Assessing the evidence for treating developmental plasticity and phenotypic flexibility as different phenomena

Phenotypic plasticity has long played a central role in eco-evolutionary theory, but it was not until 20 years ago that it was proposed that the term encompasses two distinct phenomena—developmental plasticity and phenotypic flexibility. While this terminology has since been adopted by some, the question of whether they are distinct phenomena remains contentious and they are both frequently lumped under the umbrella of ‘plasticity.’ Here, we treat the dichotomy between developmental plasticity and phenotypic flexibility as a hypothesis, put forth a set of predictions that follow from this hypothesis, and review the support for this hypothesis in the literature. We predict that, if they result from separate phenomena, developmentally plastic and phenotypically flexible traits should differ in: (1) the environmental context under which they evolve, (2) their mechanisms of regulation, (3) their costs of production, (4) how selection acts on them and (5) their influence on a population's evolutionary trajectory. In general, most evidence supports treating developmental plasticity and phenotypic flexibility as separate phenomena, but much remains to be learned, and few studies have specifically investigated their potential differences. In particular, explorations of their regulation, as well as the costs of trait production and reversal are needed. Given the hypothesized link between developmental plasticity, phenotypic flexibility and resiliency in the face of rapid environmental change, this is an urgent topic that will further our understanding of phenotypic evolution across environmental contexts. Read the free Plain Language Summary for this article on the Journal blog.