Phenological assortative mating in flowering plants: the nature and consequences of its frequency dependence

It has long been supposed that variation in mating phenology leads to assortative mating, but its inherent frequency dependence has not been examined. When plants in a population vary in their flowering schedule, the phenotypic (and genetic) composition of the mating pool changes over the season; this causes phenological assortative mating even if pollen is exchanged at random during each interval of the season. Phenotype frequencies govern this temporal shift in the mating pool and this makes phenological assortative mating frequency dependent. We studied phenological assortative mating in four steps. First, we derived a method to estimate the phenotypic correlation between mates from flowering schedules; this correlation, symbolized by rho , is the standard measure of assortative mating. Next, in a one-locus, two-allele system, we showed that the correlation between mates decreases as one or the other allele approaches fixation, but increases as the population deviates from Hardy-Weinberg proportions. Third, we showed that unlike assortative mating based on fixed preferences, the level of phenological assortment in one generation influences the level of assortment in the next, such that rho reaches an equilibrium value that depends on allele frequencies. Finally, we contrasted the effects of frequency-dependent, phenological assortative mating on directional selection to the effects of fixed levels of assortative mating. When allelic effects were additive, frequency dependence slightly accelerated the selection response, compared with a fixed rho . When one allele was dominant, phenological assortment slightly decelerated the selection response. Similarities between assortative mating through phenology and through habitat preference can cause the latter also to be frequency dependent.