Résumé
1. Evolutionary adaptation as a response to climate change is expected for
fitness-related traits affected by climate and exhibiting genetic
variance. Although the relationship between warmer spring temperature and
earlier timing of reproduction is well documented, quantifications and
predictions of the impact of global warming on natural selection acting on
phenology in wild populations remain rare. If global warming affects
fitness in a similar way across individuals within a population, or if
fitness consequences are independent of phenotypic variation in
key-adaptive traits, then no evolutionary response is expected for these
traits. 2. Here we quantified the selection pressures acting on laying
date during a 24-year monitoring of blue tits in southern Mediterranean
France, a hot spot of climate warming. We explored the temporal
fluctuation in annual selection gradients and we determined its
temperature-related drivers. 3. We first investigated the month-specific
warming since 1970 in our study site and tested its influence on selection
pressures using a model averaging approach. Then, we quantified the
selection strength associated with temperature anomalies experienced by
the blue tit population. 4. We found that natural selection acting on
laying date significantly fluctuated both in magnitude and in sign across
years. After identifying a significant warming in spring and summer, we
showed that warmer daily maximum temperatures in April were significantly
associated with stronger selection pressures for reproductive timing. Our
results indicated an increase in the strength of selection by 46% for
every +1°C anomaly. 5. Our results confirm the general assumption that
recent climate change translates into strong selection favouring earlier
breeders in passerine birds. Our findings also suggest that differences in
fitness among individuals varying in their breeding phenology increase
with climate warming. Such climate driven influence on the strength of
directional selection acting on laying date could favour an adaptive
response in this trait, since it is heritable.