Par Timothée BONNET, de l’Institute of Evolutionary Biology and Environmental Studies, University of Zürich-Irchel, Suisse

The understanding of evolution in wild animal populations is limited by difficulties to define and measure fitness when reproductive success is the result of many interacting factors and stochastic processes. Using the monitoring of a snow vole (Chionomys nivalis) population, I show that, despite recent claims, latent fitness at the individual level is meaningful in small populations, as variance in reproductive success cannot be thoroughly explained by stochastic processes. Thereby, I look for the phenotypic and genetic causes of the variation in fitness in this population, combining molecular tools and quantitative genetics methods. Eventually, I investigate the paradox of the maintenance of genetic variation, testing the hypothesis of temporally fluctuating selection. Phenotypic selection does fluctuate, but this does not translate into a fluctuation of genetic change and cannot explain the maintenance of genetic variation. In the future, I will build on these results to quantify the importance of individual-level variation for population-level processes and resilience to environmental changes.