Timing of Flowering: Opposed Selection on Different Fitness Components and Trait Covariation

The timing of reproduction influences how organisms interact with the environment and can have important fitness effects. In plants, the evolution of flowering phenology is often interpreted as the response to selection from mutualists, although antagonistic interactions may also be important. We examined direct and indirect phenotypic selection on the start of flowering via mutualistic and antagonistic interactions in the perennial herb Lathyrus vernus over 7 years. Flowering start influenced seed set, predispersal seed predation, and risk of grazing. These effects were in opposed directions and partly influenced different components of fitness. Combining information about effects on fitness components with information about links between fitness components and average lifetime fitness, in terms of population growth rate, showed that earlier flowering was associated with higher lifetime fitness in all years. These relationships were, however, mediated largely by variation in flower number, and direct selection on first flowering date was more variable among years. We conclude that long-term studies correcting for indirect selection and environmental covariance are needed to understand selection on reproductive phenology and that demographic approaches are necessary to assess selection mediated by several agents and influencing several components of fitness.