Describe how parasites may exert negative frequency-dependent selection on their hosts.

Parasites can exert negative frequency-dependent selection on their hosts by targeting the most common host genotypes. This means that as a particular host genotype becomes more common within a population, it becomes a more attractive target for the parasite. As a result, the fitness of the common host genotype decreases, leading to a decrease in its frequency within the population. This creates a dynamic where less common host genotypes have a selective advantage, as they are less likely to be targeted by the parasite. This can lead to a more diverse host population, as less common genotypes are favored by negative frequency-dependent selection. Overall, this can have a stabilizing effect on the host population, preventing any one genotype from becoming too dominant.