Final answer:
Directional selection, diversifying selection, and frequency-dependent selection all play a role in maintaining polymorphism over time.
Step-by-step explanation:
When it comes to the maintenance of polymorphism over time, there are several types of selection that can play a role. These include directional selection, diversifying selection, and frequency-dependent selection.
Directional selection occurs when the environment changes and selects for phenotypes at one end of the spectrum of existing variation. An example of this is the evolution of the peppered moth in response to industrial pollution. Diversifying selection occurs when two or more distinct phenotypes are selected for, while the intermediate phenotypes are less fit. This can happen when environmental changes favor individuals on either end of the phenotypic spectrum.
Frequency-dependent selection favors phenotypes that are either common or rare in a population. For example, in certain forms of lizards, different throat-color patterns have different reproductive strategies, with each form being favored in different situations.