Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Jessica Griffiths “Discuss the evolution of developmental plasticity as an adaptation to environmental heterogeneity” Developmental plasticity is an evolutionary adaptation allowing individuals to ‘match’ their phenotype to their surroundings if the environment changes predictably within the course of the organism’s lifecycle. Phenotypic plasticity includes all types of environmentally-induced changes (morphological, behavioural, and physiological) which may not be permanent throughout the course of an organism’s lifespan. If the optimal phenotype for a particular environment changes with different environmental conditions then phenotypic plasticity can increase fitness, and so will subsequently be selected for. It plays an important role in many organisms, allowing them to survive and strive in heterogeneous environments. Immobile organisms have a greater requirement for phenotypic plasticity than mobile organisms, since mobile organisms are able to physically move themselves away from unfavourable environmental conditions. Higher plants often display a high degree of phenotypic plasticity. They are a sessile life form; therefore there is a high demand for them to be able to adjust both their growth and metabolism to accommodate seasonal changes in light and temperature. Their developmental plasticity often involves the allocation of more resources to the roots in soils that are buried in areas containing low concentrations of nutrients, and also alterations of leaf size and thickness. For each plant there is a degree of phenotypic plasticity which is constrained by the overall genotype. Dandelions, for example, are well-known for exhibiting considerable phenotypic plasticity in morphology when growing in sunny environments, in contrast to shady environments. During development, leaves adjust morphology to match available light intensity: shade leaves are thin to maximise leaf area, whilst sun leaves increase the layers of palisade cells. Chloroplast ultrastructure is also plastic – shady chloroplasts have a smaller stromal volume and larger granal stacks, whilst sun chloroplasts have a much greater proportion of stromal lamellae and reduced granal stacking. This enables sun leaves, which are exposed to direct sunlight, to minimise excess photon energy absorption to prevent photoinhibition, and maximise the energy transmitted through the lead to successive layers of chloroplasts and leaves underneath. In contrast, shade leaves experience more diffuse light. Spreading out the chloroplasts in thinner leaves, with higher internal air spaces, maximises the interception of light energy and minimises structural costs. As a plant which is asexual and able to produce seeds without the need for fertilisation or meiosis, there is little genetic variation between parents and their offspring. They therefore resort to utilising highly plastic phenotypes to ensure that it is successful as a weed. Schistocerca gregaria, desert locusts, exhibit extreme phenotypic plasticity. Depending on population density, they are able to transform between a solitary phase and a large swarming gregarious phase. Tactile stimulation of the hind femora causes the release of serotonin – an evolutionarily conserved mediator of neuronal plasticity – responsible for this drastic behavioural change underlying swarm formation. This is an example of a positive feedback response – increased population density results in increased tactile stimulation of the hind femur, and therefore increased aggregation behaviour. Other environmental cues which are often involved in adaptive phenotypic plasticity include: temperature, photoperiods, nutrition, wave action, and presence of predators. Changes in metabolic, developmental and physiological 1 Jessica Griffiths pathways in response to these environmental cures are typically regulated by hormones and neuro-hormones. Phenotypic plasticity can therefore provide an effective means of adaptation when a predictive cue exists that predicts the forthcoming environment at a stage in development where changes in phenotype can still be regulated. The increase in fitness conferred by phenotypic plasticity depends upon the predictability and reliability of environmental conditions. It is particularly important that ectothermic organisms are able to predict variable environments over temporal and spatial scales since all aspects of their internal physiology depend upon external environmental conditions. For example, ectotherms adjust the composition of their phospholipid cell membranes, thereby changing the strength of the van der Waals interactions between the phospholipid molecules. This maintains cell membrane fluidity, which is essential for cell function. It has been hypothesised that the thermal variation of an environment is directly proportional to plastic capacity. This insinuates that species which have evolved in the warm, relatively constant climates at each of the tropics have little capacity for phenotypic plasticity compared to those living in more variable temperature climates. However, studies on Drosophila at varying latitudes shows that they do not exhibit a clear pattern of plasticity, suggesting that this “climatic variability hypothesis” only applies to certain taxa. Bicyclus anynana is a small brown butterfly found primarily in Eastern Africa. It exhibits distinctive seasonal variation in the size of its eyespots. During the African dry season they have vastly reduced eye spots, compared to the rainy season, where they are much larger. Larvae which are growing during the wet season have phenotypic traits resembling those of butterflies in the dry season, whilst larvae growing during the dry season have characteristics of butterflies in the wet season. This increases the fitness of the butterflies since it allows the butterflies to remain inconspicuous and camouflaged during the dry season among bare branches, utilising crypsis as a predator defence mechanism. In contrast, the bright eyespots can be used to misdirect predator attacks during the rainy season, where crypsis is ineffective due to the increase in green vegetation. This phenotypic plasticity increases the butterfly’s chances of survival in each of the seasons; the eventual morphology of the butterfly is determined by environmental cues received during early developmental stages. Physa virgate, a type of freshwater snail, protects itself against predators by changing their shells to make them more rotund when they detect the presence of predators (such as the bluegill sunfish). This increases their resistance against being crushed, thereby increasing their chances of survival. However, these snails are not capable of distinguishing between chemical cues from predatory and non-predatory sunfish. Therefore they will often change their shell shape and growth in the absence of a predator. This makes them more susceptible to attack by other predators, and compromises their ability to reproduce, decreasing their fitness. This illustrates that phenotypic plasticity is not necessarily adaptive in all cases – it may actually be maladaptive. Fitness increases may also be limited by the various costs of plastic responses (e.g. maintenance of machinery to detect the environmental changes and the energy expended synthesising new proteins). 2 Jessica Griffiths Phenotypic plasticity is a fascinating evolutionary adaptation. It is often more advantageous than relying upon genetic polymorphism alone. Genetic polymorphism is where different genotypes generate different phenotypes. In many cases, it confers a genetic ‘load’ since individuals are mismatched with respect to their phenotype and type of environment. This load can be particularly high when a single environment dominates for a prolonged period of time. In contrast, the dependence of the phenotype upon environmental cues received during development in phenotypic plasticity usually results in the mature organism being well-suited to its surroundings. Phenotypic plasticity is a very important concept; it alters the biotic and abiotic interactions between organisms and their environments, and can therefore impact many different levels of ecological organisation, including predation, coexistence and competition. 3