Balancing selection

Sickle-shaped red blood cells

Balancing selection refers to forms of natural selection which work to maintain genetic polymorphisms (or multiple alleles) within a population. Balancing selection is in contrast to directional selection which favors a single allele. A balanced polymorphism is a situation in which balancing selection within a population is able to maintain stable frequencies of two or more phenotypic forms. Evidence for balancing selection can be found by increased levels of genetic variation between alleles or haplotypes in a species. Note that balancing selection will not always result in an observable phenotypic difference because the genotype may not be one-to-one with the phenotype.

There are several major mechanisms (which are not exclusive within any given population) by which natural selection preserves this variation and consequently may produce a balanced polymorphism. The two most well studied are heterozygote advantage (overdominance) and frequency dependent selection. A less well studied alternative is environmental heterogeneity.

Heterozygote advantage (Overdominant selection)

In heterozygote advantage, an individual who is heterozygous at a particular gene locus has a greater fitness than a homozygous individual. A well-studied case of heterozygote advantage is that of sickle cell anemia. This can be seen in human populations with the locus for a certain amino acid present in the hemoglobin protein (an important component in blood). Individuals who are homozygous for the recessive allele at this locus are inflicted with sickle-cell disease, a disease in which red blood cells are grossly misshapen and which often results in a reduced lifespan.

An individual heterozygous at this locus will not suffer from sickle-cell disease but because of slightly irregularly shaped blood cells they are resistant to malaria. This resistance is favored by natural selection in tropical regions where malaria, a common and deadly sickness caused by the protozoan parasite Plasmodium (such as P. falciparum, P. vivax, P. ovale or P. malariae) is present and so the heterozygote has an evolutionary edge. It is in this way that natural selection preserves stable frequencies of both the heterozygote and the homozygote dominant phenotypes.

Frequency-dependent selection

The second important mechanism by which natural selection can preserve two or more phenotypic forms is known as frequency-dependent selection. Frequency-dependent selection is a form of selection in which the relative fitness of a specific phenotype declines if the frequency of that phenotype becomes too high. An example of this type of selection is between parasites and their hosts.

Frequency-dependent selection has been observed in the banding and color polymorphism in the European land snails, Cepaea nemoralis, where thrushes preferentially predate the most common morph. Frequency-dependent selection also appears in the form of mate preference, a type of sexual selection.

Example 1

Suppose that a parasite can recognize one of two receptors in its host, receptor $Alpha$ or receptor $Beta$, if many parasites with receptor $Alpha$ exist then hosts with receptor $Beta$ will be selected for, and this will subsequently increase the selective pressure on parasites which use receptor $Beta$ and this relationship will continue rocking back and forth.

Example 2

Evolutionary psychologist, Steven Pinker, uses the following example to illustrate the concept of balancing selection:

"A better way of thinking about genetic diversity is that if everyone were a tinker, it would pay to have tailor genes, and the tailor genes would start to make an inroad, but then as society filled up with tailor genes, the advantage would shift back to the tinkers. A result would be an equilibrium with a certain proportion of tinkers and a certain proportion of tailors. Biologists call this process balancing selection: two designs for an organism are equally fit, but in different physical or social environments, including the environments that consist of other members of the species."

Environmental heterogeneity

When environment conditions fluctuate, this may give the normally selected-against organism some form of advantage. An example would be the Biston betularia peppered moth, which has both dark and white polymorphic states. The lighter form is more effectively camouflaged when it rests against tree trunks which have light coloured bark or are covered with lichen, whereas the darker form is more obvious in these situations and is thus likely to be subject to predation by birds. The number of light coloured individuals in a population is normally much greater than the number of darker individuals. In areas subject to pollution by industrial activity, the environment is changed; soot blackens tree trunks and sulphur dioxide kills lichens, so the lighter form is at a selective disadvantage and the darker form gains the advantage of better camouflage. The proportion of darker forms rises accordingly. It is worth stating that the balance is tilted in the other direction when the source of pollution is removed.