Evidence for balancing selection can be found in the number of alleles in a population which are maintained above mutation rate frequencies. All modern research has shown that this significant genetic variation is common in panmictic populations. It is the field experience of Darwin, Wallace and others, that natural populations in the wild are extraordinarily varied. Museum collections of single species tell the same story.
Mechanisms of balancing selection[change | change source]
Heterozygote advantage[change | change source]
In heterozygote advantage, or heterotic balancing selection, an individual who is heterozygous at a particular gene locus has a greater fitness than a homozygous individual. Polymorphisms maintained by this mechanism are balanced polymorphisms.
A well-studied case is that of sickle cell anemia in humans, a hereditary disease that damages red blood cells. Sickle cell anemia is caused by the inheritance of a variant hemoglobin gene (HgbS) from both parents. In these individuals, hemoglobin in red blood cells is extremely sensitive to oxygen deprivation, and this causes shorter life expectancy.
A person who inherits the sickle cell gene from one parent, and a normal hemoglobin gene (HgbA) from the other, has a normal life expectancy. The heterozygote is resistant to the malarial parasite which kills a large number of people each year. The heterozygote frequency is kept high because of the fierce selection against both the homozygotes.
Frequency-dependent selection[change | change source]
Frequency-dependent selection occurs when the fitness of a phenotype depends on its frequency.
In positive frequency-dependent selection the fitness of a phenotype increases as it becomes more common. In negative frequency-dependent selection the fitness of a phenotype increases as it becomes less common. For example, in prey switching, rare morphs of prey are fitter because predators concentrate on more frequent morphs.
Fitness varies in time and space[change | change source]
Selection acts at different levels[change | change source]
The fitness of a genotype may depend on the fitness of other genotypes in the population: this covers many natural situations where the best thing to do (from the point of view of survival and reproduction) depends on what other members of the population are doing at the time.
References[change | change source]
- King R.C. Stansfield W.D. & Mulligan P.K. 2006. A dictionary of genetics, 7th ed. Oxford. p44
- Ford, E.B. (1940). "Polymorphism and taxonomy". In Julian Huxley (ed.). The new systematics. Oxford: Clarendon Press. pp. 493–513.
- Panmixis = random mating, the state in which mating takes place at random in a population.
- Frequency-dependent selection happens when, for example, predators learn to take the most common colour variant rather than the less common form.
- Heredity. 2009. Encyclopædia Britannica. Chicago.
- Allison A.C. 1956. The sickle-cell and Haemoglobin C genes in some African populations. Ann. Human Genet. 21, 67-89.
- Sickle cell anemia. 2009. Encyclopædia Britannica. Chicago.
- Ford E.B. 1965. Genetic polymorphism. MIT Press. p26, Heterozygous advantage.
- Maynard Smith J. 1998. Evolutionary genetics. Oxford. p75 and Chapter 7.