Balanced polymorphism

Balanced Polymorphism

Sickle-shaped red blood cells

A balanced polymorphism is a situation in which natural selection within a population is able to maintain stable frequencies of two or more phenotypic forms. There are two major mechanisms by which natural selection preserves this variation and consequently produces a balanced polymorphism. The first mechanism is known as heterozygote advantage; in which an individual who is heterozygous at a particular gene locus has a greater reproductive success than a homozygous individual. This can be seen in human populations with the locus for a certain protein present in hemoglobin (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 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 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. 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. An example follows: suppose that a certain 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.

References

Campbell, Neil A. & Reece, Jane B. (2002). Biology (6th ed.). Benjamin Cummings. ISBN 0-8053-6624-5.