The stasis that wasn't: Adaptive body mass evolution is opposite to phenotypic selection in a wild rodent population

Despite being heritable and under selection, traits often do not appear to evolve as predicted by evolutionary theory. Indeed, conclusive evidence for contemporary adaptive evolution remains elusive in wild vertebrate populations, and stasis seems to be the norm. This so-called stasis paradox highlights our inability when it comes to predicting evolutionary change. This is especially concerning in the context of rapid anthropogenic environmental changes, and its underlying causes are therefore hotly debated. Applying a quantitative genetic framework to data from the long-term monitoring of a wild rodent population, we here show that stasis is an illusion. Indeed, the population has evolved to become lighter, and this genetic change is an adaptive response related to climate. Importantly however, both this evolutionary change and the selective pressures driving it are not apparent on the phenotypic level. We thereby demonstrate that natural populations can show a rapid and adaptive evolutionary response in response to novel selective pressures, but that these may easily go undetected and may not match phenotypic estimates of selection. Hence, evolutionary stasis is likely to be less common than it appears.