Balancing selection maintains a dynamic state of balance between advantageous and disadvantageous alleles. At first this may sound contradictory, however, think about the sickle-cell allele. You learned that individuals who are heterozygous for the allele show an increase in fitness when living in areas of high malaria infestation. Because these individuals are more fit, the allele persists even though the individuals who are homozygous for the allele have a marked disadvantage. At the population level, the sickle-cell allele is advantageous even though individuals with homozygous alleles are at a disadvantage. The heterozygotic advantage keeps the allele in the population through balancing selection.
Another example of balancing selection is frequency-dependent selection. This type of selection is common among traits that affect predator-prey relationships. If a trait provides the prey with an ability to avoid predation, then individuals with this trait are less likely to be eaten and their fitness is increased. However, in many instances predators can adapt to changes in prey characteristics. If the prey allele frequency becomes high for such a trait, for example a cryptic coloration, then there is a greater chance that a predator species will eventually adapt to be able to detect that cryptic coloration, and having the trait will no longer increase the fitness of the prey. However, prey with a different coloration may become more fit because the predators do not see that coloration as a potential meal. In this system, a phenotype only leads to increased fitness in the prey population if it is maintained in the population at a low level; once it becomes too common, it loses its effectiveness.