Phenological mismatch drives selection on elevation, but not on slope, of breeding time plasticity in a wild songbird

Phenotypic plasticity is an important mechanism for populations to respond to fluctuating environments, yet may be insufficient to adapt to a directionally changing environment. To study whether plasticity can evolve under current climate change, we quantified selection and genetic variation in both the elevation (RN_E ) and slope (RN_S ) of the breeding time reaction norm in a long-term (1973-2016) study population of great tits (Parus major). The optimal RN_E (the caterpillar biomass peak date regressed against the temperature used as cue by great tits) changed over time, whereas the optimal RN_S did not. Concordantly, we found strong directional selection on RN_E , but not RN_S , of egg-laying date in the second third of the study period; this selection subsequently waned, potentially due to increased between-year variability in optimal laying dates. We found individual and additive genetic variation in RN_E but, contrary to previous studies on our population, not in RN_S . The predicted and observed evolutionary change in RN_E was, however, marginal, due to low heritability and the sex limitation of laying date. We conclude that adaptation to climate change can only occur via micro-evolution of RN_E, but this will necessarily be slow and potentially hampered by increased variability in phenotypic optima.