To select against the a allele, we first will assume complete dominance, i.e., that the deleterious effects of the a allele are only observed in the aa homozygote. In other words, the amount of change is proportional to the amount of genetic variation in the population, as we showed last lecture. However, we will refer back to mutation selection balance several times during the course, so it is essential that you have a feel for dynamics of this evolutionary interaction. Consider two populations, x and y with frequencies of the A allele of p x and p y . If we considered a grid or array of populations and focus on one of those populations as the recipient population with all other populations contributing equally to it, then p y would be replaced by the average p for all the other populations.
Many scenarios are possible.
Depending on the relative strengths of these two opposing forces, an equilibrium condition can arise. So, back to the fish.