Phenotypic plasticity refers to environment-dependent trait expression (Dewitt and Scheiner, 2004).11 Plasticity typically refers to the consistent expression of phenotypes in different environments. Traits that change unpredictably in different environments are usually said to be ‘noisy’ rather than plastic. Various terms are used to describe traits with the same phenotype in all environments, including ‘aplastic’, ‘non-plastic’, ‘fixed’, ‘constant’, ‘canalized’, and ‘environmentally insensitive’. Knowledge of phenotypic plasticity is important in virtually all areas of basic and applied biology. Researchers in applied fields (such as agriculture, medicine, public health, wildlife management, and conservation biology) have a vested interest in knowing how traits are or will be expressed under specific conditions. Ecologists are interested in how the expression of traits in different environmental conditions and habitats might affect population and community dynamics. And evolutionary biologists are interested in how traits with environmentally-conditional expression have and will evolve. The widespread interest in phenotypic plasticity has made it a prominent focus of biological research.

Phenotypic plasticity is an especially active research area in ecology and evolution with a brimming literature that has advanced the understanding of organismal variation, adaptation, and speciation (Sarkar, 2004; Pfennig, 2021). Most advances, especially recently, are based on highly simplified biological scenarios such as dichotomous environments or linear environmental gradients. Here we advocate a path for taking modern plasticity research in a far more biologically relevant direction.

Phenotypic plasticity, like any trait, can be heritable and respond to any evolutionary force. What makes plasticity unique is that it manifests only in a variable environment and is thus automatically complex. The key to addressing plasticity's ineluctable complexity, we contend, is a simple but comprehensive conceptual framework that can be used to address questions about phenotypic plasticity (including connections among areas of development, behavior, genetics, ecology, and evolution) with far more depth and realism than current literature.

The framework (Figure 1) involves four independent components: (1) patterns of plasticity; (2) environment encounters; (3) fitness consequences; and (4) inheritance. The first two components are needed to predict realized patterns of expression, the first three determine population dynamics, and all four contribute to evolution. Below, we describe each component in turn, highlighting key concepts and practices that enable researchers to enrich the understanding of phenotypic plasticity and its evolution in nature. While none of these four components is new, we have not seen them presented together in a systematic way, as here. We contend that widespread use of this structured quartet of concepts would drive modern studies of phenotypic plasticity in a much more productive, profound, connected, and comprehensible direction.

Figure 1Open in figure viewerPowerPoint

The four fundamental elements of phenotypic plasticity and their roles in determining patterns of phenotypic expression realized in nature, ecology (population or community dynamics), and evolution.