Final answer:
Bees impose a directional selection on flower size, preferring larger flowers and leading to an evolution towards larger sizes.
Step-by-step explanation:
In the case of flower size, bees impose a directional selection. This means that bees tend to prefer larger flowers and are more likely to visit and pollinate them.
As a result, over time, the population of flowers may evolve to have larger sizes as smaller flowers are less successful in reproducing.