What is Phenotypic Plasticity?
Carl Schlichting (6) defined phenotypic plasticity as the ability of an organism with a given genotype to adjust its phenotypic response to variations in the environment. An organism’s genotypes contains the entire set of genes in its DNA responsible for that individual’s physical traits. A phenotype is the observed expression of a gene from the genotype of an organism. One genotype displays numerous genes with multiple phenotype alleles for that gene. Many phenomenon in organisms are being analyzed as phenotypic plasticity including, heat shock reaction, enzyme induction, predator-induced defense, environmentally induced transcription and translation and general stress responses. Under certain environmental conditions, such as temperature and other stressors, different phenotypic responses occur.
Biologist recognize phenotypic plasticity as a developmental process that occurs continuously from fertilization to adulthood (7). Phenotypic responses due to phenotypic plasticity applies to the physiology, morphology, biochemistry and life history of an organism (6). These responses can manifest in two general manners, either by a developmental conversion or a phenotypic modulation (9). Phenotypic modulation is a gradual response to environmental cues, whereas developmental conversion is a phenotype that seems to switch without having an intermediate form. If an organism does not display a plastic response of a physical trait it does not necessarily mean that the organism does not possess plastic traits. Some mechanistic processes of development can also be altered due to environmental cues and phenotypic plasticity. These are simple alterations that could be happening as a plastic response such as growth rate. There is no change in the morphology of the organism, but it is actually experiencing phenotypic plasticity.
Reaction Norms of Phenotypic Plasticity
When observing phenotypic plasticity it is difficult to quantify the effects environmental cues are having on traits. In order to begin to understand the volume and intensity of phenotypic plasticity an organism possesses, a normal base line for traits must be produced. The base line measure of phenotype normality is known as the norm of reaction, otherwise known as reaction norm. A reaction norm is the pattern of a phenotypic expression of a single genotype across a range of environments. In an open course conducted by Yale University professor, Stephen Stearns, he gives an in-depth explanation of reaction norms. You may watch the video by clicking here.
Reaction norms are used for defining properties of a single genotype. It describes the set of phenotypes that a single genotype can express as environments vary. These reaction norms are established to better visualize the change in phenotypes of a genotype. Reaction norms are determined by plotting values for specific traits across multiple environments or treatments (8) Each genotype will exhibit a different reaction norm, or different response to the environment. Establishing phenotypic reaction norms are also based on genetic variation. Genetic variation contains plasticity in itself. The environment acts on genetic variation to alter different species’ reaction norms.
How Does Phenotypic Plasticity Evolve?
Phenotypic plasticity of a trait is under its own independent genetic system of control and is distinct from the trait it alters. As the gene for a trait can mutate and evolve, so can the plasticity of a trait. The evolution of plasticity will be independent from any evolutionary changes the gene for the trait it alters experiences. The theory of independed evolution is supported by witnessing varied phenotypic plasticity responses in individuals of the same population. Various members of the same species in a specific habitat display differing reaction norms when compared to one another.
There are three general forces responsible for evolutionary changes in phenotypic plasticity. The evolution of plasticity is due to disruption of the genetic system, selection, or drift (6). The first form of evolution is through a disruption in the genetic system. This could be due to inbreeding or crossbreeding. This form of evolution has shown to have alterations in the response of plants to the environment. If a plastic response is superior in relation to a specific environmental system the evolution can be attributed to selection. In a situation where the evolution of plasticity is due to random chance and not selection or change in another trait due to selection, then the evolution is thought to be solely driven by drift. Evolution through drift can also be assessed by the degree of similarity of plasticity between individuals. If the individuals are more related and have increased similarity in plasticity then their evolution was due to genetic drift. The emergence of different plasticity patterns can be attributed to any combination of these forces.
The fact that plants have not evolved to have an unlimited phenotypic response to the environment shows that there are some evolutionary constraints. A lack of genetic variation, resource limitations of the environment, or structural constraints of the organism itself could contribute to the sources of the evolutionary road block.
Phenotypic Plasticity in Plants
Prior to the 1980’s, biologist researched the amount of phenotypic plasticity an organism has, but did not focus on other aspects of phenotypic plasticity. In 1986, Schlichting set out to examine the severity and directionality of plastic responses. The stationary life-style of plants requires them to live with the environmental conditions of their immediate surroundings their entire life. This element of their life history makes them a great model organism to study phenotypic plasticity. As well as being stationary, plants are also unable to choose the fuel they metabolize and use for energy and are generally simple specimens to maintain a consistent genotype between different replicate individuals for experimentation.
There have been a number of cases where plasticity is observed in plants such as, differences in leaf forms of the same plant (1,2,3) and response to predators (4,5). One study examined the impact climate change was having on plants species in Thoreau’s Woods located in Concord, Massachusetts. The exploration and results were published in the National Academy of Science. Over a 100 year period the change of temperature in Thoreau’s Woods was recorded using observational surveys and the results were coupled with an examination of the 473 different plant species that inhabited the woods. The compiled data lead to results consistent with the presence of phenotypic plasticity. The study found that the species varied dramatically in their flowering times in response to climate change. Some species were flowering earlier, some later, and some had not change of flowering time in the season (10). The temperate plants with an ability to differentiate the changing in temperatures possessed more virulence that those species lacking this ability. The correlation between flowering time and the abundance of individuals in a species provides evidence of a plasticity mechanism
Why is Phenotypic Plasticity Important?
Any individuals who studies biological sciences should learn the concept of phenotypic plasticity due to its complexity and the impact it has on the evolutionary theory of genetics. The phenotypic plasticity of species can also give us insight to how changes in the environment is effecting organisms, it addresses the nature vs. nurture controversy, and aids in proper species identification once skewed by the unknown plasticity of species.
On an organismal level, phenotypic plasticity alleviates stress. Phenotypic plasticity enables the organism to change its physiology, morphology, biochemistry and life history in response to a change in environmental conditions. These adaptations are internal systematic responses to external stimuli in order to help an organism maintain homeostasis. All sensory modalities, such as visual and thermal, received stimuli from environmental factors and initiate plastic responses. Plastic responses can be specific or generalized. Some plants possess receptor proteins that detect only their common natural enemy (11) Plastic defense is a specific response to predation and change from cues given by different predators in the environment. Factors such as temperature and nutrition five a more general response that influences the entire organism’s phenotype. When a plasticity matches a change in the environment it becomes beneficial to an individual.
The overall theory of phenotypic plasticity shows us that genes and the environment are always linked. Though the genome provides the code for a trait, the environment determines how the gene expresses a genotype. These differences lead to an individual with more fitness that can lead to a population adaptation and the successful propagation of species in out ever changing environment.
- Cook, C. D. K. (1968). Phenotypic plasticity with particular reference to three amphibious plant species. In Mod- ern Methods in Plant Taxonomy, ed. V. Heywood, pp. 97-111.
- Cook, S. A., Johnson, M. P. (1968). Adaptation to heterogeneous environ- ments. I. Variation in heterophylly in Ranunculus flammula L. Evolution 22, 496-516.
- Deschamp, P. A., Cooke, T. J. (1985). Leaf dimorphism in the aquatic an- giosperm Callitriche heterophylla. J. Bot. 72, 1377-87.
- Hendrix, S. D. (1979). Compensatory reproduction in a biennial herb following insect defloration. Oecologia 42, 107-18.
- Lubchenco, J., Cubit, J. (1980). Heteromorphic life histories of certain marine algae as adaptations to variations in herbivory. Ecology 61, 676-87.
- Schlichting, C. D. (1986). The Evolution of Phenotypic Plasticity in Planta. Rev. Ecol. Syst. 17, 667-693.
- Schlichting, C. D. and H. Smith. (2002). Phenotypic plasticity: linking molecular mechanisms with evolutionary outcomes. Ecol. 16, 189–211.
- Schlichting, C. D. and M. Pigliucci. 1998. Phenotypic Evolution a Reaction Norm Perspective. Sinauer Associates, Sunderland, MA.
- Smith-Gill, S. (1983). Developmental Plasticity: Developmental Conversion versus Phenotypic Modulation.American Zoologist, 23(1), 47-55.
- Willis, C., Ruhfel, B., Primack, R., Miller-Rushing, A., & Davis, C. (2008). Phylogenetic Patterns of Species Loss in Thoreau’s Woods Are Driven by Climate Change.Proceedings of the National Academy of Sciences of the United States of America, 105(44), 17029-17033.
- Zhao, J., L. C. Davis and R. Verpoorte. (2005). Elicitor signal transduction leading to production of plant secondary metabolites. Biotechnology Advances, 23, 283–333.