maybe it changes, like those moths that got eaten by owls in england or something.

before the industry came in the lighter ones were more common, but when the industry came in the smog covered the trees and the darker ones survived more cause the camo worked better.

there is also a genotype-phenotype issue. I might be a little off, but i'll give this a shot: Some plants have a phenotype(explicit physical characteristics) which expresses dependent upon environmental demands. So although the the genotype(implicit genetic structure) has potential traits these traits only express in a given environment. So we could have the predisposition for multiple types but only one expresses explicitly based upon the way the functions organize themselves naturally.