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Balancing selection - Wikipedia, the free encyclopedia
Evidence for balancing selection can be found by increased levels of genetic ... Note that balancing selection will not always result in an observable phenotypic ...
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balancing selection: Definition from Answers.com
balanced polymorphism ( ?bal?nst ¦päli?mör?fiz?m ) ( genetics ) Maintenance in ... Evidence for balancing selection can be found by increased levels of genetic ...
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Balancing selection refers to forms of natural selection which work to maintain genetic polymorphism (biology) (or multiple alleles) within a population. Balancing selection is in contrast to directional selection which favor 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 phenotype 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 genotype-phenotype distinction 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 In heterozygote advantage, an individual who is heterozygous at a particular gene locus (genetics) has a greater fitness (biology) than a homozygous individual.A well-studied case of heterozygote advantage is that of Sickle-cell disease. 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 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 falciparum) 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. 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.

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

Environmental heterogeneity In the case of environmental heterogeneity, 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.

References

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

Balancing selection
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Balancing selection - Wikipedia, the free encyclopedia
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 ...

Balancing selection at the prion protein gene consistent with ...
Kuru is an acquired prion disease largely restricted to the Fore linguistic group of the Papua New Guinea Highlands, which was transmitted during endocannibalistic feasts.

Definition: balancing selection from Online Medical Dictionary
The Online Medical Dictionary is a searchable dictionary of definitions from medicine, science and technology.

BioMed Central | Full text | Directional and balancing selection in ...
Edward J Hollox 1 and John AL Armour 2. 1 Department of Genetics, University of Leicester, Leicester, UK. 2 Institute of Genetics, University of Nottingham, Nottingham, UK

A hallmark of balancing selection is present at the promoter region of ...
ORIGINAL ARTICLE A hallmark of balancing selection is present at the promoter region of interleukin 10 JNWilson 1,2, KRockett 1, BKeating 1, MJallow 3, MPinder 3, FSisay-Joof 3 ...

Balancing selection - definition of Balancing selection in the Medical ...
balanced polymorphism. n. A system of genes in which two alleles are maintained in stable equilibrium because the heterozygote is more fit than either of the homozygotes.

Balancing selection - Psychology Wiki
Balancing selection refers to forms of natural selection which work to maintain genetic polymorphisms (or multiple alleles) within a population. Balancing selection is in ...

Presentation No. 47 - Patterns of Nucleotide Variability within and ...
WORKSHOP ABSTRACT NO: 47. Patterns of Nucleotide Variability within and around G6pd, a Locus Under Balancing Selection in Humans. MA Saunders, JD Jensen, MF Hammer, MW Nachman

Selection - Wikipedia, the free encyclopedia
Distinct from patterns of selection are mechanisms of selection; for example, disruptive selection often is the result of disassortative sexual selection, and balancing selection ...