Polymorphism in biology and zoology is the occurrence of two or more clearly different morphs or forms, also referred to as alternative phenotypes, in the population of a species. In order 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). Three mechanisms may cause polymorphism: \n* Genetic polymorphism - where the phenotype of each individual is genetically determined. \n* A conditional development strategy, where the phenotype of each individual is set by environmental cues. \n* A mixed development strategy, where the phenotype is randomly assigned during development. Polymorphism as used in zoology and biology involves morphs of the phenotype, and the term polyphenism can be used to clarify that the different forms arise from the same genotype. The term genetic polymorphism is also used somewhat differently by geneticists and molecular biologists to describe certain mutations in the genotype, such as SNPs (with detection methods RFLPs and AFLPs), that may not always correspond to a phenotype but always corresponds to a branch in the genetic tree. See . Polymorphism is common in nature; it is related to biodiversity, genetic variation and adaptation; it usually functions to retain variety of form 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 (see mimicry), 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 make-up allows for different morphs, and the switch mechanism that determines which morph is shown is environmental. In genetic polymorphism, the genetic make-up determines the morph. Ants exhibit both types in a single population. 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, in Obelia there are feeding individuals, the gastrozooids; the individuals capable of asexual reproduction only, the gonozooids, blastostyles and free-living or sexually reproducing individuals, the medusae.

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