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Justin Gerlach Evolution and Behaviour Discuss the evolution of developmental plasticity as an adaptation to environmental heterogeneity? Phenotypic plasticity is a way in which organisms can adapt to environmental heterogeneity. Phenotypic plasticity occurs when fluctuating environmental conditions influence the developmental trajectory of the organism, to yield phenotypic variation. A single genotype can take different developmental trajectories due to an environmental cue, which predicts what environment the adult will find itself in, resulting in the different phenotypes and an adult form that is well adapted to its environment. In order for phenotypic plasticity to be defined as an adaptation it must increase relative fitness, and therefore will be selected for by natural selection. Phenotypic plasticity can be considered adaptive when the different phenotypes each have their highest relative fitness in the environment in which they tend to be expressed. In adaptive developmental phenotypic plasticity there is greater survival and increased fitness for individuals of each phenotype displayed that is better suited to the environment experienced, therefore the phenotypic plasticity that causes this confers greater fitness. Phenotypic plasticity can allow adaptation to environmental heterogeneity particularly when there is a predictive cue of the environment changing that can trigger altered development accordingly to result in suitable phenotypes with greater relative fitness in that environment. This is particularly successful when the predictive cue occurs at a point in the life cycle of the organism that allows development to be altered to produce the required phenotype, such as during ontogeny. A suitable environmental cue is one that accurately predicts the forthcoming environment in which selection occurs. Examples of possible environmental cues include temperature, photoperiods and nutrition. The mechanism for detection of the environmental cue and regulation of the developmental pathway to produce the suitable phenotype for the environment often involves circulating hormones or neuro-hormones, altered expression of transcription factors that leads to differential expression of other groups of genes, and modification of the epigenome. These butterfly pupae, which developed with a diet containing no carotenoids, give an example of phenotypic plasticity. The pupae on the left developed in the dark, compared to in the light for the pupae on the right. The pupae in the each row have a different genotype from those on the other, but have identical genotypes to one another, yet show phenotypic variation in their colour dependent on the light levels they developed in, displaying phenotypic plasticity. Justin Gerlach Evolution and Behaviour An example of phenotypic plasticity that is adaptive is observed in the wet season (top) and dry season (bottom) forms of the seasonally dimorphic gaudy commodore butterfly, Precis octavia . The different forms are induced by different temperatures during development, and they have greater fitness in the different seasons by enabling crypsis and attracting mates respectively. However, phenotypic plasticity is not always adaptive, for example in the phenotypic plasticity shown by the scarring effect. An example of the scarring effect is when if an embryo is not provided with sufficient nutrition a small organism will develop. The distinction between phenotypic plasticity that is adaptive and that is not adaptive is not entirely clear. A smaller child is less metabolically active, therefore better adapted to the environment, so phenotypic plasticity and the scarring effect can overlap. Phenotypic plasticity can be classed as an adaptation when it is selected for by natural selection. Adaption is the process by which an adaptive trait is selected for and therefore evolves in response to a changing or novel environment. In the case of phenotypic plasticity, the adaptive trait that evolves is the ability of organisms to vary their phenotype to increase survival in a changed environment. Among individuals the level of phenotypic plasticity displayed is variable, but those organisms that have a greater level of phenotypic plasticity will be better adapted to the environment therefore have increased survival, mating success, fecundity and therefore relative fitness. Selected for by natural selection, the alleles enabling phenotypic plasticity will increase in frequency over time, leading to a population with increased phenotypic plasticity that is better adapted to variable environmental conditions. Reaction norms can be used as a way to illustrate the genetics of the evolution of phenotypic plasticity. Reaction norms show the response of the phenotype to the environment; the pattern of variation in the development of the adult phenotype that is caused by variation in environmental factors. Reaction norms can be plotted; with a steeper slope indicating a greater level of plasticity when the environmental factor is plotted against the phenotype. Genetic variation is required for the evolution of adaptive phenotypic plasticity; where different genotypes confer a different level of ability to causes changes in phenotype in response to the environment. Justin Gerlach Evolution and Behaviour Here the environmental factor (E) is plotted against the mean phenotype of individuals in these different environmental conditions (P). Genotypes B and C have the same amount of phenotypic plasticity, as they have parallel reaction norms. A has greater phenotypic plasticity, shown by the steeper gradient. Given that different genotypes have different reaction norms, responses to changes in the environmental factor vary in their extent. Therefore the population can evolve phenotypic plasticity, as there is potential for a population containing these genotypes to evolve a change in plasticity in response to natural selection. Therefore phenotypic plasticity is an adaptive trait: an aspect of the developmental pattern of the organism which enables or enhances the probability of that organism surviving and reproducing, that is selected for by natural selection. Phenotypic plasticity can evolve to become more effective if alleles result in an improved matching of phenotype to the environment at an appropriate time in the life cycle, and an improved ability to perceive and respond to environmental cues. These alleles are selected for as they lead to suitable phenotypes being displayed in the correct environment, and reduce the occurrence of individuals with phenotypes mis-matched to their environment, therefore increasing fitness. In conclusion, since phenotypic plasticity can result in increased relative fitness in different environmental conditions, and be selected for by natural selection, it is an adaptation of organisms to environmental heterogeneity. It is a vitally important adaptation as it enables organisms to inhabit variable environments, and increases survival of plants, which are unable to move and therefore must respond to environmental changes in order to survive. The contribution of phenotypic plasticity to adaptive radiation, speciation and diversification is controversial. The first of the two main contrasting views is that the environment does not influence the genes passed on to offspring, therefore plasticity is not relevant to evolution as it reduces the effects of selection by enabling phenotypes to be variable without genotypic variation. The effect of plasticity on radiation is likely dependent on the level of plasticity for a trait, as high levels of plasticity would result in minimal evolution, as the phenotype is highly variable without the genotype varying, while non-plastic organisms have Justin Gerlach Evolution and Behaviour to evolve in order to adapt. Therefore, the level of plasticity reflects the likelihood of radiation. However, phenotypic plasticity is important as a starting point of radiations, and an origin of phenotypic differences between species, since plasticity enables survival in different environments, which can lead to reproductive isolation and speciation. REFERENCES The role of developmental plasticity in evolutionary innovation. Armin P. Moczek , Sonia Sultan , Susan Foster , Cris Ledón-Rettig , Ian Dworkin , H. Fred Nijhout , Ehab Abouheif , David W. Pfennig DOI: 10.1098/rspb.2011.0971 Published 10 August 2011 Pfennig et al. 2010 Phenotypic plasticity's impacts on diversification and speciation. TREE 25: 439-467 [http://www.sciencedirect.com/science/article/pii/S0169534710001059] Ecological Epigenetics: An Introduction to the Symposium Cris C. Ledón-Rettig Integr. Comp. Biol. (2013) 53 (2):307-318.doi: 10.1093/icb/ict053First published online: May 20, 2013