Phenotypic Plasticity

daphniaSome organisms can change their appearance, physiology, and development in response to changes in the environment. This is called phenotypic plasticity, and some examples of phenotypically plastic organisms include the Junonia octavia butterflies described in my natural selection lecture, or water fleas that develop a spiny helmet in the presence of predators, as shown in the picture above (left: exposed to predators; right: same species and sex, not exposed to predators; source: Agrawal (2001)). More mundane examples include the changes experienced by bodybuilding humans, or even the effect of learning on the brain – it’s a ubiquitous phenomenon, but here I’ll only be concentrating on the sort of phenotypic plasticity that leads to set phenotypes: bodybuilders have complete control over their muscle sizes and a genius can atrophy their brain at will, but Junonia octavia can only be blue or orange, nothing else.

The most phenotypically plastic organisms are plants. Unlike animals, plants are forced to stay put and must weather any and all conditions thrown at them by the environment with no option of an immediate migration or a run for shelter. Hence, they’ve evolved to be plastic in their physiology and development, with the best example of this being heterophylly, the ability of many wetland plants to change leaf structure and physiology in drought and in flood conditions (hetero = different; phylly = related to leaf).

How do such things work? A genotype is not a blueprint and what results as a final form is the product of both hardwired developmental processes combined with the effect of external factors such as temperature, interactions, or nutrition. Look at the examples I named above: the butterflies develop their colours in response to temperature; the water fleas in response to predators; the plants in response to abiotic changes. These do not somehow trigger sudden reversible mutations, but the culmination of their physiological effects enable the crossing of a threshold, called the reaction norm, which leads to different genes getting turned on and off, leading eventually to the modified phenotypes.

Intuitively, we can guess at how such mechanisms can evolve: they allow the organism to adapt to varying conditions in real-time, potentially allowing for an optimal phenotype for every condition rather than have to settle for one phenotype for that may just be average in most conditions. But such intuitions are not scientifically sound, merely thoughts. A look through the history of phenotypic plasticity research shows two divergent lines of thought:

  • Phenotypic plasticity evolves as stated above, by selecting for complete phenotypes and hardwired reaction norms. This view can be seen fully-formulated as early as Via & Lande (1985).
  • There are gene regulation mechanisms that respond to changing environmental conditions, and these regulatory mechanisms and they genes they affect are what are selected for in the evolution of phenotypic plasticity. In other words, the capacity to be plastic, not the end product of the plasticity, is selected for. Arguments for this view and evidence for such genes can be found as early as Pigliucci (1996).

The reality is that both of these views are valid. The second point describes genotype by environment interactions (GxE interactions), and treats phenotypic plasticity as a trait in and of itself. Since all traits evolve, the second point is valid almost by definition, and this is reflected in the ubiquity of phenotypic plasticity as noted at the end of the first paragraph in this post.

However, it’s also true that some plasticities are set, for example the Junonia octavia butterflies with the blue and orange morphs for the different seasons. Such cases are evidence for the first point, and are collected as a special type of phenotypic plasticity called adaptive phenotypic plasticity: the product of the plasticity provides a distinct advantage, and so it’s set that whenever specific conditions arise, the track towards the alternative phenotype is embarked upon.

The practical significance of phenotypic plasticity shouldn’t be underestimated. As an example, plants, the most phenotypically plastic of all, will probably be relying on that capacity to get through global warming (Valladares et al., 2007). Thankfully, phenotypic plasticity is one of the larger areas of current research from developmental biology, ecology, and evolution.

pigliucci_book

For more on phenotypic plasticity, the eponymous 2001 book by Pigliucci, Phenotypic Plasticity: Beyond Nature and Nurture, may be old and somewhat outdated, but still a useful reference. I also forgot while writing this that I’ve already written a post on phenotypic plasticity, but at least it does take on a different tack…

References:

Agrawal AA. 2001. Phenotypic Plasticity in the Interactions and Evolution of Species. Science 294, 321-326.

Pigliucci M. 1996. How organisms respond to environmental changes: from phenotypes to molecules (and vice versa). TrEE 11, 168-173.

Valladares F, Gianoli E & Gómez JM. 2007. Ecological limits to plant phenotypic plasticity. New Phytologist 176, 749-763.

Via S & Landa R. 1985. Genotype-Environment Interaction and the Evolution of Phenotypic Plasticity. Evolution 39, 505-522.

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