Download Discuss the evolution of developmental plasticity as an adaptation to

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