OK. I don't get it.
As far as I can tell, the "daughter species" gets isolated geographically from the "parent species." Over time, members of the daughter species mutate. Eventually the cumulative effect of the variations makes it impossible for members of the daughter species to mate with members of the parent species.
So far so good?
My problem is, somewhere along the line one member of the daughter species mutated enough to become reproductively isolated from the parent species. But at the same time that creature must necessarily be reproductively isolated from the other members of the daughter species, unless an opposite sex member of the daughter species mutated comparably simultaneously.