Balancing selection is a form of natural selection that actively maintains multiple versions, or alleles, of a gene within a population. Unlike other selective pressures that may favor a single, optimal version of a gene, this process ensures that different alleles are preserved in the gene pool at frequencies higher than what would be expected by random chance. This prevents any one allele from becoming fixed or lost, promoting genetic diversity.
The Core Principle of Maintaining Variation
A deep gene pool with multiple alleles acts as a buffer, providing a population with the flexibility to adapt to changing conditions. A trait that is advantageous in one environment might be less so in another, or an allele that is neutral today could become beneficial in the face of a new disease or climate shift. By keeping these alternative alleles present, balancing selection ensures the population has the raw material needed to respond to future environmental challenges, enhancing its long-term survival prospects.
Mechanisms of Balancing Selection
Two primary mechanisms drive balancing selection, each working in a distinct way to maintain allele diversity. The first is heterozygote advantage, and the second is negative frequency-dependent selection. Both processes prevent a single allele from dominating by conferring a reproductive or survival advantage to different genetic combinations under specific circumstances.
Heterozygote advantage, sometimes called overdominance, occurs when an individual with two different alleles for a particular gene (a heterozygote) has higher fitness than an individual with two identical alleles (a homozygote). In this scenario, both homozygous forms have some form of disadvantage. An individual inheriting two copies of one allele may suffer from one condition, while an individual with two copies of the other allele may face a different challenge. The heterozygote, however, benefits from having one copy of each, often gaining a protective effect that outweighs any minor disadvantages.
Negative frequency-dependent selection favors rare traits. In this model, an allele’s fitness is inversely related to its commonness; the rarer an allele is, the more advantageous it becomes. A classic analogy is a predator-prey relationship. If a predator develops a search image for the most common type of prey, that common phenotype becomes a liability. The rarer forms, which the predator does not recognize as easily, will have a higher survival rate, and their numbers will increase. As they become more common, the predator may shift its attention to them, making them the new target and giving an advantage back to the now-rare phenotype.
Real-World Examples of Balancing Selection
One of the most well-documented cases of heterozygote advantage involves sickle cell anemia and malaria resistance. The gene for hemoglobin has a normal allele and a sickle cell allele. Individuals who are homozygous for the sickle cell allele suffer from severe sickle cell disease. Those who are homozygous for the normal allele have healthy red blood cells but are highly susceptible to malaria. Individuals who are heterozygous—carrying one of each allele—have a significant survival advantage in regions where malaria is common. Their red blood cells are not ideal hosts for the malaria parasite, conferring resistance to the disease, while they do not suffer the severe effects of sickle cell anemia.
The diversity of Major Histocompatibility Complex (MHC) genes in vertebrates is another example of balancing selection. MHC molecules present pathogen fragments to the immune system, and a wider variety of MHC molecules allows for the recognition of more pathogens. This is an advantage in the constant evolutionary race against pathogens, as heterozygous individuals with multiple MHC types have a more robust immune response. This genetic diversity helps populations withstand constantly evolving diseases.
Negative frequency-dependent selection is illustrated by the scale-eating cichlid fish, Perissodus microlepis. These fish have mouths that are asymmetrically twisted to either the left or the right, which helps them attack their prey from one side to scrape off scales. Their prey, other fish, learn to anticipate attacks from the more common side. This means that if right-mouthed fish are more numerous, their prey will become more vigilant on their left flank, making it easier for the rarer, left-mouthed fish to successfully feed. As the left-mouthed fish thrive and increase in number, the prey will adjust their defensive behavior, giving the advantage back to the now-rarer, right-mouthed fish.
Distinguishing Balancing Selection from Other Evolutionary Pressures
Understanding balancing selection is clearer when contrasted with other modes of natural selection. Two other major evolutionary pressures are directional selection and stabilizing selection. While balancing selection actively maintains diversity, these other processes often work to reduce it.
Directional selection occurs when one extreme phenotype is favored over others, causing the allele frequency in a population to shift consistently in one direction. This happens in response to a changing environment. An example is the evolution of darker-colored peppered moths in England during the Industrial Revolution. As pollution darkened the tree bark, moths with darker wings were better camouflaged from predators and had higher survival rates, causing the allele for dark coloration to increase in frequency.
Stabilizing selection, on the other hand, favors the intermediate phenotype and selects against extreme variations. This is the most common form of natural selection in stable environments where the current average trait is the most adaptive. Human birth weight is an example; babies that are much larger or smaller than average have historically had lower survival rates. This selective pressure narrows the range of variation around the optimal intermediate phenotype.