Phenotypic Plasticity

Phenotypic plasticity is the capacity of a single genotype to produce a variety of phenotypes under different environmental conditions. The response of a genotype to a particular environmental factor is called its “norm of reaction.” Some norms of reaction reflect unavoidable consequences of adverse situations, like the progressive stunting of fish when developing at progressively higher densities. Others reflect the influence of physical or chemical factors in an organism’s environment, like the slowing of development in ectothermic animals as temperatures decrease. Many responses to an environmental stimulus enhance fitness, like the production of toxic chemicals in the leaves of some plants when attacked by herbivores. Plasticity takes many forms. Some plasticity is reversible, like the remodeling of muscle and bone in vertebrates in response to physical stress or the ability of an animal to learn and, in some cases, unlearn. Irreversible plasticity is reflected in developmental transitions like the response of flowering time to temperature in plants. For over a century, biologists have devoted enormous effort to understanding plasticity. Some of the earliest studies focused on how much phenotypic variation among populations might be due to plasticity. The widespread prevalence of plasticity led to asking whether observed norms of reaction, characteristic of particular populations, were the signature of natural selection in a variable environment. A classic case study was the demonstration that water fleas produce a thicker carapace, often with long projections, when they develop in the presence of a predator, the larvae of the phantom midge. The thickened carapace and its projections protect the water flea from predation by midges. In the absence of midges, the water fleas do not develop the thickened carapace. The theory for how norms of reaction could evolve through natural selection and the tests of these theories in a staggering array of organisms has produced an enormous literature. In recent years, ecologists have examined the role of plasticity in helping organisms survive and persist in stressful conditions and how it facilitates the establishment and expansion of populations in novel environments. Evolutionary biologists have asked when plasticity will promote or preclude further evolutionary change. The recognition that norms of reaction could reach across generations via maternal effects opened an enormous opportunity for new research. The search for the mechanistic bases of norms of reaction—that is, their genetic control—has led to the integration of epigenetics into ecology and evolutionary biology.