Figure 1.

Hypothetical surface profile representation of the timing of a trait and its interaction with the quality of its expression (e.g. size, colour and complexity). Examples for specific traits could be bird song, or ornaments in birds and mammals. Sexual selection would favour an early expression of this trait, while fecundity/viability selection may act against an early display. Likewise, sexual selection would promote a high quality of trait expression, while fecundity/viability selection would act against the expression of the highest quality traits. Consequently, only individuals of the highest quality would be able to sustain the costs of displaying this trait early and at high quality, but would gain maximal reproductive success. Specifics of surface profile depend on parametrization of the model, and this representation serves mainly illustrative and not quantitative purposes. For additional explanations, see text. (Online version in colour.)