Gene flow drives periods of both evolutionary stasis and change in a wild bird population.

The struggle to explain the absence of evolutionary change in heritable, fitness-related traits of free-living populations has become emblematic of our ignorance of the evolutionary process in the wild. Yet while this 'paradox of stasis' has been the subject of much discussion, it is primarily identified a posteriori and rarely subject to direct quantification. We examined the evolution of clutch size in neighbouring populations of great tits (Parus major) before, during and after an experimental evolutionary perturbation. Interannual genetic changes in clutch size were directionally consistent with selection (refuting absolute stasis) but were smaller than predicted by adaptive evolutionary models (supporting relative stasis). Yet accounting for the contribution of post-selective immigration (i.e., explicitly recognising the distinction between adaptation and evolution) greatly improved our forecast accuracy and this tendency toward overprediction. Indeed, attributing interannual genetic change to its contributing demographic processes revealed immigration to be key to explaining periods of both evolutionary stasis and change, with its evolutionary impact varying across time and space in a manner analogous to the dynamism of natural selection. Our multi-decadal study of clutch size evolution thus shows that recognising the impact of gene flow is crucial to explaining contemporary evolutionary change and stasis of a key determinant of life history.