Natural Selection is a theory. Minimum viable population numbers also contradict the idea of a single random mutation creating a new species. A single random mutation is the same as having only one of a species left. One theory is that a single organism could experience a mutation, then breed back with the general population that does not share that mutation, producing hybrids, and those hybrids breeding with the general population causing that mutation to become recessive, and then something unknown triggering that recessive gene in a large enough population to create a sustainable population of that new species. Research is ongoing. Some reading for you:
https://www.sciencedaily.com/releases/2009/07/090720163716.htm
"Most evolutionary changes happen in tiny increments. But when it comes to traits like the number of wings on an insect, or limbs on a primate, there is no middle ground. How are these sorts of large evolutionary leaps made? According to new research, such changes may at least sometimes be the result of random fluctuations, or noise (nongenetic variations), working alongside a phenomenon known as partial penetrance."