is one that cannot successfully mate with it's antecedants. Then you are out of the realm of "microevolution." Can the blind cave salamanders successfully mate with normal salamanders? If not, you have a new species.

As for mutations in the genetic code, this happens continuously through environmental damage or through simple replication error. If such a mutation is harmless or advantageous to the organism, it survives and gets passed to the offspring. If it is damaging, it reduces the individual's chances of survival and reproduction and dies out.

Thus, mutations are random, but the random mutations are edited by their effect on the robustness of the resulting individual.