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Encyclopedia > Balancing selection
Sickle-shaped red blood cells
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 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 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. Micrograph of sickled red blood cells, taken from the NIH (US government agency) site at http://www. ... The Galápagos Islands hold 13 species of finches that are closely related and differ most markedly in the shape of their beaks. ... In biology, polymorphism can be defined as the occurrence in the same habitat of two or more forms of a trait in such frequencies that the rarer cannot be maintained by recurrent mutation alone. ... An allele is any one of a number of viable DNA codings occupying a given locus (position) on a chromosome. ... In population genetics, directional selection (sometimes referred to as positive selection) occurs when natural selection favors a single allele and therefore allele frequency continuously shifts in one direction. ... Individuals in the mollusk species Donax variabilis show diverse coloration and patterning in their phenotypes. ... The gene pool of a species or a population is the complete set of unique alleles that would be found by inspecting the genetic material of every living member of that species or population. ... The genotype is the specific genetic makeup (the specific genome) of an individual, in the form of DNA. Together with the environmental variation that influences the individual, it codes for the phenotype of that individual. ... The genotype-phenotype distinction refers to the fact that while genotype and phenotype of an organism are related, they do not necessarily coincide. ...


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. A heterozygote advantage (heterozygous advantage or overdominance) describes the case in which the heterozygote genotype has a higher relative fitness than either the homozygote dominant or homozygote recessive genotype. ... Frequency dependent selection is the term given to an evolutionary process where the fitness of a phenotype is dependent on its frequency relative to other phenotypes in a given population. ...

Contents

Heterozygote advantage

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 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. Heterozygote cells are diploid or polyploid and have different alleles at a locus (position) on homologous chromosomes. ... This stylistic schematic diagram shows a gene in relation to the double helix structure of DNA and to a chromosome (right). ... In biology and evolutionary computation, a locus is the position of a gene (or other significant sequence) on a chromosome. ... Fitness (often denoted in population genetics models) is a central concept in evolutionary theory. ... Homozygote cells are diploid or polyploid and have the same alleles at a locus (position) on homologous chromosomes. ... Sickle-cell disease is a general term for a group of genetic disorders caused by sickle hemoglobin (Hgb S or Hb S). ... 3-dimensional structure of hemoglobin. ... Human blood smear: a - erythrocytes; b - neutrophil; c - eosinophil; d - lymphocyte. ... A disease or medical condition is an abnormality of the body or mind that causes discomfort, dysfunction, distress, or death to the person afflicted or those in contact with the person. ... Human red blood cells Red blood cells are the most common type of blood cell and the vertebrate bodys principal means of delivering oxygen from the lungs or gills to body tissues via the blood. ...


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. Malaria is an infectious disease that is widespread in tropical and subtropical regions. ... Malaria is an infectious disease that is widespread in tropical and subtropical regions. ... Protozoa (in Greek protos = first and zoon = animal) are single-celled creatures with nuclei that show some characteristics usually associated with animals, most notably mobility and heterotrophy. ... A parasite is an organism that spends a significant portion of its life in or on the living tissue of a host organism and which causes harm to the host without immediately killing it. ... Binomial name Plasmodium falciparum Welch, 1897 Plasmodium falciparum is a protozoan parasite, one of the species of Plasmodium that cause malaria in humans. ...


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 is the term given to an evolutionary process where the fitness of a phenotype is dependent on the relative frequency of other phenotypes in a given population. ... A parasite is an organism that spends a significant portion of its life in or on the living tissue of a host organism and which causes harm to the host without immediately killing it. ...


Frequency-dependent selection has been observed experimentally in the banding and colour polymorphism in European land snails, Cepaea nemoralis, by Sheppard and Kwok.[citation needed] Frequency-dependent selection often appears in the form of mate preference, a type of sexual selection. The name snail applies to most members of the molluscan class Gastropoda that have coiled shells. ... Binomial name Cepaea nemoralis (Linnaeus, 1758) The grove snail or brown-lipped snail (Cepaea nemoralis) is one of the most common species of land snail in Europe. ... Illustration from The Descent of Man and Selection in Relation to Sex by Charles Darwin showing the Tufted Coquette Lophornis ornatus, female on left, ornamented male on right. ...


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. Binomial name Biston betularia Linnaeus, 1758 Subspecies The Peppered Moth (Biston betularia) is a temperate species of night-flying moth often used by educators as an example of natural selection. ...


References

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

External links

  • Sheppard and Kwok

  Results from FactBites:
 
Balancing selection - Wikipedia, the free encyclopedia (526 words)
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 phenotypic forms.
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.
  More results at FactBites »

 
 

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