Here is the definition of "polymorphic" from www.dictionary.com:
"The occurrence of different forms, stages, or types in individual organisms or in organisms of the same species, independent of sexual variations."
Polymorphisms --
A salamander named the eastern newt has an aquatic larval stage followed by a terrestrial, air-breathing stage (the red eft), followed by an aquatic adult stage.
In honey bees, queens and workers have different appearances and functions in the hive.
The ermine changes from white in winter to brownish in summer and back, as do the ptarmigan and snowshoe hare.
California king snakes occur in banded and striped forms. Graybanded king snakes occur in blairi and alterna phases.
I don't think that "polymorphism" is applied to differences that become common because of human selection.
Paul Hollander