Quantitative analyses of stochastic influences on the response to phenotypic selection in a small passerine, the...

Stochastic influences arising from demographic and environmental stochasticity strongly affect eco-evolutionary dynamics. Demographic stochasticity causes reduction in the long-run population growth rates at small population sizes and generates genetic drift. The effects of stochastic environmental fluctuations on fitness are more difficult to assess because they may introduce stochasticity in the selection processes. Here, we examined stochastic variation in the response to selection on 3 breeding parameters in a collared flycatcher (Ficedula albicollis) population using density-dependent and frequency-dependent environmental covariances among individuals with different phenotypes. Environmental fluctuations induced considerable variation among individual fitness dependent on their phenotype and population size. These stochastic effects on fitness were strongest for individuals with large clutches or few fledglings, and in years with few conspecifics. Contrastingly, we found no evidence that the stochastic environmental effects on individual fitness varied with laying date. Furthermore, we found that phenotype-specific effects of environmental fluctuations were less correlated for fledgling number than for laying date and clutch size, resulting in less correlation in fitness between pairs with similar fledgling numbers. Our analyses showed that the stochastic component in the response to selection on clutch size and laying date caused by environmental stochasticity at the carrying capacity was of the same order as the component due to genetic drift. This means that stochasticity can affect phenotypic evolution even in large populations via stochastic selection generated by environmental fluctuations.