Even if you consider that the radiation and FEV didn't induced any change in animals DNA, you have to take into account that the world has changed.
The climate has changed, new hazards appeared, the human population almost disapeared and was non existants for a while, some entire species have disapeared (some others species lost their food, some lost their predators), some various chemicals were released into the air, the many dead bodies spread some proteins, etc... To survive into that new world, species had to adapt.
We should take into account not only the Fallout world as we know, but also the Fallout world during the nuclear winter, the few years/decades in which it wasn't possible for any human to survive outside, and yet life kept going...