Scandinavia and the World

Comments #9818782:

Who wants to live forever 17 7, 10:16pm

To achieve significant number of mutations, plants and seeds had to be subjected to radiation levels that killed about half of them. Type, duration and conditions of exposure also had to be tweaked to achieve useful results, but process in general is still very random.

Random mutations in animals are likely to produce non-viable offspring, and high enough dose will probably kill or render subject sterile well before it has a chance to reproduce. Even if mutant specimen manages to reproduce, there is no guarantee that it's particular mutation will be replicated in next generation or generation after that. Apparently slight morphological differences and higher rates of albinism were discovered in exclusion zone, but I don't think there were mentions of something drastic like formation of new species or subspecies.

Also, long-term accumulation of radioactivity happens in animals through consumption of plant matter (or other animals) that picked up caesium-137 and strontium-90 from environment. Both isotopes are treated by body as potassium and calcium respectively, as such they are distributed through and accumulated in appropriate tissues. Americium-241 will become a bigger problem over time, but since it produces alpha-radiation it's more immediately lethal.