Balancing selection

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 favor a single allele. Particular mechanisms of balancing selection (which are not exclusive within any given population) include:

1. heterozygote advantage or overdominance
2. frequency dependent selection
3. environmental heterogenity

A well-studies case of heterozygote advantage is sickle cell anemia. In some tropical regions (most significantly West Africa), heterozygotes are selected over homozygotes of both the sickle cell allele and the normal allele, as heterozygotes have increased resistance to malaria, a common and deadly sickness caused by the bacterium Plasmodium falciparum.

Frequency-dependent selection was observed in the banding and colour polymorphism in European land snails, Cepaea Nemoralis, by Sheppard and Kwok. Frequency-dependent selection often appears in the form of mate preference, a type of sexual selection.

In the case of environmental heterogenity when the environment conditions fluctuate, it 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. During snowfall, when the fields are covered with snow, it is more likely that the white forms are selectively favoured. The balance is tilted in the other direction when the snow disappears.

External links

• Sheppard and Kwok