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Phenotypic plasticity is the ability of an organism to change its phenotype in response to changes in the environment. Fundamental to the way in which organisms cope with environmental variation, phenotypic plasticity encompasses all types of environmentally-induced changes (e.g. morphological, physiological, behavioural, phenological) that may or may not be permanent throughout an individual’s lifespan. The term was originally used to describe developmental effects on morphological characters, but is now more broadly used to describe all phenotypic responses to environmental change, such as acclimation.

Generally, phenotypic plasticity is more important for immobile organisms (e.g. plants) than mobile organisms (e.g. most animals), as mobile organisms can often move away from unfavourable environments. 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.

Plants
Phenotypic plasticity in plants includes the allocation of more resources to the roots in soils that contain low concentrations of nutrients and the alteration of leaf size and thickness. The transport proteins present in roots are also changed depending on the concentration of the nutrient and the salinity of the soil. Some plants, Mesembryanthemum crystallinum for example, are able to alter their photosynthetic pathways to use less water when they become water- or salt-stressed.

Temperature
Plastic responses to temperature are essential among ectothermic organisms, as all aspects of their physiology are directly dependent on their thermal environment. As such, thermal acclimation entails phenotypic adjustments that are found commonly across taxa, such as changes in the lipid composition of cell membranes. Temperature change influences the fluidity of cell membranes by affecting the motion of the fatty acyl chains of glycerophospholipids. Because maintaining membrane fluidity is critical for cell function, ectotherms adjust the phospholipid composition of their cell membranes such that the strength of van der Waals forces within the membrane is changed, thereby maintaining fluidity across temperatures.

Diet
The digestive system of some animals can exhibit phenotypic plasticity in response to changes in diet quality, nutrient composition , or energetic demands.

Poor quality diets, those that contain a large amount of non-digestible material, have lower concentrations of nutrients, so animals must process a greater total volume of poor-quality food to extract the same amount of energy as they would from a higher quality diet. Many species, including: prairie voles, Mongolian gerbils , Japanese quail , wood ducks , and mallards respond to poor quality diets by increasing their food intake, enlarging digestive organs, and increasing the capacity of the digestive tract. Poor quality diets also result in lower concentrations of nutrients in the lumen of the intestine, which can cause a decrease in the activity of digestive enzymes.

Shifts in dietary nutrient composition, the proportion of lipids, proteins and carbohydrates, may occur during development (i.e. weaning) or with seasonal changes in the abundance of food types. These diet changes can elicit phenotypic plasticity in the activity of particular digestive enzymes on the brush border of the small intestine. For example, in the first few days after hatching nestling house sparrows (Passer domesticus) transition from insect diet, high in protein and lipids, to a seed based diet that contains mostly carbohydrates; this diet change is accompanied by two-fold increase in the enzyme maltase which digests carbohydrates Periods of increased energy demand (i.e. lactation) often require greater energy intake and induce phenotypic plasticity in the digestive system that are similar to changes produced by poor quality diets — increased food intake and larger digestive organs. Unlike changes in diet quality, increases in energy demand does not cause changes in the activity of digestive enzymes because nutrient concentrations in the intestinal lumen are determined by food quality and remain constant.

Parasitism
Infection with parasites can induce phenotypic plasticity as a means to compensate for the detrimental effects caused by parasitism. Commonly, invertebrates respond to parasitic castration or increased parasite virulence with fecundity compensation in order to increase their reproductive output, or fitness. For example, water fleas, (Daphnia magna), exposed to microsporidian parasites produce more offspring in the early stages of exposure to compensate for future loss of reproductive success. A reduction in fecundity may also occur, as a means of re-directing nutrients to an immune response or to increase longevity of the host. This particular form of plasticity has been shown in particular cases to be mediated by host molecules (e.g. schistosomin in snails Lymnaea stagnalis) that regulate reproduction in the host.

Hosts can also respond to parasitism through plasticity in physiology aside from reproduction. Using a vertebrate example, house mice infected with intestinal nematodes experience decreased glucose transport rate in the intestine. To compensate for this, they increase the total mass of mucosal cells, cells responsible for glucose transport, in the intestine. This allows infected mice to maintain the same capacity for glucose uptake and body size as uninfected mice.

Phenotypic plasticity can also be observed as changes in behaviour. In response to infection, both vertebrates and invertebrates practice self-medication, which can be considered a form of adaptive plasticity. Primates infected with intestinal worms engage in leaf-swallowing, in which they ingest large, rough, whole leaves that physically dislodge parasites from the intestine. Additionally, the leaves irritate the gastric mucosa, which promotes the secretion of gastric acid and increases gut motility, effectively flushing parasites from the system. woolly bear caterpillars (Grammia incorrupta) infected with tachinid flies increase their survival by ingesting plants containing toxins known as pyrrolizidine alkaloids. The physiological basis for this change in behaviour is unknown; however, it is possible that, when activated, the immune system sends signals to the taste system that trigger plasticity in feeding responses during infection.

Evolution of Phenotypic Plasticity
Plasticity is thought to be an evolutionary adaptation to environmental variation, as it allows individuals to ‘fit’ their phenotype to different environments. If the optimal phenotype in a given environment changes with environmental conditions, the ability of individuals to express different traits should be advantageous and thus selected for. Hence, phenotypic plasticity can evolve if fitness is increased by changing phenotype. However, the fitness benefits of plasticity can be limited by the energetic costs of plastic responses (e.g. synthesizing new proteins, adjusting expression ratio of isozyme variants, maintaining sensory machinery to detect changes) as well as the predictability and reliability of environmental cues (see Beneficial acclimation hypothesis).

Freshwater snails (Physa virgata), provide an example of when phenotypic plasticity can be either adaptive or maladaptive. In the presence of a predator, bluegill sunfish, these snails make their shell shape more rotund and reduce growth. This makes them more crush-resistant and better protected from predation. However, these snails cannot tell the difference in chemical cues between the predatory and non-predatory sunfish. Thus, the snails respond inappropriately to non-predatory sunfish by producing an altered shell shape and reducing growth. These changes, in the absence of a predator, make the snails susceptible to other predators and limit fecundity. Therefore, these freshwater snails produce either an adaptive or maladaptive response to the environmental cue depending on whether the predatory sunfish is actually present.

Given the profound ecological importance of temperature and its predictable variability over large spatial and temporal scales, adaptation to thermal variation has been hypothesized to be a key mechanism dictating the capacity of organisms for phenotypic plasticity. The magnitude of thermal variation is thought to be directly proportional to plastic capacity, such that species that have evolved in the warm, constant climate of the tropics have a lower capacity for plasticity compared to those living in variable temperate habitats. Termed the “climatic variability hypothesis”, this idea has been supported by several studies of plastic capacity across latitude in both plants and animals. However, recent studies of (“Drosophila”) species have failed to detect a clear pattern of plasticity over latitudinal gradients, suggesting this hypothesis may not hold true across all taxa or for all traits. Some researchers propose that direct measures of environmental variability in factors such as precipitation are better predictors of plasticity than latitude alone.

Plasticity in a Changing World
Unprecedented rates of climate change are predicted to occur over the next 100 years as a result of human activity. Phenotypic plasticity is a key mechanism with which organisms can cope with a changing climate, as it allows individuals to respond to change within their lifetime. This is thought to be particularly important for species with long generation times, as evolutionary responses via natural selection may not produce change fast enough to mitigate decreases in fitness.

The North American Red Squirrel (Tamiasciurus hudsonicus) has experienced an increase in average temperature over this last decade of almost 2°C. This increase in temperature has caused an increase in abundance of white spruce cones, the main food source for winter and spring reproduction. In response, the mean lifetime parturition date of this species has advanced by 18 days. Food abundance showed a significant effect on the breeding date with individual females, indicating a high amount of phenotypic plasticity in this trait.