Polymorphism (biology)

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.
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 polyphenism can be used to clarify that the different forms arise from the same genotype. Genetic polymorphism is a term used somewhat differently by geneticists and molecular biologists to describe certain mutations in the genotype, such as 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 biodiversity, genetic variation, and adaptation. Polymorphism usually functions to retain a variety of forms in a population living in a varied environment. The most common example is sexual dimorphism, which occurs in many organisms. Other examples are mimetic forms of butterflies, and human hemoglobin and 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 zooids, within the same organism. It is a characteristic feature of cnidarians.
For example, Obelia has feeding individuals, the gastrozooids; the individuals capable of asexual reproduction only, the gonozooids, blastostyles; and free-living or sexually reproducing individuals, the medusae.
Balanced polymorphism refers to the maintenance of different phenotypes in population.

Terminology

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

Polymorphism does not cover characteristics showing continuous variation, though this has a heritable component. Polymorphism deals with forms in which the variation is discrete or strongly bimodal or polymodal.
Morphs must occupy the same habitat at the same time; this excludes geographical races and seasonal forms. The use of the words "morph" or "polymorphism" for what is a visibly different geographical race or variant is common, but incorrect. The significance of geographical variation is that it may lead to allopatric speciation, whereas true polymorphism takes place in panmictic populations.
The term was first used to describe visible forms, but it has been extended to include cryptic morphs, for instance blood types, which can be revealed by a test.
Rare variations are not classified as polymorphisms, and mutations by themselves do not constitute polymorphisms. To qualify as a polymorphism, some kind of balance must exist between morphs underpinned by inheritance. The criterion is that the frequency of the least common morph is too high simply to be the result of new mutations or, as a rough guide, that it is greater than 1%.
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, and for classical genetics by John Maynard Smith. The shorter term morphism was preferred by the evolutionary biologist Julian Huxley.
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. Phenotypic traits and characteristics are also possible descriptions, though that would imply just a limited aspect of the body.
In the taxonomic nomenclature of zoology, the word "morpha" plus a Latin name for the morph can be added to a binomial or trinomial name. However, this invites confusion with geographically variant ring species or subspecies, especially if polytypic. Morphs have no formal standing in the ICZN. In botanical taxonomy, the concept of morphs is represented with the terms "variety", "subvariety" and "form", which are formally regulated by the ICN. Horticulturists sometimes confuse this usage of "variety" both with cultivar and with the legal concept "plant variety".

Mechanisms

Three mechanisms may cause polymorphism:

Genetic polymorphism – where the phenotype of each individual is genetically determined
* An example of this can be seen in the blue-tailed damselfly
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. 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.

Genetics

Genetic 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. The later definition by Cavalli-Sforza & Bodmer 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%.

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 functionally silent differences in DNA sequence between individuals that make each human genome unique.
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. 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

Heterosis : "Heterosis: the heterozygote at a locus is fitter than either homozygote".
Frequency dependent selection: The fitness of a particular phenotype is dependent on its frequency relative to other phenotypes in a given population. Example: prey switching, where rare morphs of prey are actually fitter due to predators concentrating on the more frequent morphs.
Fitness varies in time and space. Fitness of a genotype may vary greatly between larval and adult stages, or between parts of a habitat range.
Selection acts differently at different levels. The fitness of a genotype may depend on the fitness of other genotypes in the population: this covers many natural situations where the best thing to do depends on what other members of the population are doing at the time.
Pleiotropism

Most genes have more than one effect on the phenotype of an organism. 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 characteristic, yet a change in fitness is recorded. In such cases, the gene's subsurface 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 genetic abnormality and its manifestations is neither apparent nor understood.

Epistasis

occurs when the expression of one gene is modified by another gene. For example, gene A only shows its effect when allele B1 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. 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.