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. Here we show that a wild rodent population has evolved to become lighter, but that both this evolutionary change and the selective pressure that drives it are not apparent on the phenotypic level. Thereby we demonstrate that understanding and predicting the response of wild populations to environmental change requires an explicitly (quantitative) genetic approach, and that natural populations can show a rapid and adaptive, but easily missed, evolutionary response.