Difference between revisions of "Polymorphism" - New World Encyclopedia

From New World Encyclopedia
Line 33: Line 33:
 
Polymorphism is the expression of genetic diversity 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. In many polymorphic species, we observe 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.
 
Polymorphism is the expression of genetic diversity 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. In many polymorphic species, we observe 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==
+
==Selected Bibliography==
  
 
*Clegg, C.J., and D.G. Mackean. 2000. ''Advanced Biology Principle & Applications''. Second Edition, John Murray Ltd. London. ISBN 0719576709
 
*Clegg, C.J., and D.G. Mackean. 2000. ''Advanced Biology Principle & Applications''. Second Edition, John Murray Ltd. London. ISBN 0719576709
 
*Silverside, Alan J. 2002. [http://www-biol.paisley.ac.uk/bioref/Genetics/Primula_heterostyly.html ''Heterostyly in the Cowslip (Primula veris L.)'']. Biological Sciences, University of Paisley. Retrieved May 6, 2007.
 
  
 
*Taylor, D.J., N.P.O. Green, and G.W. Stout. 2003. ''Biological Science''. Third Edition, Editor R. Soper, Cambridge University Press, UK. ISBN 0521639239
 
*Taylor, D.J., N.P.O. Green, and G.W. Stout. 2003. ''Biological Science''. Third Edition, Editor R. Soper, Cambridge University Press, UK. ISBN 0521639239
Line 45: Line 43:
  
 
*[http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/P/Polymorphisms.html ''Polymorphisms'']. Retrieved May 11, 2007.
 
*[http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/P/Polymorphisms.html ''Polymorphisms'']. Retrieved May 11, 2007.
 +
 +
*[http://www-biol.paisley.ac.uk/bioref/Genetics/Primula_heterostyly.html ''Heterostyly in the Cowslip (Primula veris L.)'']. Biological Sciences, University of Paisley. Retrieved May 6, 2007.
  
 
{{credit|113468662}}
 
{{credit|113468662}}

Revision as of 10:35, 11 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 gene 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. This is due to a single gene with two alleles. The moths with genotypes CC and Cc are melanic (Biston betularia carbonaria), and moths with genotype cc are pale (Biston betularia typica). In a period of 100 years, melanic forms have been found to predominate in the polluted area because of the phenomenon of industrial melanism. Now, the genotypic frequencies of both 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 one 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 relative lengths of the pistil and the stamens. For instance, some cowslip plants (Primula veris) have "pin flowers", in which the style (the elongated middle part of the pistil) reaches the end of the corolla tube causing stigma (tip of the pistil) project out of the corolla tube and the stamens remain hidden in the halfway of the tube. Other cowslip plants have "thrum flowers", in which the stamens reach the end of the corolla tube and the stigma is hidden inside. Also, thrum flowers produce bigger pollen grains than pin flowers. This polymorphism promotes outbreeding through cross-pollination.

Neutral Polymorphism

Variations in an organism's DNA sequence without having any phenotypic effect are called neutral polymorphisms. Examples of such polymorphism may include single-nucleotide polymorphisms (SNPs) and restriction-fragment-length polymorphisms (RFLPs). In the SNPs, one of the nucleotides of a codon undergoes change without it becoming a codon of different amino acid, as there are more than one codon for most amino acids. RFLP is a polymorphism usually due to SNP and is characterized by variation in the length of the DNA sequence when cut by the restriction enzyme. However, some SNPs can have phenotypic expression in homozygous condition, (e.g. sickle-cell gene).

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 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. In many polymorphic species, we observe 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.

Selected Bibliography

  • Clegg, C.J., and D.G. Mackean. 2000. Advanced Biology Principle & Applications. Second Edition, John Murray Ltd. London. ISBN 0719576709
  • Taylor, D.J., N.P.O. Green, and G.W. Stout. 2003. Biological Science. Third Edition, Editor R. Soper, Cambridge University Press, UK. ISBN 0521639239

External link

  • CEPH Center for the Study of Human Polymorphisms. Retrieved May 11, 2007.

Credits

New World Encyclopedia writers and editors rewrote and completed the Wikipedia article in accordance with New World Encyclopedia standards. This article abides by terms of the Creative Commons CC-by-sa 3.0 License (CC-by-sa), which may be used and disseminated with proper attribution. Credit is due under the terms of this license that can reference both the New World Encyclopedia contributors and the selfless volunteer contributors of the Wikimedia Foundation. To cite this article click here for a list of acceptable citing formats.The history of earlier contributions by wikipedians is accessible to researchers here:

The history of this article since it was imported to New World Encyclopedia:

Note: Some restrictions may apply to use of individual images which are separately licensed.