polymorphismpolymorphicmorphpolymorphismsmorphsmorphotypemonomorphicdimorphiccolor morphcolour morph
In biology and zoology, 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.wikipedia
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Other examples are mimetic forms of butterflies (see mimicry), and human hemoglobin and blood types. Polymorphism was crucial to research in ecological genetics by E. B. Ford and his co-workers from the mid-1920s to the 1970s (similar work continues today, especially on mimicry). In elaborate cases of mimicry, such as the African butterfly Papilio dardanus, female morphs mimic a range of distasteful models, often in the same region.
Mimics may have different models for different life cycle stages, or they may be polymorphic, with different individuals imitating different models, such as in Heliconius butterflies.
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.
One study on genetic variations between different species of Drosophila suggests that, if a mutation changes a protein produced by a gene, the result is likely to be harmful, with an estimated 70 percent of amino acid polymorphisms that have damaging effects, and the remainder being either neutral or marginally beneficial.
seasonal formsseasonal polyphenismpolyphenic
The term polyphenism can be used to clarify that the different forms arise from the same genotype.
When polyphenic forms exist at the same time in the same panmictic (interbreeding) population they can be compared to genetic polymorphism.
Batesian mimicry in butterflies and heterostyly in angiosperms are good examples.
Heterostyly is a unique form of polymorphism and herkogamy in flowers.
heterozygous advantageevolutionary trade-offEvolutionary trade-offs
The mechanism of heterozygote advantage assures the population of some alternative alleles at the locus or loci involved.
Polymorphism can be maintained by selection favoring the heterozygote, and this mechanism is used to explain the occurrence of some kinds of genetic variability.
frequency dependent selectionFrequency-dependentnegative frequency dependent selection
Frequency-dependent selection can lead to polymorphic equilibria, which result from interactions among genotypes within species, in the same way that multi-species equilibria require interactions between species in competition (e.g. where α ij parameters in Lotka-Volterra competition equations are non-zero).
It is a characteristic feature of cnidarians.
Polymorphism refers to the occurrence of structurally and functionally more than two different types of individuals within the same organism.
Balanced polymorphismbalancingmaintained by natural selection for competing factors
The mechanisms that conserve it are types of balancing selection.
In this way genetic polymorphism is conserved.
Batesian mimicmimicBatesian mimics
Batesian mimicry in butterflies and heterostyly in angiosperms are good examples.
Some mimetic populations have evolved multiple forms (polymorphism), enabling them to mimic several different models and thereby to gain greater protection.
Form and phase are sometimes also 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.).
As opposed to morphs (see below), a subpopulation usually consists of a single form only at any given point of time.
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.
Zooids can exhibit polymorphism.
E.B. FordEdmund Brisco FordFord, E. B.
For example, there are the terms established in ecological genetics by E.B. Ford (1975), and for classical genetics by John Maynard Smith (1998).
E.B. Ford worked for many years on genetic polymorphism.
Apostatic selection, whereby a predator consumes a common morph whilst overlooking rarer morphs is possible and does occur.
It operates on polymorphic species, species which have different forms.
Polymorphism was crucial to research in ecological genetics by E. B. Ford and his co-workers from the mid-1920s to the 1970s (similar work continues today, especially on mimicry).
Examples might be: flowering time, drought tolerance, polymorphism, mimicry, avoidance of attacks by predators.
The Duffy antigen is a protein located on the surface of red blood cells, encoded by the FY (DARC) gene.
Polymorphisms in this gene are the basis of the Duffy blood group system.
Sir Julian HuxleyJulian Sorell HuxleyJulian S. Huxley
It was the preferred term of the evolutionary biologist Julian Huxley (1955).
neutralneutral evolutionneutral theory
The work started at a time when natural selection was largely discounted as the leading mechanism for evolution, 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 proposal of the neutral theory was followed by an extensive "neutralist-selectionist" controversy over the interpretation of patterns of molecular divergence and polymorphism, peaking in the 1970s and 1980s.
Papilio dardanus ceneaPapilio dardanus dardanusP. dardanus
In elaborate cases of mimicry, such as the African butterfly Papilio dardanus, female morphs mimic a range of distasteful models, often in the same region.
The species shows polymorphism in wing appearance, though this is limited to females, which are often given as an example of Batesian mimicry in insects.
In botanical taxonomy, the concept of morphs is represented with the terms "variety", "subvariety" and "form", which are formally regulated by the ICN.
CainA.J. CainArthur James Cain
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.
With P.M. Sheppard, Cain studied the ecological genetics of colour and banding polymorphisms in snails.
In 1973, M. J. D. White, then at the end of a long career investigating karyotypes, gave an interesting summary of the distribution of chromosome polymorphism.
Some species are polymorphic for different chromosome structural forms.
MHCmajor histocompatibility complex (MHC)HLA loci
The genes of the major histocompatibility complex (MHC) are highly polymorphic, and this diversity plays a very important role in resistance to pathogens.
Diversity of antigen presentation, mediated by MHC classes I and II, is attained in at least three ways: (1) an organism's MHC repertoire is polygenic (via multiple, interacting genes); (2) MHC expression is codominant (from both sets of inherited alleles); (3) MHC gene variants are highly polymorphic (diversely varying from organism to organism within a species).
R.A. FisherR. A. FisherFisher
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.
In ecological genetics he and E. B. Ford showed how the force of natural selection was much stronger than had been assumed, with many ecogenetic situations (such as polymorphism) being maintained by the force of selection.
modern synthesismodern evolutionary synthesisevolutionary synthesis
The results had a considerable effect on the mid-century evolutionary synthesis, and on present evolutionary theory.
In 1940, he was the first to describe and define genetic polymorphism, and to predict that human blood group polymorphisms might be maintained in the population by providing some protection against disease.
nominate subspeciesnominotypical subspeciesnominate
However, this invites confusion with geographically variant ring species or subspecies, especially if polytypic.