Abstract

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 (RNE) and slope (RNS) of the breeding time reaction norm in a long‐term (1973–2016) study population of great tits (Parus major). The optimal RNE (the caterpillar biomass peak date regressed against the temperature used as cue by great tits) changed over time, whereas the optimal RNS did not. Concordantly, we found strong directional selection on RNE, but not RNS, 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 RNE but, contrary to previous studies on our population, not in RNS. The predicted and observed evolutionary change in RNE 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 RNE, but this will necessarily be slow and potentially hampered by increased variability in phenotypic optima.

Highlights

  • Phenotypic plasticity is an important mechanism by which an individual can adapt its phenotype in response to fluctuating environmental conditions (Pigliucci 2001; Schlichting and Pigliucci 1998)

  • CHANGE IN OPTIMAL REACTION NORM OVER TIME We found that the optimal reaction norm for laying date changed to earlier dates over three distinct periods (Fig. 1A); the optimal phenotype (LDθ) advanced by 7.72 days from the first to the second period, and by 8.45 days from the first to the third period

  • SELECTION ON THE GREAT TIT REACTION NORM Along with a change to earlier dates in the elevation of the optimal reaction norm, we found statistical evidence for directional selection on the elevation of the great tit reaction norm in period 2; that is, lifetime reproductive success (LRS) covaried negatively with the elevation, indicating selection for a lower reaction norm (Table 2)

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Summary

Introduction

Phenotypic plasticity is an important mechanism by which an individual can adapt its phenotype in response to fluctuating environmental conditions (Pigliucci 2001; Schlichting and Pigliucci 1998). Quantifying variation in reaction norms is imperative for understanding evolutionary processes because it can elucidate whether populations are capable of responding to such directional selection. Predicting such responses may be difficult when G × E leads to nonlinear changes in genetic variation across environments (Tomkins et al 2004; Turelli and Barton 2004; Kokko and Heubel 2008), or when genetic variation and selection are negatively correlated with one another (Wood and Brodie III 2016; but see Ramakers et al 2018). The extent to which phenotypic plasticity modulates evolutionary processes will be highly context dependent (Hoffman and Merila 1999)

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