Résumé
Observed phenotypic responses to selection in the wild often differ from
predictions based on measurements of selection and genetic variance. An
overlooked hypothesis to explain this paradox of stasis is that a skewed
phenotypic distribution affects natural selection and evolution. We show
through mathematical modelling that, when a trait selected for an optimum
phenotype has a skewed distribution, directional selection is detected
even at evolutionary equilibrium, where it causes no change in the mean
phenotype. When environmental effects are skewed, Lande and Arnold’s
(1983) directional gradient is in the direction opposite to the skew. In
contrast, skewed breeding values can displace the mean phenotype from the
optimum, causing directional selection in the direction of the skew. These
effects can be partitioned out using alternative selection estimates based
on average derivatives of individual relative fitness, or additive genetic
covariances between relative fitness and trait (Robertson-Price identity).
We assess the validity of these predictions using simulations of selection
estimation under moderate samples size. Ecologically relevant traits may
commonly have skewed distributions, as we here exemplify with avian laying
date – repeatedly described as more evolutionarily stable than expected –,
so this skewness should be accounted for when investigating evolutionary
dynamics in the wild.