What is Polymorphism?
[[File:Jaguar head shot.jpg|thumb|right|Light-morph [[Jaguar]] (typical)]] [[File:Black jaguar.jpg|thumb|right|Dark-morph or [[Melanism|melanistic]] Jaguar (about 6% of the South American population)]] [[File:Hugorme.jpg|right|thumb|250px|The [[adder]] ''Viperus berus'': normal and melanistic colour patterns]] '''Polymorphism''' <ref>{{lang-el|πολύ}} = many, and ''μορφή'' = form, figure, silhouette</ref> in biology is when two or more clearly different [[phenotype]]s exist in the same population of a species.<ref name=Ford1975/> The words ''forms'' or ''morphs'' are sometimes used.<ref>Polymorphism as described here involves morphs of the [[phenotype]]. The term is also used somewhat differently by molecular biologists to describe certain point mutations in the [[genotype]], such as [[single nucleotide polymorphism|SNPs]] (see also [[RFLP]]s). This usage is not discussed on this page.</ref> Polymorphism is common in nature. The most common example is [[sexual dimorphism]], which occurs in many organisms. Another example is [[sickle-cell anaemia]]. In order to be classified as such, morphs must occupy the same habitat at the same time and belong to a population with [[random mating]].<ref name=Ford1965>[[E.B. Ford|Ford E.B.]] 1965. ''Genetic polymorphism''. Faber & Faber, London.</ref> == The switch == The mechanism which decides which of several morphs an individual displays is called the ''switch''. This switch may be genetic, or it may be environmental. Taking sex determination as the example, in man the determination is genetic, by the XY [[sex determination]] system. In [[Hymenoptera]] ([[ants]], [[bees]] and [[wasp]]s), sex determination is by haplo-diploidy: the females are all [[diploid]], the males are [[haploid]]. However, in some animals an environmental trigger determines the sex: [[alligator]]s are a good example. In ants the distinction between workers and guards is environmental, by the feeding of the grubs. Polymorphism with an environmental trigger is called [[polyphenism]]. The polyphenic