These are not mutations...these are all within the range of genetic variation that already exists in the human species. Every so many births there will be a six-fingered child and every so many births there will be a child that will grow up at the low end of the height scale. None of these confer any particular reproductive advantage, especially among humans (in which cultural factors matter more than physical adaptations to environmental factors, and culturally people tend not to prefer mates that have six fingers or are very short), so their incidence remains very low. But if you look at another trait, such as sickle-cell morphology of red blood cells, then this does confer a selective advantage to people living in different environmental situations. Sickle-shaped cells result in anema which by itself is not a very good thing but which represents a selective advantage in places of the world where malaria is prevalent. Why? Because the parasite transmitted from the mosquito cannot effectively attack cells that are sickle-shaped, which thus prevent an infection.
But having an odd trait that gives selective advantage does not mean that that those that have the trait will "evolve into their own species". That requires a reproductive barrier, such that what was previously a single interbreeding population becomes split into two gene pools that no longer interbreed. The trait itself could pose a barrier to reproduction (perhaps it is a sexual turn-off, for instance), but the barrier could be due to geographical isolation or other factors. When the two gene pools are isolated over an extended period of time, then they can drift in their own directions, such that traits that might occur 1 in a million births in the original population might occur in 1 in a 100 in one population and not in the other. A trait would become more common for one isolated population because it is advantageous to it in a way it is not in the other population. These distinct populations that do not interbreed would then be distinct species with different morphologies.