Consider the following hypothetical scenario: An ancestral species of duck had a varied diet that included aquatic plants and terrestrial plants and insects. These ducks spent time on both land and water. Individuals of this species varied in the amount of webbing in their feet, with some individuals having more webbing and some having less. As many years went by, the environment changed such that the aquatic food sources were much more plentiful than those on land. Many generations later, almost all ducks had more webbing on their feet. How is this best explained?