Thanks
@krobra for that article! That made it so clear. I also see almost all tigers and no pipevines at my house. I think pipevine grows around here, but I'm afraid to add it to the garden for fear of it taking over! Perhaps an explanation is that although there may not be pipevine butterflies around, the predators probably don't know that. They think the the dark form females are the bad-tasting pipevines and so the dark tigers continue to survive and reproduce. For a trait to decrease or go away, that trait would have to be selected against, and this trait seems to be beneficial.
Edit: Forgot to say, that half and half tiger is about the coolest butterfly I have ever seen!
