1. Department of Population Biology, Evolutionary Biology Centre, Uppsala University, Norbyvägen 18d, SE-752 36 Uppsala, Sweden
2. Institute of Cell, Animal and Population Biology, University of Edinburgh, Edinburgh EH9 3JT, UK
3. Department of Zoology, University of Oxford, South Parks Road, Oxford OX1 3PS, UK
4. Department of Animal Ecology, Evolutionary Biology Centre, Uppsala University, Norbyvägen 18d, SE-752 36 Uppsala, Sweden

Correspondence to: J. Merilä¹ Correspondence and requests for materials should be addressed to J.M. (e-mail: Email: juha.merila@ebc.uu.se).

Abstract

Microevolution is expected to be commonplace, yet there are few thoroughly documented cases of microevolution in wild populations^{1, 2}. In contrast, it is often observed that apparently heritable traits under strong and consistent directional selection fail to show the expected evolutionary response^{3, 4}. One explanation proposed for this paradox is that a genetic response to selection may be masked by opposing changes in the environment^{5, 6}. We used data from a 20-year study of collared flycatchers (Ficedula albicollis) to explore selection on, and evolution of, a heritable trait: relative body weight at fledging ('condition'). Despite consistent positive directional selection, on both the phenotypic and the additive genetic component (breeding values, estimated from an animal model) of condition, the mean phenotypic value of this trait in the population has declined, rather than increased, over time. Here we show that, despite this decline, the mean breeding value for condition has increased over time. The mismatch between response to selection at the levels of genotype and phenotype can be explained by environmental deterioration, concealing underlying evolution. This form of cryptic evolution may be common in natural environments.