Difference between revisions of "Polymorphism" - New World Encyclopedia

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In [[biology]], '''polymorphism''' (from [[Greek_language|Greek]]: ''poly'' "many", ''morph'' "form") can be defined as discontinuous [[genetic variation]] that results in the occurrence of several different forms or types of individuals among the members of a single species. The most obvious example of polymorphism is the separation of most higher organisms into male and female sexes. Another classic example is the different [[melanic]] and non-melanic [[Morph (zoology)|morphs]] of the [[peppered moth]]. A polymorphism that persists over many generations is usually maintained because no one form has an overall advantage or disadvantage over the others in terms of [[natural selection]]. The different forms are called morphs, which are the [[phenotype]] expression of an organism's [[genotype]]: the specific manifestation of a trait. Both natural and [[artificial selection]] cause different polymorphisms; they occur when organisms with different morphs reproduce with different degrees of success.
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In [[biology]], '''polymorphism''' (from [[Greek_language|Greek]]: ''poly'' "many", ''morph'' "form") is defined as the existence of two or more forms of the same species within the same population, and can apply to biochemical, morphological and behavioral characteristics. The different forms are called morphs, and are the [[phenotype|phenotyp]]ic expression of an organism's [[genotype]]. One of the characteristic features of any natural population is its genetic or biodiversity. Basically, discontinuous [[genetic variation]] results in the occurrence of several different forms or types of individuals in a species. Polymorphism is a universal phenomenon and are found in every species of organisms. Thus, it is the basis for the operation of both natural as well as [[artificial selection]], as out of the different morphs, one or another is allowed to reproduce with higher degrees of success in the given environmental condition. Hence, polymorphism plays a significant role in the process of [[natural selection]].
  
==Examples==
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==Types of Polymorphism==
  
In some cases of polymorphism, the different morphs are distinct.  An example from [[botany]] is [[heterostyly]], in which flowers occur in different forms having different arrangements of the [[Flower#Flower anatomy|pistil]] and the [[Flower#Flower anatomy|stamen]]s.  For instance, some cowslip plants (''[[Primula veris]]'') have "pin flowers", in which the [[Flower#Flower anatomy|style]] (the stalk of the pistil) reaches the end of the [[Flower#Flower anatomy|corolla]] tube and the stamens extend only halfway up the tube and thus are hidden.  Other cowslip plants have "thrum flowers", in which the stamens reach the end of the corolla tube and the style is hidden inside.  Also, thrum flowers produce bigger [[pollen]] grains than pin flowers.  This polymorphism prevents [[inbreeding]].
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===Balanced Polymorphism===
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This occurs when different forms coexist within a given population in equilibrium, i.e., at a constant proportion from generation to generation. A polymorphism that persists over many generations is usually maintained because no one form has an overall advantage or disadvantage over the others in terms of [[natural selection]], or in another words all the forms possess, for one reason or another, survival value for the species. There are various types of balanced or stable polymorphism.
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====Sexual dimorphism====
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In higher invertebrates as well as in vertebrates, the male and female sex organs are carried by separate individual, i.e., whether male or female, only one sex occurs in an individual. Thus, this is the most obvious example of polymorphism. Further, many animals exhibit well marked sexual dimorphism (male and female individuals having different morphological forms). The male and female populations in animals and plants constitute balanced polymorphism, as their proportion remains constant from generation to generation, unless there is an artificial preference for a particular sex.
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====Allelic polymorphism====
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When the allelic genes (genes occurring at the same locus of two homologous chromosomes) have different phenotypic expression, then polymorphism results based on the number of allelic genes and their proportion in the genetic pool. In many cases of polymorphism, the different morphs are distinct. A classic example of this type of polymorphism is the existence of [[melanic]] and non-melanic [[Morph (zoology)|morphs]] in the [[peppered moth]] ''Biston betularia''. Because of the phenomenon of industrial melanism, peppered moth evolved some 100 years ago into a black form (''Biston betularia carbonaria'') in addition to the existing lighter form (''Biston betularia typica''). This is due to a single gene with two alleles. The moths of genotype CC and Cc are melanic, and moths of genotype cc are pale. Melanic forms have been found to predominate in polluted area. The genotypic frequencies of the various forms exhibit equilibrium since each form has a selective advantage of equal intensity.
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In humans, the existence of the A, B, AB and O blood groups is the example of balanced polymorphism. Whilst the genotypic frequencies within different population may vary, they remain constant from generation to generation within that population. This is because none of them have a selective advantage over the other. In some cases of polymorphism, there is continuous variationFor instance, normal human [[hair color]], even within the single "habitat" of [[northern Europe]], ranges continuously from black through reddish and brownish shades to nearly white.  Little is known about any adaptive value of this polymorphism.
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====Caste System====
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Social insects (e.g., bees, wasps, ants and termites) have members differentiated into distinct castes for division of labor. However, division of labor is accompanied by the structural, functional and behavioral specialization leading to a clear balanced polymorphism. In these insects, caste determination takes place due to the genetics as well as the extrinsic factors like nutrition. Reproductive cast king or male develop parthenogenetically from unfertilized haploid eggs, where as the queens, workers and soldiers are diploid female differentiated from fertilized eggs. Differentiation of queens from other two takes place just because of quality and quantity of food served. For example, in case of bees, royal jelly given to the female larva causes her to develop into queen, while honey and pollen (bee bread) causes her to develop into worker.
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====Cnidarian Polymorphism====
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Coelenterates are noted for their remarkable phenomenon of polymorphism. This is associated with the division of labor. Basically, coelenterates have two types of individuals or '''zooids''' namely '''polyps''' and '''medusae'''; polyps are sessile (fixed) and are nutritive in function, where as medusae are saucer-shaped, motile, reproductive in function and are produce asexually by gonozooids or blastostyles attached to the colony nourished by polyps or gastrozooids. Most hydrozoans (e.g.,''Obelia'', ''Tubularia'') show dimorphism. But the siphonophores like ''Physalia'' exhibit the highest degree of polymorphism. They have three kinds of polyploids (namely gastrozooids for nutrition, dactylozooids for defence and gonozooids for the asexual production of medusae) and four types of medusoids (namely pneumatophore for floating, nectophore for swimming, phyllozooid for protection and gonophore for bearing gonads).
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====Heterostyly====
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An example of polymorphism from [[botany]] is [[heterostyly]], in which flowers occur in different forms having different arrangements of the [[Flower#Flower anatomy|pistil]] and the [[Flower#Flower anatomy|stamen]]s.  For instance, some cowslip plants (''[[Primula veris]]'') have "pin flowers", in which the [[Flower#Flower anatomy|style]] (the stalk of the pistil) reaches the end of the [[Flower#Flower anatomy|corolla]] tube and the stamens extend only halfway up the tube and thus are hidden.  Other cowslip plants have "thrum flowers", in which the stamens reach the end of the corolla tube and the style is hidden inside.  Also, thrum flowers produce bigger [[pollen]] grains than pin flowers.  This polymorphism prevents [[inbreeding]].
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====Neutral Polymorphism====
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Still other polymorphisms are variations in an organism's [[DNA]] sequence that may or may not affect its [[phenotype]].  Examples include [[Single nucleotide polymorphism|single-nucleotide polymorphisms]] (SNPs) and [[Restriction fragment length polymorphism|restriction-fragment-length polymorphisms]].  Polymorphisms that do not affect the phenotype are sometimes called neutral polymorphisms.
  
In other cases of polymorphism, there is continuous variation.  For instance, normal human [[hair color]], even within the single "habitat" of [[northern Europe]], ranges continuously from black through reddish and brownish shades to nearly white.  Little is known about any adaptive value of this polymorphism.
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===Transient Polymorphism===
  
Another example of a polymorphic species is the [[Peppered moth]].
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In transient polymorphism, one form is gradually being replaced by another. As the name implies, it represents a temporary situation as a by-product of directional natural selection. As for example, during the course of industrial melanism, melanic form of peppered moth gradually predominated the non-melanic form in the trees of Manchester due to selective pecking of the latter (lighter form) by the birds.
  
Still other polymorphisms are variations in an organism's [[DNA]] sequence that may or may not affect its [[phenotype]]. Examples include [[Single nucleotide polymorphism|single-nucleotide polymorphisms]] (SNPs) and [[Restriction fragment length polymorphism|restriction-fragment-length polymorphisms]]. Polymorphisms that do not affect the phenotype are sometimes called neutral polymorphisms.
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==Conclusion==
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Polymorphism is the expression of genetic diversity of a population of a species. This gives each species a flexibility of adaptation in the environment. Further, this is the population level ground for the action of natural selection leading to change in allelic frequencies and then micro-evolution. Polymorphism is not only the variability the individuals of a species can enjoy in the environment, but also a division of labor for effective social organization and utilization of natural resources. We have clearly seen in the polymorphic species that the social organization integrate the discrete members as if they are different organs of a single biological organism, so as to ensure the welfare of the whole population and the species as a whole.
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==References==
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Clegg, C.J. and Mackean, D.G. ''Advanced Biology Principle & Applications''. Second Edition, John Murray Ltd. London. 2000 ISBN 0719576709
  
 
==See also==
 
==See also==

Revision as of 13:26, 6 May 2007

In biology, polymorphism (from Greek: poly "many", morph "form") is defined as the existence of two or more forms of the same species within the same population, and can apply to biochemical, morphological and behavioral characteristics. The different forms are called morphs, and are the phenotypic expression of an organism's genotype. One of the characteristic features of any natural population is its genetic or biodiversity. Basically, discontinuous genetic variation results in the occurrence of several different forms or types of individuals in a species. Polymorphism is a universal phenomenon and are found in every species of organisms. Thus, it is the basis for the operation of both natural as well as artificial selection, as out of the different morphs, one or another is allowed to reproduce with higher degrees of success in the given environmental condition. Hence, polymorphism plays a significant role in the process of natural selection.

Types of Polymorphism

Balanced Polymorphism

This occurs when different forms coexist within a given population in equilibrium, i.e., at a constant proportion from generation to generation. A polymorphism that persists over many generations is usually maintained because no one form has an overall advantage or disadvantage over the others in terms of natural selection, or in another words all the forms possess, for one reason or another, survival value for the species. There are various types of balanced or stable polymorphism.

Sexual dimorphism

In higher invertebrates as well as in vertebrates, the male and female sex organs are carried by separate individual, i.e., whether male or female, only one sex occurs in an individual. Thus, this is the most obvious example of polymorphism. Further, many animals exhibit well marked sexual dimorphism (male and female individuals having different morphological forms). The male and female populations in animals and plants constitute balanced polymorphism, as their proportion remains constant from generation to generation, unless there is an artificial preference for a particular sex.

Allelic polymorphism

When the allelic genes (genes occurring at the same locus of two homologous chromosomes) have different phenotypic expression, then polymorphism results based on the number of allelic genes and their proportion in the genetic pool. In many cases of polymorphism, the different morphs are distinct. A classic example of this type of polymorphism is the existence of melanic and non-melanic morphs in the peppered moth Biston betularia. Because of the phenomenon of industrial melanism, peppered moth evolved some 100 years ago into a black form (Biston betularia carbonaria) in addition to the existing lighter form (Biston betularia typica). This is due to a single gene with two alleles. The moths of genotype CC and Cc are melanic, and moths of genotype cc are pale. Melanic forms have been found to predominate in polluted area. The genotypic frequencies of the various forms exhibit equilibrium since each form has a selective advantage of equal intensity. In humans, the existence of the A, B, AB and O blood groups is the example of balanced polymorphism. Whilst the genotypic frequencies within different population may vary, they remain constant from generation to generation within that population. This is because none of them have a selective advantage over the other. In some cases of polymorphism, there is continuous variation. For instance, normal human hair color, even within the single "habitat" of northern Europe, ranges continuously from black through reddish and brownish shades to nearly white. Little is known about any adaptive value of this polymorphism.

Caste System

Social insects (e.g., bees, wasps, ants and termites) have members differentiated into distinct castes for division of labor. However, division of labor is accompanied by the structural, functional and behavioral specialization leading to a clear balanced polymorphism. In these insects, caste determination takes place due to the genetics as well as the extrinsic factors like nutrition. Reproductive cast king or male develop parthenogenetically from unfertilized haploid eggs, where as the queens, workers and soldiers are diploid female differentiated from fertilized eggs. Differentiation of queens from other two takes place just because of quality and quantity of food served. For example, in case of bees, royal jelly given to the female larva causes her to develop into queen, while honey and pollen (bee bread) causes her to develop into worker.

Cnidarian Polymorphism

Coelenterates are noted for their remarkable phenomenon of polymorphism. This is associated with the division of labor. Basically, coelenterates have two types of individuals or zooids namely polyps and medusae; polyps are sessile (fixed) and are nutritive in function, where as medusae are saucer-shaped, motile, reproductive in function and are produce asexually by gonozooids or blastostyles attached to the colony nourished by polyps or gastrozooids. Most hydrozoans (e.g.,Obelia, Tubularia) show dimorphism. But the siphonophores like Physalia exhibit the highest degree of polymorphism. They have three kinds of polyploids (namely gastrozooids for nutrition, dactylozooids for defence and gonozooids for the asexual production of medusae) and four types of medusoids (namely pneumatophore for floating, nectophore for swimming, phyllozooid for protection and gonophore for bearing gonads).

Heterostyly

An example of polymorphism from botany is heterostyly, in which flowers occur in different forms having different arrangements of the pistil and the stamens. For instance, some cowslip plants (Primula veris) have "pin flowers", in which the style (the stalk of the pistil) reaches the end of the corolla tube and the stamens extend only halfway up the tube and thus are hidden. Other cowslip plants have "thrum flowers", in which the stamens reach the end of the corolla tube and the style is hidden inside. Also, thrum flowers produce bigger pollen grains than pin flowers. This polymorphism prevents inbreeding.

Neutral Polymorphism

Still other polymorphisms are variations in an organism's DNA sequence that may or may not affect its phenotype. Examples include single-nucleotide polymorphisms (SNPs) and restriction-fragment-length polymorphisms. Polymorphisms that do not affect the phenotype are sometimes called neutral polymorphisms.

Transient Polymorphism

In transient polymorphism, one form is gradually being replaced by another. As the name implies, it represents a temporary situation as a by-product of directional natural selection. As for example, during the course of industrial melanism, melanic form of peppered moth gradually predominated the non-melanic form in the trees of Manchester due to selective pecking of the latter (lighter form) by the birds.

Conclusion

Polymorphism is the expression of genetic diversity of a population of a species. This gives each species a flexibility of adaptation in the environment. Further, this is the population level ground for the action of natural selection leading to change in allelic frequencies and then micro-evolution. Polymorphism is not only the variability the individuals of a species can enjoy in the environment, but also a division of labor for effective social organization and utilization of natural resources. We have clearly seen in the polymorphic species that the social organization integrate the discrete members as if they are different organs of a single biological organism, so as to ensure the welfare of the whole population and the species as a whole.

References
ISBN links support NWE through referral fees

Clegg, C.J. and Mackean, D.G. Advanced Biology Principle & Applications. Second Edition, John Murray Ltd. London. 2000 ISBN 0719576709

See also

  • CEPH

External link

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