Dynamics of allelic frequencies under different selection pressures (averages over 10 simulations). Plain line: central locus (Growth9); dotted lines: radial loci (Growth4, Growth6 and Growth12). Initial frequencies are set to f(H) = 4/7 for all loci, which seems to be one of the most likely situations (see Figure 3). (A) Upwards directional selection; (B) downwards directional selection; (C) downwards directional selection including a decrease of the fitness for low-weight individuals, simulating physiological constraints: the normal truncation selection is performed, and then the fitness of low-weight individuals is modified (if the weight is lower than 450 g, then the fitness is 0, and fitness increases linearly between 450 and 550 g; above 550 g, the fitness is maximal and set to 1). In this last situation, the selection intensity has been decreased (40 individuals randomly drawn among the 200 best phenotypes) to avoid too much genetic drift (selection in both sides sometimes lead to much less than 40 fertile adults with the 'standard' selection strength).