Balancing selection

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Balancing selection refers to a number of selective processes by which multiple alleles (different versions of a gene) are actively maintained in the gene pool of a population at frequencies larger than expected from genetic drift alone. Balancing selection is rare compared to purifying selection. [1] It can occur by various mechanisms, in particular, when the heterozygotes for the alleles under consideration have a higher fitness than the homozygote. [2] In this way genetic polymorphism is conserved. [3]

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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 ubiquitous in panmictic populations.

There are several mechanisms (which are not exclusive within any given population) by which balancing selection works to maintain polymorphism. The two major and most studied are heterozygote advantage and frequency-dependent selection.

Mechanisms

Heterozygote advantage

Sickle-shaped red blood cells. This non-lethal condition in heterozygotes is maintained by balancing selection in humans of Africa and India due to its resistance to the malarial parasite. Sicklecells.jpg
Sickle-shaped red blood cells. This non-lethal condition in heterozygotes is maintained by balancing selection in humans of Africa and India due to its resistance to the malarial parasite.
Malaria versus sickle-cell trait distributions Malaria versus sickle-cell trait distributions.png
Malaria versus sickle-cell trait distributions

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. [4] Due to unexpected high frequencies of heterozygotes, and an elevated level of heterozygote fitness, heterozygotic advantage may also be called "overdominance" in some literature.

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 an allele (HgbS) of the hemoglobin gene from both parents. In such individuals, the hemoglobin in red blood cells is extremely sensitive to oxygen deprivation, which results in shorter life expectancy. A person who inherits the sickle cell gene from one parent and a normal hemoglobin allele (HgbA) from the other, has a normal life expectancy. However, these heterozygote individuals, known as carriers of the sickle cell trait, may suffer problems from time to time.

The heterozygote is resistant to the malarial parasite which kills a large number of people each year. This is an example of balancing selection between the fierce selection against homozygous sickle-cell sufferers, and the selection against the standard HgbA homozygotes by malaria. The heterozygote has a permanent advantage (a higher fitness) wherever malaria exists. [5] [6] Maintenance of the HgbS allele through positive selection is supported by significant evidence that heterozygotes have decreased fitness in regions where malaria is not prevalent. In Surinam, for example, the allele is maintained in the gene pools of descendants of African slaves, as the Surinam suffers from perennial malaria outbreaks. Curacao, however, which also has a significant population of individuals descending from African slaves, lacks the presence of widespread malaria, and therefore also lacks the selective pressure to maintain the HgbS allele. In Curacao, the HgbS allele has decreased in frequency over the past 300 years, and will eventually be lost from the gene pool due to heterozygote disadvantage. [7]

Frequency-dependent selection

Frequency-dependent selection occurs when the fitness of a phenotype is dependent on its frequency relative to other phenotypes in a given population. 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 decreases as it becomes more common. For example, in prey switching, rare morphs of prey are actually fitter due to predators concentrating on the more frequent morphs. As predation drives the demographic frequencies of the common morph of prey down, the once rare morph of prey becomes the more common morph. Thus, the morph of advantage now is the morph of disadvantage. This may lead to boom and bust cycles of prey morphs. Host-parasite interactions may also drive negative frequency-dependent selection, in alignment with the Red Queen hypothesis. For example, parasitism of freshwater New Zealand snail (Potamopyrgus antipodarum) by the trematode Microphallus sp. results in decreasing frequencies of the most commonly hosted genotypes across several generations. The more common a genotype became in a generation, the more vulnerable to parasitism by Microphallus sp. it became. [8] Note that in these examples that no one phenotypic morph, nor one genotype is entirely extinguished from a population, nor is one phenotypic morph nor genotype selected for fixation. Thus, polymorphism is maintained by negative frequency-dependent selection.

Fitness varies in time and space

The fitness of a genotype may vary greatly between larval and adult stages, or between parts of a habitat range. [9] Variation over time, unlike variation over space, is not in itself enough to maintain multiple types, because in general the type with the highest geometric mean fitness will take over, but there are a number of mechanisms that make stable coexistence possible. [10]

More complex examples

Species in their natural habitat are often far more complex than the typical textbook examples.

Grove snail

The grove snail, Cepaea nemoralis , is famous for the rich polymorphism of its shell. The system is controlled by a series of multiple alleles. Unbanded is the top dominant trait, and the forms of banding are controlled by modifier genes (see epistasis).

Grove snail, dark yellow shell with single band Schneckesnail1.jpg
Grove snail, dark yellow shell with single band

In England the snail is regularly preyed upon by the song thrush Turdus philomelos, which breaks them open on thrush anvils (large stones). Here fragments accumulate, permitting researchers to analyse the snails taken. The thrushes hunt by sight, and capture selectively those forms which match the habitat least well. Snail colonies are found in woodland, hedgerows and grassland, and the predation determines the proportion of phenotypes (morphs) found in each colony.

Two active grove snails Cepaea nemoralis active pair on tree trunk.jpg
Two active grove snails

A second kind of selection also operates on the snail, whereby certain heterozygotes have a physiological advantage over the homozygotes. Thirdly, apostatic selection is likely, with the birds preferentially taking the most common morph. This is the 'search pattern' effect, where a predominantly visual predator persists in targeting the morph which gave a good result, even though other morphs are available.

The polymorphism survives in almost all habitats, though the proportions of morphs varies considerably. The alleles controlling the polymorphism form a supergene with linkage so close as to be nearly absolute. This control saves the population from a high proportion of undesirable recombinants.

In this species predation by birds appears to be the main (but not the only) selective force driving the polymorphism. The snails live on heterogeneous backgrounds, and thrush are adept at detecting poor matches. The inheritance of physiological and cryptic diversity is preserved also by heterozygous advantage in the supergene. [11] [12] [13] [14] [15] Recent work has included the effect of shell colour on thermoregulation, [16] and a wider selection of possible genetic influences is also considered. [17]

Chromosome polymorphism in Drosophila

In the 1930s Theodosius Dobzhansky and his co-workers collected Drosophila pseudoobscura and D. persimilis from wild populations in California and neighbouring states. Using Painter's technique, [18] they studied the polytene chromosomes and discovered that all the wild populations were polymorphic for chromosomal inversions. All the flies look alike whatever inversions they carry, so this is an example of a cryptic polymorphism. Evidence accumulated to show that natural selection was responsible:

Drosophila polytene chromosome Drosophila polytene chromosomes 2.jpg
Drosophila polytene chromosome
  1. Values for heterozygote inversions of the third chromosome were often much higher than they should be under the null assumption: if no advantage for any form the number of heterozygotes should conform to Ns (number in sample) = p2+2pq+q2 where 2pq is the number of heterozygotes (see Hardy–Weinberg equilibrium).
  2. Using a method invented by L'Heretier and Teissier, Dobzhansky bred populations in population cages, which enabled feeding, breeding and sampling whilst preventing escape. This had the benefit of eliminating migration as a possible explanation of the results. Stocks containing inversions at a known initial frequency can be maintained in controlled conditions. It was found that the various chromosome types do not fluctuate at random, as they would if selectively neutral, but adjust to certain frequencies at which they become stabilised.
  3. Different proportions of chromosome morphs were found in different areas. There is, for example, a polymorph-ratio cline in D. robusta along an 18-mile (29 km) transect near Gatlinburg, TN passing from 1,000 feet (300 m) to 4,000 feet. [19] Also, the same areas sampled at different times of year yielded significant differences in the proportions of forms. This indicates a regular cycle of changes which adjust the population to the seasonal conditions. For these results selection is by far the most likely explanation.
  4. Lastly, morphs cannot be maintained at the high levels found simply by mutation, nor is drift a possible explanation when population numbers are high.

By 1951 Dobzhansky was persuaded that the chromosome morphs were being maintained in the population by the selective advantage of the heterozygotes, as with most polymorphisms. [20] [21] [22]

See also

Related Research Articles

An allele, or allelomorph, is a variant of the sequence of nucleotides at a particular location, or locus, on a DNA molecule.

<span class="mw-page-title-main">Dominance (genetics)</span> One gene variant masking the effect of another in the other copy of the gene

In genetics, dominance is the phenomenon of one variant (allele) of a gene on a chromosome masking or overriding the effect of a different variant of the same gene on the other copy of the chromosome. The first variant is termed dominant and the second is called recessive. This state of having two different variants of the same gene on each chromosome is originally caused by a mutation in one of the genes, either new or inherited. The terms autosomal dominant or autosomal recessive are used to describe gene variants on non-sex chromosomes (autosomes) and their associated traits, while those on sex chromosomes (allosomes) are termed X-linked dominant, X-linked recessive or Y-linked; these have an inheritance and presentation pattern that depends on the sex of both the parent and the child. Since there is only one copy of the Y chromosome, Y-linked traits cannot be dominant or recessive. Additionally, there are other forms of dominance, such as incomplete dominance, in which a gene variant has a partial effect compared to when it is present on both chromosomes, and co-dominance, in which different variants on each chromosome both show their associated traits.

Population genetics is a subfield of genetics that deals with genetic differences within and among populations, and is a part of evolutionary biology. Studies in this branch of biology examine such phenomena as adaptation, speciation, and population structure.

Allele frequency, or gene frequency, is the relative frequency of an allele at a particular locus in a population, expressed as a fraction or percentage. Specifically, it is the fraction of all chromosomes in the population that carry that allele over the total population or sample size. Microevolution is the change in allele frequencies that occurs over time within a population.

Ecological genetics is the study of genetics in natural populations. It combines ecology, evolution, and genetics to understand the processes behind adaptation. It is virtually synonymous with the field of molecular ecology.

<span class="mw-page-title-main">Polymorphism (biology)</span> Occurrence of two or more clearly different morphs or forms in the population of a species

In biology, polymorphism is the occurrence of two or more clearly different morphs or forms, also referred to as alternative phenotypes, in the population of a species. To be classified as such, morphs must occupy the same habitat at the same time and belong to a panmictic population.

Frequency-dependent selection is an evolutionary process by which the fitness of a phenotype or genotype depends on the phenotype or genotype composition of a given population.

Disassortative mating is a mating pattern in which individuals with dissimilar phenotypes mate with one another more frequently than would be expected under random mating. Disassortative mating reduces the mean genetic similarities within the population and produces a greater number of heterozygotes. The pattern is character specific, but does not affect allele frequencies. This nonrandom mating pattern will result in deviation from the Hardy-Weinberg principle.

<span class="mw-page-title-main">Index of evolutionary biology articles</span>

This is a list of topics in evolutionary biology.

A heterozygote advantage describes the case in which the heterozygous genotype has a higher relative fitness than either the homozygous dominant or homozygous recessive genotype. Loci exhibiting heterozygote advantage are a small minority of loci. The specific case of heterozygote advantage due to a single locus is known as overdominance. Overdominance is a rare condition in genetics where the phenotype of the heterozygote lies outside of the phenotypical range of both homozygote parents, and heterozygous individuals have a higher fitness than homozygous individuals.

<span class="mw-page-title-main">Overdominance</span>

Overdominance is a phenomenon in genetics where the phenotype of the heterozygote lies outside the phenotypical range of both homozygous parents. Overdominance can also be described as heterozygote advantage regulated by a single genomic locus, wherein heterozygous individuals have a higher fitness than homozygous individuals. However, not all cases of the heterozygote advantage are considered overdominance, as they may be regulated by multiple genomic regions. Overdominance has been hypothesized as an underlying cause for heterosis.

<span class="mw-page-title-main">Disruptive selection</span> Natural selection for extreme trait values over intermediate ones

In evolutionary biology, disruptive selection, also called diversifying selection, describes changes in population genetics in which extreme values for a trait are favored over intermediate values. In this case, the variance of the trait increases and the population is divided into two distinct groups. In this more individuals acquire peripheral character value at both ends of the distribution curve.

In genetics, underdominance, also known as homozygote advantage, heterozygote disadvantage, or negative overdominance," is the opposite of overdominance. It is the selection against the heterozygote, causing disruptive selection and divergent genotypes. Underdominance exists in situations where the heterozygotic genotype is inferior in fitness to either the dominant or recessive homozygotic genotype. Compared to examples of overdominance in actual populations, underdominance is considered more unstable and may lead to the fixation of either allele.

In biology, a cline is a measurable gradient in a single characteristic of a species across its geographical range. Clines usually have a genetic, or phenotypic character. They can show either smooth, continuous gradation in a character, or more abrupt changes in the trait from one geographic region to the next.

Marker assisted selection or marker aided selection (MAS) is an indirect selection process where a trait of interest is selected based on a marker linked to a trait of interest, rather than on the trait itself. This process has been extensively researched and proposed for plant- and animal- breeding.

<span class="mw-page-title-main">Zygosity</span> Degree of similarity of the alleles in an organism

Zygosity is the degree to which both copies of a chromosome or gene have the same genetic sequence. In other words, it is the degree of similarity of the alleles in an organism.

Human genetic resistance to malaria refers to inherited changes in the DNA of humans which increase resistance to malaria and result in increased survival of individuals with those genetic changes. The existence of these genotypes is likely due to evolutionary pressure exerted by parasites of the genus Plasmodium which cause malaria. Since malaria infects red blood cells, these genetic changes are most common alterations to molecules essential for red blood cell function, such as hemoglobin or other cellular proteins or enzymes of red blood cells. These alterations generally protect red blood cells from invasion by Plasmodium parasites or replication of parasites within the red blood cell.

Host–parasite coevolution is a special case of coevolution, where a host and a parasite continually adapt to each other. This can create an evolutionary arms race between them. A more benign possibility is of an evolutionary trade-off between transmission and virulence in the parasite, as if it kills its host too quickly, the parasite will not be able to reproduce either. Another theory, the Red Queen hypothesis, proposes that since both host and parasite have to keep on evolving to keep up with each other, and since sexual reproduction continually creates new combinations of genes, parasitism favours sexual reproduction in the host.

This glossary of genetics and evolutionary biology is a list of definitions of terms and concepts used in the study of genetics and evolutionary biology, as well as sub-disciplines and related fields, with an emphasis on classical genetics, quantitative genetics, population biology, phylogenetics, speciation, and systematics. It has been designed as a companion to Glossary of cellular and molecular biology, which contains many overlapping and related terms; other related glossaries include Glossary of biology and Glossary of ecology.

References

  1. ↑ Charlesworth, Deborah; Willis, John H. (November 2009). "The genetics of inbreeding depression". Nature Reviews Genetics. 10 (11): 783–796. doi:10.1038/nrg2664. ISSN   1471-0064. PMID   19834483. S2CID   771357.
  2. ↑ King, R.C.; Stansfield, W.D.; Mulligan, P.K. (2006). A dictionary of genetics (7th ed.). Oxford: Oxford University Press. p.  44.
  3. ↑ Ford, E.B. (1940). "Polymorphism and taxonomy". In J. Huxley (ed.). The New Systematics. Oxford: Clarendon Press. pp. 493–513.
  4. ↑ Heredity. 2009. Encyclopædia Britannica. Chicago.
  5. ↑ Allison A.C. 1956. The sickle-cell and Haemoglobin C genes in some African populations. Ann. Human Genet.21, 67-89.
  6. ↑ Sickle cell anemia. 2009. Encyclopædia Britannica. Chicago.
  7. ↑ David Wool. 2006. The Driving Forces of Evolution: Genetic Processes in Populations. 80-82.
  8. ↑ Koskella, B. and Lively, C. M. (2009), EVIDENCE FOR NEGATIVE FREQUENCY-DEPENDENT SELECTION DURING EXPERIMENTAL COEVOLUTION OF A FRESHWATER SNAIL AND A STERILIZING TREMATODE. Evolution, 63: 2213–2221. doi : 10.1111/j.1558-5646.2009.00711.x
  9. ↑ Ford E.B. 1965. Genetic polymorphism, p26, Heterozygous advantage. MIT Press 1965.
  10. ↑ Bertram, Jason; Masel, Joanna (20 March 2019). "Different mechanisms drive the maintenance of polymorphism at loci subject to strong versus weak fluctuating selection". Evolution. 73 (5): 883–896. doi:10.1111/evo.13719. hdl: 10150/632441 . PMID   30883731. S2CID   83461372.
  11. ↑ Cain A.J. and Currey J.D. Area effects in Cepaea. Phil. Trans. R. Soc. B 246: 1-81.
  12. ↑ Cain A.J. and Currey J.D. 1968. Climate and selection of banding morphs in Cepaea from the climate optimum to the present day. Phil. Trans. R. Soc. B 253: 483-98.
  13. ↑ Cain A.J. and Sheppard P.M. 1950. Selection in the polymorphic land snail Cepaea nemoralis (L). Heredity4:275-94.
  14. ↑ Cain A.J. and Sheppard P.M. 1954. Natural selection in Cepaea. Genetics 39: 89-116.
  15. ↑ Ford E.B. 1975. Ecological genetics, 4th ed. Chapman & Hall, London
  16. ↑ Jones J.S., Leith B.N. & Rawlings P. 1977. Polymorphism in Cepaea: a problem with too many solutions. Annual Review of Ecology and Systematics8, 109-143.
  17. ↑ Cook L.M. 1998. A two-stage model for Cepaea polymorphism. Phil. Trans. R. Soc. B 353, 1577-1593.
  18. ↑ Painter T.S. 1933. "A new method for the study of chromosome rearrangements and the plotting of chromosome maps". Science78: 585–586.
  19. ↑ Stalker H.D and Carson H.L. 1948. "An altitudinal transect of Drosophila robusta". Evolution1, 237–48.
  20. ↑ Dobzhansky T. 1970. Genetics of the evolutionary process. Columbia University Press N.Y.
  21. ↑ [Dobzhansky T.] 1981. Dobzhansky's genetics of natural populations. eds Lewontin RC, Moore JA, Provine WB and Wallace B. Columbia University Press N.Y.
  22. ↑ Ford E.B. 1975. Ecological genetics. 4th ed. Chapman & Hall, London.
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