Fitness consequences and heritability aspects of emergence date in Phlox drummondii

1 The fitness consequences of emergence date in Phlox drummondii were determined for three populations in central Texas (USA). We experimentally manipulated the emergence time of seedlings to extend the distribution of emergents across time. 2 In all years (1989-92) significant differences in fitness were found between populations and between emergence dates. Fitness decreased with later seedling emergence in five of six population-by-year combinations tested. 3 The frequency of juvenile mortality also differed with emergence date in five cases. 4 Later emerging plants flowered sooner than would be expected based on the range of emergence dates, but this was not sufficient to overcome the penalty of late emergence on flower and fruit production. Total flowers per flowering plant always decreased with later emergence, and total fruits and seed-set were negatively correlated with emergence date for two of the three populations. 5 Fitness functions were used to describe the fitness relationship between emergence dates. In all cases individual fitness declined non-linearly at later emergence dates, and in many cases the earliest emergents had the highest expected fitness. The variance in fitness differed among years and sites, and ranged from 0.043 to 0.766 seeds individual(-2). 6 The degree to which days to seedling emergence was heritable in two further native populations of P. drummondii was measured by conducting divergent artificial selection for two generations. The estimates of realized heritability (0.081 and 0.15) could not be distinguished from zero. 7 Phlox drummondii therefore appears to have little genetic capacity to respond to natural or artificial selection on the timing of emergence.