Download phenotypic-plasticity-essay 23 kb phenotypic-plasticity

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
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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).
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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.
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