Natural selection will make alleles with high fitness more common in the population and will remove those alleles with low fitness. So one question people sometimes have is, why do so many genes have multiple alleles? Well, there are multiple answers to that question, but one answer that happens sometimes is called balancing selection. Balancing selection is defined as selection that favors more than one allele in the population. So when we think of the effect on allele frequency, remember, the effect on allele frequency is our measure of evolution.
This process will maintain multiple alleles. It will prevent any one allele from being driven to what we call fixation, becoming the only allele in that population, and it will keep alleles from being removed from the population. We're going to see two common types here: frequency-dependent selection and heterozygote advantage.
Frequency-dependent selection is defined as when selection favors the less common phenotype. For example, consider these snails with different shell patterns. The hypothesis is that birds learn to recognize specific shell patterns of these snails called Cepaea nemoralis. These shell patterns help with camouflage, but if a bird learns a shell pattern, like the common yellowish shells, it will hunt those snails until they become less common. Then the birds will shift their focus to a different, now more common, shell pattern. In this way, the rare phenotype is always favored, preventing any phenotype from becoming most common, thus allowing multiple alleles to be maintained.
The second type, heterozygote advantage, occurs when heterozygotes have higher fitness than either homozygote. Since heterozygotes have two different alleles, this keeps both alleles in the population. A classic example of heterozygote advantage involves malaria and sickle cell. In areas where malaria is prevalent, heterozygote advantage is observed with the sickle cell allele.
Regarding sickle cell, we have three basic phenotypes. Normal hemoglobin refers to AA homozygotes, heterozygotes have the sickle cell trait, and SS homozygotes have sickle cell disease. Sickle cell disease is associated with high childhood mortality rates and shorter lifespans for those untreated. If there is malaria present, individuals with normal hemoglobin (AA homozygotes) have a medium risk of death from malaria but no risk from sickle cell, leading to medium fitness. Individuals with sickle cell disease (SS homozygotes) have high risks from both malaria and sickle cell, resulting in low fitness.
However, individuals with the sickle cell trait (AS heterozygotes) have low risk of death from both malaria and sickle cell. These mild symptoms of sickle cell may protect them from malaria, resulting in high fitness. When heterozygotes mate, they can produce both homozygote forms, keeping both alleles in the population. All three genotypes are expected to be seen in the population, although heterozygotes have the highest fitness. This scenario illustrates why all alleles are retained, despite one genotype having clear fitness advantages. We will explore more about these types of balancing selection soon.