulation. This force introduces a alleles into the population, changing both the genotypic and allelic frequencies.
4. Assortative Mating changes only genotype frequencies. In this simulation, positive assortative mating, decreases the frequency of heterozygotes while increasing the frequency of the homozygotes. This decrease in heterozygotes can best be observed via the DeFinetti diagram (see below).
5. Genetic Drift introduces stochasticity into the population. For this simulation the size of the population undergoing genetic drift is 40. For obvious computational reasons, the size of the population "without drift" is 400, instead of infinity.
Combining two or more forces complicates things somewhat. In general, combining two opposing forces (such as selection and migration) produces an equilibrium in the population. Combining two complimentary forces amplifies their effect.
The DeFinetti diagram is useful when determining if a population is in H-W Equilibrium. In this diagram, each of the three genotype frequencies is plotted along one of the sides of the diagram. The curve through the diagram represents the H-W Equilibrium genotypic frequencies. When the population frequencies lie below this line positive assortative mating may be at play. Negative assortative mating is possible when the population frequencies lie above this line. Genetic drift causes the population to deviate noticeable from this line.