If you are just taking a snapshot in time, that might just work. The problem over time, however, is that the descent, of each species so defined, is a long chai

If you are just taking a snapshot in time, that might just work. The problem over time, however, is that the descent, of each species so defined, is a long chain of breeding. There is a sorites problem in dividing it up into one species begetting another, as it is unlikely you will find offpring that could not breed with their parents' generation.