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  Polymorphism (Greek, ‘many-shapedness’), in the life sciences, is the occurrence together in one population of two or more genetically distinct types of individual of the same species. The study of such phenomena is important for an understanding of the mechanism of speciation. Polymorphism arises because the forces of selection favour both types equally; if the environment changes, then one or other might be placed in a favourable position and become dominant, or the two types might become still more different, leading ultimately to speciation. Two forms of polymorphism occur: stable and transient. In the former the various forms, known as morphs, exist in stable proportions and all individuals appear to interbreed without preference. Examples of this situation are the maintenance of different blood groups within human populations and sickle-cell anaemia in populations exposed to malaria; individuals who suffer from sickle-cell anaemia are resistant to malaria and are therefore more likely to reach reproductive age and so pass on the gene. Many species exhibit sexual dimorphism, where one or other sex is larger and more colourful, and exhibits behavioural differences—the difference between peacocks and peahens provides a clear example of this. Henry Bates, studying mimicry in the mid-19th century, observed that bees exhibit polymorphism with respect to the various castes present in the hive workers, drones and queen. Transient polymorphism is the situation where the balance between forms is not even and one type appears to be increasing in abundance at the expense of the other. RB

See also evolution; genetics; natural selection.Further reading John Maynard-Smith, The Theory of Evolution.



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