Abstracts (first author)
Quantifying the demographic cost of selection in a changing environment
The lag load, or reduction in mean fitness caused by incomplete adaptive tracking of a moving phenotypic optimum, is a key quantity in models of evolution in changing environments. Despite its central theoretical importance, empirical studies quantifying the lag load and the factors affecting it are lacking. Here we explore these issues in a Dutch population of great tits Parus major, where warming springs have generated a mismatch between annual breeding time and the timing of an important food resource, providing an ideal opportunity to examine how the resulting directional selection for earlier breeding time impacts population mean fitness and related demographic parameters. First, we show that interannual variation in mismatch over almost four decades has surprisingly not affected population growth, despite it having led to intensified directional selection. We demonstrate an important mechanism contributing to this uncoupling, whereby fitness losses associated with mismatch are counteracted by fitness gains due to relaxed competition. Next, we parameterised a quantitative genetic model to predict the theoretical ‘critical rate of environmental change’, beyond which increased maladaptation leads to population extinction. Results imply that even ‘mild’ rates of climate change would be close to the critical rate. However, individual-based simulations that account for both evolutionary processes and density dependence revealed that the expected time to extinction, although highly uncertain, is on the order or centuries, rather than decades. These findings imply that microevolution would only rescue the population from mild rates of sustained climate change. On the other hand, they also illustrate that considerable maladaptation can be demographically tolerated in the short term through density dependence, without immediate population declines, effectively ‘buying time’ for microevolution to restore adaptation until the environment (climate) stabilises.