Polymorphism (biology)

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For other uses, see Polymorphism.

Dark-morph or melanistic
jaguar (about 6% of the South American population)
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Darwin's finches by John Gould
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In biology, polymorphism[1] 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 (one with random mating).[2]

Put simply, polymorphism is when there are two or more possibilities of a trait on a gene. For example, there is more than one possible trait in terms of a jaguar's skin colouring; they can be light morph or dark morph. Due to having more than one possible variation for this gene, it is termed 'polymorphism'. However, if the jaguar has only one possible trait for that gene, it would be termed "monomorphic". For example, if there was only one possible skin colour that a jaguar could have, it would be termed monomorphic.

The term

single nucleotide polymorphisms that may not always correspond to a phenotype, but always corresponds to a branch in the genetic tree. See below
.

Polymorphism is common in nature; it is related to

blood types
.

According to the theory of evolution, polymorphism results from evolutionary processes, as does any aspect of a species. It is

heritable and is modified by natural selection
. In polyphenism, an individual's genetic makeup allows for different morphs, and the switch mechanism that determines which morph is shown is environmental. In genetic polymorphism, the genetic makeup determines the morph.

The term polymorphism also refers to the occurrence of structurally and functionally more than two different types of individuals, called

cnidarians.[2]
For example,
medusae
.

Balanced polymorphism refers to the maintenance of different phenotypes in population.

Terminology

Monomorphism means having only one form. Dimorphism means having two forms.

Nomenclature

Polymorphism crosses several discipline boundaries, including ecology, genetics, evolution theory, taxonomy, cytology, and biochemistry. Different disciplines may give the same concept different names, and different concepts may be given the same name. For example, there are the terms established in ecological genetics by

E.B. Ford (1975),[4] and for classical genetics by John Maynard Smith (1998).[7] The shorter term morphism was preferred by the evolutionary biologist Julian Huxley (1955).[8]

Various synonymous terms exist for the various polymorphic forms of an organism. The most common are morph and morpha, while a more formal term is morphotype. Form and phase are sometimes used, but are easily confused in zoology with, respectively, "form" in a population of animals, and "phase" as a color or other change in an organism due to environmental conditions (temperature, humidity, etc.). Phenotypic traits and characteristics are also possible descriptions, though that would imply just a limited aspect of the body.

In the

viticultural usage, rice agriculture jargon, and informal gardening lingo) and with the legal concept "plant variety" (protection of a cultivar as a form of intellectual property
).

Mechanisms

Three mechanisms may cause polymorphism:[9]

  • Genetic polymorphism
    – where the phenotype of each individual is genetically determined
  • A conditional development strategy, where the phenotype of each individual is set by environmental cues
  • A mixed development strategy, where the phenotype is randomly assigned during development

Relative frequency

Endler's survey of natural selection gave an indication of the relative importance of polymorphisms among studies showing natural selection.[10] The results, in summary: Number of species demonstrating natural selection: 141. Number showing quantitative traits: 56. Number showing polymorphic traits: 62. Number showing both Q and P traits: 23. This shows that polymorphisms are found to be at least as common as continuous variation in studies of natural selection, and hence just as likely to be part of the evolutionary process.[citation needed]

Genetics

Genetic polymorphism

Main article: Gene polymorphism

Since all polymorphism has a genetic basis, genetic polymorphism has a particular meaning:

  • Genetic polymorphism is the simultaneous occurrence in the same locality of two or more discontinuous forms in such proportions that the rarest of them cannot be maintained just by recurrent mutation or immigration, originally defined by Ford (1940).[6][11]: 11  The later definition by Cavalli-Sforza & Bodmer (1971) is currently used: "Genetic polymorphism is the occurrence in the same population of two or more alleles at one locus, each with appreciable frequency", where the minimum frequency is typically taken as 1%.[12][13]

The definition has three parts: a) sympatry: one interbreeding population; b) discrete forms; and c) not maintained just by mutation.

In simple words, the term polymorphism was originally used to describe variations in shape and form that distinguish normal individuals within a species from each other. Presently, geneticists use the term genetic polymorphism to describe the inter-individual, functionally silent differences in DNA sequence that make each human genome unique.[14]

Genetic polymorphism is actively and steadily maintained in populations by natural selection, in contrast to transient polymorphisms where a form is progressively replaced by another.[15]: 6–7  By definition, genetic polymorphism relates to a balance or equilibrium between morphs. The mechanisms that conserve it are types of balancing selection.

Mechanisms of balancing selection

Pleiotropism

Most genes have more than one effect on the phenotype of an organism (pleiotropism). Some of these effects may be visible, and others cryptic, so it is often important to look beyond the most obvious effects of a gene to identify other effects. Cases occur where a gene affects an unimportant visible character, yet a change in fitness is recorded. In such cases, the gene's other (cryptic or 'physiological') effects may be responsible for the change in fitness. Pleiotropism is posing continual challenges for many clinical dysmorphologists in their attempt to explain birth defects which affect one or more organ system, with only a single underlying causative agent. For many pleiotropic disorders, the connection between the gene defect and the various manifestations is neither obvious, nor well understood.[16]

"If a neutral trait is pleiotropically linked to an advantageous one, it may emerge because of a process of natural selection. It was selected but this doesn't mean it is an adaptation. The reason is that, although it was selected, there was no selection for that trait."[17]

Epistasis

Epistasis occurs when the expression of one gene is modified by another gene. For example, gene A only shows its effect when allele B1 (at another locus) is present, but not if it is absent. This is one of the ways in which two or more genes may combine to produce a coordinated change in more than one characteristic (for instance, in mimicry). Unlike the supergene, epistatic genes do not need to be closely linked or even on the same chromosome.

Both pleiotropism and epistasis show that a gene need not relate to a character in the simple manner that was once supposed.

The origin of supergenes

Although a polymorphism can be controlled by

linked genes on a single chromosome. Batesian mimicry in butterflies and heterostyly
in angiosperms are good examples. There is a long-standing debate as to how this situation could have arisen, and the question is not yet resolved.

Whereas a gene family (several tightly linked genes performing similar or identical functions) arises by duplication of a single original gene, this is usually not the case with supergenes. In a supergene some of the constituent genes have quite distinct functions, so they must have come together under selection. This process might involve suppression of crossing-over, translocation of chromosome fragments and possibly occasional cistron duplication. That crossing-over can be suppressed by selection has been known for many years.[18][19]

Debate has centered round the question of whether the component genes in a super-gene could have started off on separate chromosomes, with subsequent reorganization, or if it is necessary for them to start on the same chromosome. Originally, it was held that chromosome rearrangement would play an important role.[20] This explanation was accepted by E. B. Ford and incorporated into his accounts of ecological genetics.[4]: ch. 6 [11]: 17–25 

However, many believe it more likely that the genes start on the same chromosome.[21] They argue that supergenes arose in situ. This is known as Turner's sieve hypothesis.[22] John Maynard Smith agreed with this view in his authoritative textbook,[7] but the question is still not definitively settled.

Ecology

Selection, whether natural or artificial, changes the frequency of morphs within a population; this occurs when morphs reproduce with different degrees of success. A genetic (or balanced) polymorphism usually persists over many generations, maintained by two or more opposed and powerful selection pressures.

prefossil shells going back to the Mesolithic Holocene.[23][24]
Non-human apes have similar blood groups to humans; this strongly suggests that this kind of polymorphism is ancient, at least as far back as the last common ancestor of the apes and man, and possibly even further.

monarch in Hawaii is partly a result of apostatic selection.[25]

The relative proportions of the morphs may vary; the actual values are determined by the

alleles at the locus or loci involved. Only if competing selection disappears will an allele disappear. However, heterozygote advantage is not the only way a polymorphism can be maintained. Apostatic selection
, whereby a predator consumes a common morph whilst overlooking rarer morphs is possible and does occur. This would tend to preserve rarer morphs from extinction.

Polymorphism is strongly tied to the adaptation of a species to its environment, which may vary in colour, food supply, and predation and in many other ways including sexual harassment avoidance. Polymorphism is one good way the opportunities[vague] get to be used; it has survival value, and the selection of modifier genes may reinforce the polymorphism. In addition, polymorphism seems to be associated with a higher rate of speciation.

Polymorphism and niche diversity

G. Evelyn Hutchinson, a founder of niche research, commented "It is very likely from an ecological point of view that all species, or at least all common species, consist of populations adapted to more than one niche".[26] He gave as examples sexual size dimorphism and mimicry. In many cases where the male is short-lived and smaller than the female, he does not compete with her during her late pre-adult and adult life. Size difference may permit both sexes to exploit different niches. In elaborate cases of mimicry, such as the African butterfly Papilio dardanus, female morphs mimic a range of distasteful models called Batesian mimicry,[27] often in the same region. The fitness of each type of mimic decreases as it becomes more common, so the polymorphism is maintained by frequency-dependent selection. Thus the efficiency of the mimicry is maintained in a much increased total population. However it can exist within one gender.[4]: ch. 13 

Female-limited polymorphism and sexual assault avoidance

Female-limited polymorphism in Papilio dardanus can be described as an outcome of sexual conflict. Cook et al. (1994)[28] argued that the male-like phenotype in some females in P. dardanus population on Pemba Island, Tanzania functions to avoid detection from a mate-searching male. The researchers found that male mate preference is controlled by frequency-dependent selection, which means that the rare morph suffers less from mating attempt than the common morph. The reasons why females try to avoid male sexual harassment are that male mating attempt can reduce female fitness in many ways such as fecundity and longevity.[29][30]

The switch

The mechanism which decides which of several morphs an individual displays is called the switch. This switch may be genetic, or it may be environmental. Taking sex determination as the example, in humans the determination is genetic, by the

alligators are a famous case in point. In ants the distinction between workers and guards is environmental, by the feeding of the grubs. Polymorphism with an environmental trigger is called polyphenism
.

The polyphenic system does have a degree of environmental flexibility not present in the genetic polymorphism. However, such environmental triggers are the less common of the two methods.

Investigative methods

Investigation of polymorphism requires use of both field and laboratory techniques. In the field:

And in the laboratory:

Without proper field-work, the significance of the polymorphism to the species is uncertain and without laboratory breeding the genetic basis is obscure. Even with insects, the work may take many years; examples of Batesian mimicry noted in the nineteenth century are still being researched.

Relevance for evolutionary theory

Polymorphism was crucial to research in

evolutionary theory. The work started at a time when natural selection was largely discounted as the leading mechanism for evolution,[31][32] continued through the middle period when Sewall Wright's ideas on drift were prominent, to the last quarter of the 20th century when ideas such as Kimura's neutral theory of molecular evolution was given much attention. The significance of the work on ecological genetics is that it has shown how important selection is in the evolution of natural populations, and that selection is a much stronger force than was envisaged even by those population geneticists who believed in its importance, such as Haldane and Fisher.[33]

In just a couple of decades the work of Fisher, Ford, Arthur Cain, Philip Sheppard and Cyril Clarke promoted natural selection as the primary explanation of variation in natural populations, instead of genetic drift. Evidence can be seen in Mayr's famous book Animal Species and Evolution,[34] and Ford's Ecological Genetics.[4] Similar shifts in emphasis can be seen in most of the other participants in the evolutionary synthesis, such as Stebbins and Dobzhansky, though the latter was slow to change.[3][35][36][37]

Kimura drew a distinction between molecular evolution, which he saw as dominated by selectively neutral mutations, and phenotypic characters, probably dominated by natural selection rather than drift.[38]

Examples

Main article: List of polymorphisms

See also

References

  1. Greek
    : πολύ = many, and μορφή = form, figure, silhouette)
  2. ^
    Ford E.B.
    1965. Genetic polymorphism. Faber & Faber, London.
  3. ^ a b Dobzhansky, Theodosius. 1970. Genetics of the Evolutionary Process. New York: Columbia U. Pr.
  4. ^ a b c d e f g h Ford, E. B. 1975. Ecological Genetics (4th ed.). London: Chapman & Hall
  5. ^ a b Sheppard, Philip M. 1975. Natural Selection and Heredity (4th ed.) London: Hutchinson.
  6. ^
    ISBN 978-1-930723-72-6
    .
  7. ^ a b c d Smith, John Maynard. 1998. Evolutionary Genetics (2nd ed.). Oxford: Oxford U. Pr.
  8. doi:10.1038/hdy.1955.1
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  10. ^ Endler J.A. 1986. Natural Selection in the Wild, pp. 154–163 (Tables 5.1, 5.2; Sects. 5.2, 5.3). Princeton: Princeton U. Press.
  11. ^
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  14. ISBN 978-0-8153-4219-9
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    ISBN 978-1-4051-1117-1
  16. ISBN 9781416030805
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  17. ^ Sober E. 1984. The nature of selection: evolutionary theory in philosophical focus. Chicago. p197
  18. doi:10.1002/jez.1400320206
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  19. ^ Darlington, C. D. 1956. Chromosome Botany, p. 36. London: Allen & Unwin.
  20. ^ Darlington, C.D.; Mather, K. 1949. The Elements of Genetics, pp. 335–336. London: Allen & Unwin.
  21. PMID 1207162
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  22. ^ Turner, J. R. G. 1984. "Mimicry: The Palatability Spectrum and its Consequences". In R. I. Vane-Wright, & P. R. Ackery (eds.), The Biology of Butterflies, ch. 14. "Symposia of the Royal Entomological Society of London" ser., #11. London: Academic Pr.
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  24. ^ Cain, Arthur J. 1971. "Colour and Banding Morphs in Subfossil Samples of the Snail Cepaea". In R. Creed (ed.), Ecological genetics and Evolution: Essays in Honour of E.B. Ford. Oxford: Blackwell.
  25. doi:10.1038/hdy.1990.110
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  26. ^ Hutchinson, G. Evelyn 1965. The evolutionary theater and the evolutionary play. Yale. The niche: an abstractly inhabited hypervolume: polymorphism and niche diversity, p66–70.
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  31. Baltimore: Johns Hopkins U.
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  32. ^ Bowler, P. J. 2003. Evolution: the History of an Idea (3rd rev. & exp. ed.) Berkeley: University of California Press.
  33. ^ Cain, Arthur J.; Provine, W. B. 1991. "Genes and Ecology in History". In R. J. Berry, et al. (eds.), Genes in Ecology: The 33rd Symposium of the British Ecological Society. Oxford: Blackwell
  34. ^ Mayr, E. 1963. Animal Species and Evolution. Boston: Harvard U. Pr.
  35. ^ Stebbins, G. Ledyard 1950. Variation and Evolution in Plants. New York: Columbia U. Pr.
  36. ^ Stebbins, G. Ledyard. 1966. Processes of Organic Evolution.[clarification needed]
  37. ^ Dobzhansky, Theodosius. 1951. Genetics and the Origin of Species (3rd ed). New York: Columbia U. Pr. Note the contrast between these this edition and the original 1937 edition.
  38. ^ Kimura M. 1983. The neutral theory of molecular evolution. Cambridge.

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