Résumé
To forecast how fast populations can adapt to climate change, it is
essential to determine the evolutionary potential of different life-cycle
stages under selection. In birds, timing of gonadal development and moult
are primarily regulated by photoperiod, while laying date is highly
phenotypically plastic to temperature. We tested whether geographic
variation in phenology of these life-cycle events between populations of
great tits (Parus major) has a genetic basis, indicating that contemporary
genetic adaptation is possible. We carried out a common garden experiment
in which we bred first- and second-generation pairs in captivity
originating from eggs from Gotland (Sweden) and Hoge Veluwe (Netherlands),
two populations that showed different temperature sensitivity of laying
date in a recent meta-analysis. We recorded the phenology of egg-laying,
moult and gonadal size in early spring. We found no significant
differences in laying date between the populations, but they did differ in
moult timing and testis size. This implies that under climate change the
timing of gonadal development and moult, which are mainly regulated by
photoperiod, will not respond to increased temperature but can respond by
genetic adaptation in response to selection, while the opposite holds for
laying date, perhaps indicating that plasticity is constraining genetic
adaptation.