25/3/2022: Evolution and development: Developmental Plasticity
Developmental plasticity→ important concept in Evo.Devo → in the “Control of gene
expression” lecture we saw how the presence of the stress hormone (=transcription regulator)
can modify the expression of many different genres.
The level of hormones (and similar substances with an effect on gene expression) depends on
the environment. Therefore we should expect a coordinated environment to influence gene
expression. Developmental plasticity is the term used to describe this process:
Natural selection will favor the developmental consequences of environmental variation being
well-suited to that environment. So, in environments that are experienced frequently by a
species, we should see the evolution of developmental trajectories results in phenotypes that
are adaptive in those environments. Therefore one genotype can have multiple phenotypes
Limnetic: forage in open water
Benthic: forage at the bottom of ponds → in sedimentary environments
Populations of sticklebacks have the ability to both live in limnetic and benthic environments →
this “choice” influences the shape of the head, to have optimal fitness. Causes different facial
structures, which are locked in the genotype (even though the genotype is the same in both
variants)
Developments as an ingredient in the evolution
In this lecture, it will be discussed how this type of developmental plasticity plays a role in
evolution, specifically the adaptation of a species to its environment, the generation of
phenotypic variation that can be acted on by natural selection, and the origin of new species.
Plasticity promotes survival in a new environment
Invasion of plants that can grow in different environments → due to phenotypic plasticity in
predictable and consistent environments → results in differentiation in stem thickness, the
difference in pith cavity, and differences in internode distance → all result in sturdier or less
sturdy plants → 2 hypotheses of these differences:
- Local adaptations: due to natural selection, local adaptation to either aquatic or
terrestrial environment → these differences are due to fixed genetic differences between
aquatic and terrestrial populations. Their invasive ability is a result of rapid evolutionary
adaptations to different environments.
- Developmental plasticity: the plants change developmental patterns according to their
local environment. Their invasive ability is a result of the development being a response
to environmental cues, not rapid genetic evolution.
, How to test these hypotheses?
How to distinguish between the two hypotheses? Take seeds from each environment, grow
them for six months in a common environment to remove potential maternal effects, and then
grow the seedlings in either wet or dry environments in a controlled greenhouse.
If the differences between populations result from local adaptation, seeds from aquatic
environments will have thicker stems, etc. even when grown in a dry greenhouse. Seeds from a
terrestrial environment will have narrower stems even when grown in a wet greenhouse. But if
the differences between populations result from developmental plasticity, seeds from both
environments will have narrower stems when grown in a terrestrial environment and thicker
stems when grown in a dry environment.
Researchers grew seeds in all combinations (seeds from aquatic environments in either wet or
dry locations, seeds from terrestrial environments in either wet or dry locations). The results
were that there are no genetic effects in response to the environment: All seeds grew narrow
stems, narrow pith cavities, and short internodes in the dry environment, and the opposite in the
wet environment → with no genetic component, but changes in morphology due to
developmental plasticity.
Implication: the success of this invasive plant does not result from natural selection causing
local populations to become adapted to their environment. It results from inbuilt developmental
plasticity in which growth patterns adjust according to whether the surroundings are wet or dry.
Conclusion: This plasticity has evolved. Through evolutionary history, the plant has been
exposed to a fluctuating environment and has evolved the appropriate developmental response.
Plasticity itself has evolved → that’s why it is so invasive.
Plasticity masks cryptic genetic variation
It is very common for environmental change to result in high levels of phenotypic variation.
Developmental plasticity acts as a buffer between genetic variation and phenotypic variation (it
produces the same phenotype in the same environment even when there is genetic variation in
the population).
Developmental plasticity allows genetic variation to accumulate in a population. During times of
environmental change, this hidden (“cryptic”) genetic variation is revealed and is thought to
contribute to rapid phenotypic evolution at these times.
If a specific phenotype is triggered by the environment, then you can have many genetic
variations that live in the same environment. When there is a change in environment, genotypic
variation is revealed because there is no longer a selection for that specific phenotype by the
environment.
For example in the butterfly Zizeeria maha → a single phenotype is produced within the
population, as long as the environment is stable → novelties appear when the environment
changes (e.g. when they move to different environments themself) → environment trigger is no
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