The Nature of Selection on Flowering Time: Integrating Fitness Contributions through Male and Female Function

A central goal of evolutionary biology is to identify traits under selection and the causal mechanisms linking these traits to fitness. Most work towards this goal in plant systems has addressed the female component of fitness only. This thesis investigates the nature of selection on the timing of flowering onset - a trait of practical and theoretical importance - with equal emphasis on female and male function. The potential male fitness effects are many. First, flowering time may directly (or indirectly through correlation with another trait) influence ability to sire seed (e.g. by affecting pollen quantity). Second, because plants tend to mate assortatively by flowering time, an individual's flowering time might covary with the quality of dam it pollinates. Third, because flowering time partly determines the specific date range over which an individual reproduces, it may covary with exposure to pollinators, herbivores, abiotic stress, and other environmental characteristics that could affect fitness. I investigated these hypotheses through numerical modeling and experiments with the annual plant (Brassicaceae). Pollen quantity per flower did not vary with flowering time (age at flowering, AAF) in B. rapa, but pollen production, ovule production, and fruit set probability all decreased from the first to last flowers on plants. Numerical modeling revealed that this within-plant variation can drive selection for young AAF through male function. The model did not, however, incorporate temporal variation in the environment. I therefore manipulated the correlation between AAF and Julian date of flowering (JDF) in experimental populations of B. rapa to observe their independent effects. Selection through female function tended to favour early JDF, regardless of AAF, whereas that through male function tended to favour young AAF, regardless of JDF. Thus, seed production seems to have been heavily influenced by the environment experienced during reproduction, while siring success was directly influenced by AAF. This result was surprising given that pollen production did not vary with AAF, and that both the numerical model and an analysis of flowering schedules predicted temporal structure to siring opportunity. This work significantly expands the conceptual framework within which this key life history trait must be understood.