Pii: S1360-1385(00)01797-0

and life history depending on environmental conditions. These environmental responses are both trait and resource specific, and represent evolved characteristics that vary among genotypes, populations and species. The past 15 years have seen an explosion of interest in this capacity of a given genotype to express different phenotypes in different environments, a phenomenon known as phenotypic plasticity. Although biologists have long been aware of plasticity (indeed, this is the reason that experiments are performed under controlled environmental conditions), for much of the past century phenotypic response to environment was regarded as ‘environmental noise’ that obscured the ‘true’ genetic characteristics of the organism. Only recently has plasticity been widely recognized as a significant mode of phenotypic diversity and hence as an important aspect of how organisms develop, function and evolve in their environments. This new awareness has led to a redefinition of the genotype as a repertoire of environmentally contingent phenotypic possibilities or ‘norm of reaction’, rather than a blueprint for a single fixed outcome (Fig. 1). In general, biologists are increasingly coming to view the phenotype as the outcome of complex synergistic developmental systems, influenced by multiple interacting genes and gene products as well as by the organism’s internal and external environments. By the early 1990s, developmental and physiological plasticity had been reported in land plants, algae, marine invertebrates, insects, fish, amphibians, reptiles and small mammals. More recently, plasticity for structure, biochemistry and metabolic activity was documented in a lichen. Plasticity has been studied most intensively in plants, which typically show dramatic effects of environment on growth and development. They can also be more readily cloned (or highly inbred) and raised in alternative environments than many other organisms. Thus, much of our current knowledge of phenotypic plasticity comes from plant studies documenting the range of phenotypes that can be produced by individual genotypes in response to contrasting conditions. Initial studies of plant plasticity often focused on simple descriptors of growth and morphology such as plant size, branch number and internode length, although some early studies included directly functional aspects of plasticity such as proportional allocation to different plant tissues or assimilation rates. More recent studies have focused on those aspects of plasticity that relate directly to the functional and reproductive success of plants in their environments and hence are both ecologically and evolutionarily important. In addition, researchers are increasingly testing plasticity in experimental environments that are ecologically relevant to the study organism, rather than in arbitrary sets of contrasting conditions. This emphasis on ecologically, and therefore evolutionarily, meaningful traits and environments has opened several important new avenues of inquiry. Recent research has revealed diverse,