Résumé
Earlier phenology induced by climate change, such as the passerines'
breeding time, is observed in many natural populations. Understanding the
nature of such changes is key to predict the responses of wild populations
to climate change. Genetic changes have been rarely investigated for
laying date, though it has been shown to be heritable and under
directional selection, suggesting that the trait could evolve. In a
Corsican blue tit population, the birds' laying date has
significantly advanced over 40 years, and we here determine whether this
response is of plastic or evolutionary origin, by comparing the
predictions of the breeder's and the Robertson-Price (STS) equations,
to the observed genetic changes. We compare the results obtained for two
fitness proxies (fledgling and recruitment success), using models
accounting for their zero-inflation. Because the trait appears heritable
and under directional selection, the breeder's equation predicts that
genetic changes could drive a significant part of the phenological change
observed. We however found that fitness proxies and laying date are not
genetically correlated. The STS therefore predicts no evolution of the
breeding time, predicting correctly the absence of trend in breeding
values. Our results also emphasize that when investigating selection on a
plastic trait under fluctuating selection, part of the fitness-trait
phenotypic covariance can be due to within individual covariance. In the
case of repeated measurements, splitting within and between individual
covariance can shift our perspective on the actual intensity of selection
over multiple selection episodes, shedding light on the potential for the
trait to evolve.