Forms of phenotypic plasticity in key traits, and forms of selection on and genetic variation in such plasticity, fundamentally underpin phenotypic, population dynamic, and evolutionary responses to environmental variation and directional change. Accordingly, numerous theoretical and empirical studies have examined properties and consequences of plasticity, primarily considering traits that are continuously distributed on observed phenotypic scales with linear reaction norms. However, many environmentally sensitive traits are expressed as discrete alternative phenotypes and are appropriately characterized as quantitative genetic threshold traits. Here, we highlight that forms of phenotypic plasticity, genetic variation, and inheritance in plasticity, and outcomes of selection on plasticity, could differ substantially between threshold traits and continuously distributed traits (as are typically considered). We thereby highlight theoretical developments that are required to rationalize and predict phenotypic and microevolutionary dynamics involving plastic threshold traits, and outline how intrinsic properties of such traits could provide relatively straightforward explanations for apparently idiosyncratic observed patterns of phenotypic variation. We summarize how key quantitative genetic parameters underlying threshold traits can be estimated, and thereby set the scene for embedding dynamic discrete traits into theoretical and empirical understanding of the role of plasticity in driving phenotypic, population, and evolutionary responses to environmental variation and change.
We thank Yimen Araya-Ajoy, Russ Lande and Jon Wright for helpful comments and discussions.
J.M.R. and P.A. were supported by the Norwegian Research Council through the Centre for Biodiversity Dynamics (grant 223257), NTNU and University of Aberdeen.
- cryptic genetic variation
- gene by environment interaction
- phenotypic plasticity
- quantitive genetics
- reaction norm
- threshold trait
- quantitative genetics
- Cryptic genetic variation