Directional selection on initial flowering date in Phlox drummondii (Polemoniaceae)

The fitness effects due to initial flowering dare in Phlox drummondii were determined for three populations in central Texas (USA) over 3 yr (1990-199 2). Mean fitness (seed set) always decreased with the later initiation of f lowering. The Likelihood of a plant fruiting differed with flowering date i n five of the six instances (population by year combinations). Though plant s that initiated flowering later tended to have spent more time in the vege tative stage and tended to die later in the year than did earlier flowering plants, this was not sufficient to overcome the reproductive penalties of flowering late. Plants that initiated flowering later in the season spent l ess time in the adult phase and were smaller. The mean number of flowers, f ruits, and seeds per flowering plant always decreased with later flowering. Fruit set was negatively correlated with flowering date in four of the six population by year combinations. Nonparametric fitness functions were used to summarize predicted fitness among different initial flowering dates for each population on a yearly basis. Predicted mean fitness always declined nonlinearly with Later flowering: the earliest flowering plants always had the highest predicted fitness. These fitness functions describe directional selection for the early initiation of flowering.