The driving forces behind the evolution of plasticity have been studied, but the ways in which plasticity evolves and is maintained are still unclear. A relatively recent paper in 2020 by Warren Burggren entitled Phenotypic Switching Resulting From Developmental Plasticity: Fixed or Reversible? had me wondering about the reversibility of phenotypes resulting form plasticity during development. The paper follows a similar approach of a previous review by Beaman et al. (2016). Both papers, along with others, argue for a change in viewpoint about plasticity and the trade-offs associated with it, something that has peaked my interest.
Burggren (2020) starts out by introducing a new framework for understand plasticity, mainly a push to move away from the classical G x E model for plasticity to include more terms that contribute to plastic traits. The new model, G x (E + Epi) x (D x S) incorporates epigenetics (Epi; a mediator between genes and environment) that may be due to transgenerational inheritance, and thus environments experienced by former generations. Additionally, the model includes development (D) as a separate entity in plasticity, with a stochastic (S) element to developmental processes. I find this framework intriguing, because often stochasticity is ignored during development and life history, where it may play a relatively large role. Similarly, as our knowledge of transgenerational inheritance has increased, we now know that environmental effects on one generation can influence the biology of future generations, even without exposure to the original environmental signal. Therefore, plastic responses can be broken down into smaller categories that may have differing effects.
Shifting gears, both Burggren (2020) and Beaman et al. (2016) argue for the reversibility of developmental plasticity as a selective and adaptive trait that is overlooked. One of the fundamental questions I have had about plasticity is if developmental processes are moldable to an extent by the environment, why not keep such malleability to continue matching phenotypes to the environments experienced throughout life? Obviously, plasticity has a cost, but what trade-offs determine how much of this cost an organism takes on? One prevailing view about plasticity is that it is beneficial when the developmental environment can be predictive of later life environment, allowing for maximized fitness. However, in times when developmental environment and the environment experienced later in life do not match, organisms can be at a significant disadvantage. The argument made by these authors is that the view of plasticity as a decision made during development that remains static throughout life may be flawed in some situations. The idea of acclimation (Beaman et al. 2016) and reversible plasticity (Burggren 2020) is that in times when later life environments are different than that experienced during development that drove a particular plastic response, selection would favor organisms that have the ability to reverse that phenotype and acclimate to the new environment they are in. The cost associated with such an ability depends on various aspects of life history and how quickly environment varies in relation to life span. As Beaman et al. (2016) suggests, while the mean measure of an environmental variable may be an important driver for a plastic trait, the variation experienced in that environment may also prime an organism for dealing with such variation later in life, leading to the ability to acclimate to those environments. In this case, the benefits of keeping a plastic response would likely out way the costs.
The idea of reversible plasticity acclimation and the evolutionary drivers of these traits raises many questions for me, and both of these papers altered my way of thinking about them. What trade-offs are associated with having a plastic trait? What drives an organism to taken on these costs during development but not later in life? Is this driven by physiological constraints to plasticity later in life? How do the drivers of plasticity differ in long-lived vs short-lived species? What role does stochasticity play in plasticity? Hopefully, with future work, some of these questions will be answered.
Burggren W.W. Phenotypic Switching Resulting From Developmental Plasticity: Fixed or Reversible? Front. Physiol. 10:1634. doi: 10.3389/fphys.2019.01634 (2020).
Beaman, J. E., White, C. R. & Seebacher, F. Evolution of Plasticity: Mechanistic Link between Development and Reversible Acclimation. Trends in Ecology & Evolution 31, 237–249 (2016).
I really enjoyed this post Chris. I have been pondering the same ideas. Especially how do long lived species evolve plasticity in a stochastic environment. In our discussions during class, most plastic responses occur as a result of changes in the environment during development. This makes a lot of sense for something that has short generational time like frogs where adapting to a drought during development allows for survival to at least one episode of reproduction. However, what about long lived species such as elephants, giant groupers, alligators, etc. These species must have some plastic response to environmental stochasticity (as shown in your lab's work), but how does this aid a species that lives for 60+ years? Where their environment can go through many changes by the time they reach sexual maturity. Does plasticity act differently in these long lived species? Are longer lived species able to reverse plastic responses from an early age vs. shorter lived species like frogs who have shown negative carry over effects from larval to adulthood?
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