Despite accumulating examples of selection acting on heritable traits in the wild, predicted evolutionary responses are often different from observed phenotypic trends. Various explanations have been suggested for these mismatches. These include within-individual changes across lifespan that can create important variation in genetic architecture of traits and selection acting on them, but also potential problems with the methodological approach used to predict evolutionary responses of traits. Here, we used an 8-year data set on tree swallow (Tachycineta bicolor) to first assess the effects of differences among three nestling life-history stages on the genetic (co)variances of two morphological traits (body mass and primary feather length) and the selection acting on them over three generations. We then estimated the evolutionary potential of these traits by predicting their evolutionary responses using the breeder's equation and the secondary theorem of selection approaches. Our results showed variation in strength and direction of selection and slight changes in trait variance across ages. Predicted evolutionary responses differed importantly between both approaches for half of the trait–age combinations we studied, suggesting the presence of environmentally induced correlations between focal traits and fitness possibly biasing breeder's equation predictions. Our results emphasize that predictions of evolutionary potential for morphological traits are likely to be highly variable, both in strength and direction, depending on the life stage and method used, thus mitigating our capacity to predict adaptation and persistence of wild populations.

Pedigree of the southern Quebec tree swallow populationGenetic pedigree constructed with Cervus based on 6 microsatellite loci.Pedigree_TRSW.txtDataset of southern Quebec tree swallow nestlingsDataset used to estimate selection and quantitative genetic parameters. Column headings are nestling ids (ID), farm ids (FARM), year (YEAR), brood ids (BROOD), hour of measurement (HOUR), Julian day (JDAY), age (AGE), survival (FLEDG, 0 or 1), body mass (MASS) and feather length (WL).Data_TRSW.txt