Measuring selection on reaction norms: Lack's principle and plasticity in clutch size.

Plastic phenotypes may often be subject to conflicting demands, which should generate nonlinear selection favouring intermediate optima. However, investigating complex patterns of selection on reaction norms has been challenging. We leveraged data on clutch size from two long-term studies (25 and 19 years) of individually marked house sparrows (Passer domesticus). We used a novel multivariate technique for examining linear and nonlinear selection acting on clutch size reaction norms via the fitness components of hatchling number, nestling survival, and nestling body mass at the end of the main parental period. Reaction norm slopes were highly canalized and lacked sufficient among-female variation to detect selection. Stabilizing selection and opposing patterns of directional selection occurred on both intercepts and individual residual variation via hatchling counts and nestling body mass. We also observed changes in these gradients as a function of laying date. Our results support Lack's hypothesis, that quantity-quality trade-offs shape selection on both mean clutch size and the variability of clutch sizes across breeding seasons. Our findings are also consistent with models of adaptation on asymmetric fitness landscapes, where trade-offs near a local fitness ridge or cliff favor distinct patterns of clutch size plasticity in response to unpredictable environmental variability.