The theory of evolution says species change. This is true, and while each individual change necessarily begins with one animal who is part of an interbreedomg population, “the change” that accumulates over time does so IN the population.
For example, the big river changed course and cut through the chimpanzee population range. Chimpanzees do not swim across rivers. The changes that started to accumulate in each separate population changed one into the bonobo chimpanzee and the other into the pan chimpanzee.
There was no “first bonobo” that couldn't find a mate, there was a population of pre-bonobo chimps that accumulated DIFFERENT changes than the pre-pan chimps.
So which chimp is going to be a “dead end”? And how is this change in DNA over time going to lead to a loss of reproductive viability in either population?
But even if these two kinds of chimpanzees are in a strict sense different species, we still don’t have scientific evidence (observation of the process) that they were once a single species.