Phenotypic plasticity is the ability of an organism to change its phenotype in response to changes in the environment.[1] Such plasticity in some cases expresses as several highly morphologically distinct results; in other cases, a continuous norm of reaction describes the functional interrelationship of a range of environments to a range of phenotypes. The term was originally conceived in the context of development, but is now more broadly applied to include changes that occur during the adult life of an organism, such as behaviour.
Organisms may differ in the degree of phenotypic plasticity they display when exposed to the same environmental change. Hence, phenotypic plasticity can evolve and be adaptive if fitness is increased by changing phenotype.[2] In general, sustained directional selection is predicted to increase plasticity in that same direction.[3]
Some responses will be similar in all organisms, for example in organisms that do not thermoregulate, as temperatures change lipids in the cell membrane must be altered by creating more double bonds (when temperatures decrease) or removing them (when temperatures increase).[4]
Generally phenotypic plasticity is more important for immobile organisms (e.g. plants) than mobile organisms (e.g. animals). This is because immobile organisms must adapt to their environment or they will die, whereas mobile organisms are able to move away from a detrimental environment.[5] Examples of phenotypic plasticity in plants include the allocation of more resources to the roots in soils that contain low concentrations of nutrients and the alteration of leaf size and thickness.[6] The transport proteins present in roots are also changed depending on the concentration of the nutrient and the salinity of the soil.[7] Some plants, Mesembryanthemum crystallinum for example, are able to alter their photosynthetic pathways to use less water when they become water- or salt-stressed.[8] Nevertheless, some mobile organisms also have significant phenotypic plasticity, for example Acyrthosiphon pisum of the [Aphid] family exhibits the ability to interchange between asexual and sexual reproduction, as well as growing wings between generations when plants become too populated.[9]
In epidemiology, there exists a theory that rising incidences of coronary heart disease and type II diabetes in human populations undergoing industrialization is due to a mismatch between a metabolic phenotype determined in development and the nutritional environment to which an individual is subsequently exposed. This is known as the 'thrifty phenotype' hypothesis.[10]
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