Um, Nothing evolved here. Heres an example that could go alonmg with it. Say there were only two types of hair color for humans, brown and black. Most of the humans had had brown hair with only a few black haired humans. An animal came along and started eating all the brown haired humans but would not touch the black haired ones. After awhile blacked haired people would become more prodomiant than brown for the black didnt have anything eating them. Same thing here, nothing is evolving, it could be that just one is being eaten more than the other which is not evolving for the ones without it are NOT getting it.