Phenotypic plasticity (PP) has been defined by many authors in various ways but the central concept is, a single genotype can produce a range of phenotypes depending on the environmental variation. Various authors want to describe PP as just an environmental sensitivity for a trait. PP is not only associated with some common morphological characters but the whole life history (containing virtually any traits and the developmental process) of an individual can also show PP, because all PP result from altered underlying biochemical-physiological processes. Contrary to the common notion, PP need not necessarily be adaptive only but it often has undergone adaptive evolution. It is probably ancestral as the environment always changed and all living things are susceptible to effects of abiotic and biotic factors. The only way for an individual to adapt to a changing environment is by changing its phenotype. For the evolution of new phenotypes, it does not require evolution of new gene complexes. Epigenetic interaction and repatterning of existing genetic structure are sufficient for that. Exposing a species to extreme condition can elicit hidden phenocopies. Plasticity can be evolved and examined considering some factors (genetic, environmental, gene × environment interaction factors) effecting reaction norms and selecting one type of plasticity over another. Moreover, PP is a consequence of mutational evolution.
PP can be analyzed by ANOVA, where we can partition different types of variance (i.e. statistical measure of variation) for example, Genetic, environmental and some unexplained variance. Genotype and environmental interaction is another important factor to measure the PP as it shows genotype expression is plastic itself.
We can use reaction norms to visualize PP by plotting multiple values for a given phenotypic trait across a range or gradients of environments. Thus instead of getting a linear graph only we can even get curvilinear or discontinuous reaction norms also. Phenotypic modulation, the graded and continuous responses of phenotypes produce linear or curvilinear reaction norms which sometime assumed to be non-adaptive and reversible. Whereas, discontinuous or sigmoid reaction norms are produced by developmental conversions (discrete switches in phenotypes with no intermediate form), and are assumed to be permanent and beneficial adaptations. One problem is that we cannot get any hint of underlying physiological mechanism, evolutionary pathway or future consequences of PP from reaction norms or variance partitioning.
Virtually any factor can act as a cue or a stimuli to initiate PP and can originate internally (E.g. pathogen presence etc.) or externally (E.g. water force etc.). Cues tend to be harmless stimuli (like photoperiod) without any direct effect on the individual and induce adaptive plasticity by predicting future environmental conditions, whereas harmful selective agents like toxins are considered as stimuli though the division between these two is blurred.
Plastic responses can be highly specific as well as more general, depending on different elicitors. Learning is a good example of general plasticity which can respond to various environmental stimuli and produce a large variety of plastic responses. The species characteristics and environmental factors that favor the evolution of general vs. specific cues and responses, and the underlying physiological and ecological constraints that shape these responses are currently unknown.
Plasticities are under conflicting selective pressures and carry numerous costs and tradeoffs and some have argued that it is nearly impossible to ever know their total cost/benefit ratios. Concluding a particular trait as an adaptive one is always questionable and controversial since the tradeoff for a trait itself is relative to various contexts like time, place, presence or absence of interacting individuals. Genetic and environmental correlation are themselves plastic to the environment. “Adaptive” implies past selection, but the history of a population is often opaque hence it is difficult to know the adaptive value and “adaptiveness” cannot be a criterion for judging plasticity of a trait. Two features that often support adaptation are anticipatory plasticity (individual get some consistent abiotic cues which predict the future environment, and initiate phenotypic change before the appearance of the environmental factor, E.g. diapause) and responsive or direct or non-anticipatory plasticity (triggered after the appearance of the new environment). Another type of plasticity is called passive plasticity. Small size due to poor nutrition is a classic example of that. Active and passive plasticity can act simultaneously on the same trait in the same individual, and can be in similar or different directions.
Phylogenetic constraints clearly prohibit certain plasticities in certain taxa. Plastic response should change with ontogeny and decline with age. K-strategist and r-strategist may have their different requirements for favouring plasticity. Plasticity can be transgenerational depending on a longer cycle of environmental fluctuation and organism can alter their offspring’s relationship with environment, with profound fitness and ecosystem consequences. Some species can show plasticity throughout much of their lives, but for arthropods there is some constraints due to their discrete life stages (i.e. metamorphosis). For insects, PP in external morphology must be initiated before molting. The speed in phenotypic change should match the speed of environmental change. Behavioral and physiological PP can be rapid as well as reversible whereas altered life history can be achieved when there is a slow change of environments and tends to be permanent. Short lived insects do not need reversible phenotypes. Long lived humans and plants can have long lasting induced plasticities. Rapid, short-term physiological homeostasis can represents phenotypic plasticity (E.g. regulation of blood pH, osmolarity etc.). Even canalization, which is often considered as opposite of PP can be accomplished via PP. To hold one trait constant in changing environment it requires plasticity in another trait.
Phenotype alteration can be accomplished from a single, unchanging genome via any combination of transcription, translation, enzyme, hormone, and morphogen regulation, morphogenesis, apoptosis, and neural control, with appropriate regulation and feedbacks between subcomponents of the overall process. Hormones play a critical role in the insect’s PP. Nowadays it is relatively easy to decipher the gene or underlying physiology of PP by various molecular techniques like microarrays, gene knockouts and many other technologies.
PP changes the way we describe the evolutionary theory. Under traditional evolutionary theory, the environment acts after phenotypic variation is produced, and plays a single role i.e. to select among genetically produced variation. With PP, the environment plays a dual role by creating phenotypic variation and selecting some of them. It also emphasizes on the adaptation of individual, contrary to the theory of adaptation of population between-generation. Though, individual adaptation can lead to population adaptation. It also provides a better model to understand the evolution of organismal diversity (E.g. aposematism). Thus it can shape the structure of a community. Environmental stress act as one of the major factor for driving evolution and producing plasticity. Under PP concept, nature and nurture cannot be separated from each other. Microenvironment surrounding a gene drives the functionality of that gene. It helps to correct erroneous synonyms, disease diagnosis, pest control, bioprospecting etc. It may helps to prevent competitive exclusion of one antagonistic species, protect and store hidden genetic diversity, and may foster adaptive evolution. PP plays a role in generating novelty and thus stimulates evolutionary diversification. As the plastic responses are under selection pressure so it is acting as a factor for evolution. It can lead to speciation via traditional Mendelian processes. There can be some reciprocal interplay between phenotype and environment (E.g. by constructing reefs, corals alter local wave force, turbulence etc.). Altered environment can alter phenotypes which can further alter their environment in a continuous PP-environment feedback loop. Plasticity coevolution should be common among symbiotic species. PP makes us realize that genes and environment are forever intimately linked. Gene provide a menu of developmental possibilities and phenotypes, but the environment determines the phenotypic outcome.