Your definition of a new species, while almost universally accepted, is flawed.
How would the first of a new species procreate if it can’t breed with the species it originated from? Did two of the new species evolve simultaneously, one male and one female? Is the first of each new species capable of asexual reproduction?
It is populations that evolve, not individuals.
Let me give an example
East to West Squirrels
OK, we have lotso squirrels living all over the place from east to west. Small, but no vast empty spots (deserts, mountains).
Squirrels don’t migrate and don’t wander far from home. Mutations that occur in the east just keep piling up, as do all the others all the way to the west, but the extreme eastern ones and the extreme western ones are different.
So, eventually, the western squirrels are larger, mate only in the dry season, have black tails, etc. The eastern squirrels are smaller, have grey tails, but black ears, mate only in the fall and gestate during hiberation.
Because of the size difference they cannot mate successfully. New species