Ah...Darwin. His theory of survival is also based on natural selection not on Mutation. Cutting the things, Progressive evolution and natural selection are main cause behind the formation of new species. Mutation has very little impact+ Mutation mostly leads to a destructive form. With time evolution will happen in these rats and a new species could evolve but not all of a sudden bcoz of this mutation. Mutation and all of a sudden formation of a new species suits better for a Movie story like X-Man.