Demographic limits to the role of plasticity in adaptation to environmental shifts

Background/Question/Methods

The question of how phenotypic plasticity affects adaptation has a long history. Efforts to understand evolutionary aspects of species introductions, or other anthropogenic environmental changes have spurred more recent interest in plasticity’s role in evolutionary rescue. Several recent theoretical treatments of this role focus on the benefits to partially adaptive plasticity. In these models, variance in plasticity of a trait results in additive genetic variance of the trait increasing with environmental novelty. This increased variance enables rapid selection for increased plasticity (consistent with the Baldwin effect) that facilitates evolutionary rescue of the population. Yet, it is well established that variance reduces mean population fitness (termed ``variance load''). Further, when plasticity is partially adaptive due to imperfect environmental predictability, increasing environmental novelty reduces mean population fitness. Here, we aim to reconcile potential benefits of plasticity in novel environments with effects of reduced mean fitness due to these two factors. In particular, we ask how the benefits of plasticity are limited by the risk of population extinction due to reduced growth rate as environmental novelty increases.

Results/Conclusions

We describe a quantitative genetic model of selection on a plastic trait after a discrete environmental shift. Then, linking the quantitative genetic model to a demographic model, we numerically simulate probability of extinction (i.e., population decrease below a critical abundance) to quantify how extinction probability depends on environmental predictability, genetic variation in plasticity (i.e., GxE), and magnitude of the environmental shift.  These results clarify that the demographic imperative to avoid extinction can limit the benefit to plasticity for populations adapting to novel environments.