Evolutionary tipping points in the capacity to adapt to environmental change
Environmental variation is becoming more frequent and unpredictable as a consequence of climate change, yet we currently lack the tools to evaluate the extent to which organisms may adapt to this phenomenon. Earlier work indicates that some organisms face environmental uncertainty by employing a strategy that minimizes fitness variance across all possible environmental scenarios (conservative bet-hedging), while others have evolved a mix of strategies that takes advantage of alternative environmental conditions in a probabilistic fashion (diversification bet-hedging). Organisms are also known to cope with environmental variation through phenotypic plasticity, expressed either early in development (irreversible plasticity) or throughout life (reversible plasticity). In other cases, environmental variation has simply resulted in correlated variation in mean population traits, as natural selection favors different phenotypes over evolutionary time (also known as adaptive tracking). The conditions that promote these different forms of evolutionary response to environmental variation, or the extent to which natural populations may exhibit them as a consequence of climate change are currently unclear. Here we introduce a predictive theoretical framework that addresses these issues, and demonstrate how it can be used to explore the potential consequences of natural or human-induced alterations to environmental cycles.
We show that the parameter space determined by different combinations of predictability and timescale of environmental variation is partitioned into distinct ‘response mode regions’ where a single mode of response has a clear selective advantage over all others. We then demonstrate that although evolution can accommodate for significant environmental changes within those regions, any changes (even relatively minor ones) that involve transitions between them, will result in rapid population collapse and often extinction. Thus, boundaries between response mode regions in this parameter space are potential ‘evolutionary tipping points’, where even minor changes in the timescale or predictability of environmental variation can have dramatic and disproportionate consequences on population viability. We then discuss how different life histories and genetic architectures may influence the location of tipping points in natural systems and the likelihood of extinction during such transitions. These insights may help identify and address some of the cryptic threats to natural populations that are likely to result from any natural or human-induced environmental change.