Rethinking phenotypic plasticity and its consequences for individuals, populations and species

Abstract

Much research has been devoted to identify the conditions under which selection favours flexible individuals or genotypes that are able to modify their growth, development and behaviour in response to environmental cues, to unravel the mechanisms of plasticity and to explore its influence on patterns of diversity among individuals, populations and species. The consequences of developmental plasticity and phenotypic flexibility for the performance and ecological success of populations and species have attracted a comparatively limited but currently growing interest. Here, I re-emphasize that an increased understanding of the roles of plasticity in these contexts requires a 'whole organism' (rather than 'single trait') approach, taking into consideration that organisms are integrated complex phenotypes. I further argue that plasticity and genetic polymorphism should be analysed and discussed within a common framework. I summarize predictions from theory on how phenotypic variation stemming from developmental plasticity and phenotypic flexibility may affect different aspects of population-level performance. I argue that it is important to distinguish between effects associated with greater interindividual phenotypic variation resulting from plasticity, and effects mediated by variation among individuals in the capacity to express plasticity and flexibility as such. Finally, I claim that rigorous testing of predictions requires methods that allow for quantifying and comparing whole organism plasticity, as well as the ability to experimentally manipulate the level of and capacity for developmental plasticity and phenotypic flexibility independent of genetic variation.

Figure 1

Trends in research output on plasticity. Figure shows number of publications per year up to December 2013. Figure is based on data extracted from a topic search for ‘phenotypic plasticity' OR ‘developmental plasticity' OR ‘phenotypic flexibility' OR ‘behavio*ral plasticity' conducted on 21 March 2014 from ISI Web of Science. The literature search yielded 11 822 papers published between 1967 and December 2013. More than 1000 papers are review articles.

Figure 2

Interdependence among phenotypic dimensions, plasticity, flexibility and environmental influences jointly affect body size in snakes. Schematic representation of the many ways by which different phenotypic dimensions interact and are influenced by internal and external environmental factors and how they may jointly contribute to within- and among-individual variation in growth rate and body size of snakes. This example illustrates that individuals are complex integrated units that cannot be decomposed into a suite of independent ‘traits', and that variation in a given phenotypic dimension can be influenced by combinations of both genes, irreversible developmental plasticity and by reversible phenotypic flexibility in response to changes along different environmental factors. See text for details.