It's not easy to define species following a common basic definition that applies to all taxonomy ranks, specially when asexual reproduction takes place.
But this is not the case being discussed here.
With higher mammals that is not the case, and the definition of species is already well established: sexual reproduction results in fertile offspring.
Riddle me this, you have 2 populations containing 2 sub-populations each. Lets call them populations Aa and Bb. 'a' can mate with 'b' and produce c. c can produce viable offspring with A or B but A and B can not produce viable offspring. What do we call Aa and Bb if not distinct species? What do we call c?
What if some members A can produce viable offspring with B some of the time but not always? What then? My point is we make taxonomical distinctions for our convenience, they don't really have any hold on the natural world.
By this definition, are a chihuahua and a great dane the same species? With artificial insemination they would certainly produce fertile offspring, but they could almost certainly not do this naturally.
That seems to kick the issue down the road. It defines 'species' clearly, but it doesn't make it clear how to assess 'fertile reproduction', or why it should be the criterion.
Unfortunately we don’t have a better working definition.