As I noted before, these are just man-made definitions such as the definition of "citizen" or "adult". What's an "adult"? An 18 year old or a 21 year old?
A subspecies develops when two populations have been isolated from each other for so long that their characteristics begin to drift apart to the point that they are no longer genetically compatible to produce fertile offspring.
If the two populations of subspecies ever get back into the same geographic area and start mating regularly and producing fertile offspring, then they will revert to a common form with little difference between each other after many generations.
Since a fertile mule is a rare occurance, biologists have agreed to classify donkeys and horses as different species.
If fertile mules were to develop, they would form their own species and would share the classification of "species" with any others that they could consistently produce fertile offspring with.
A subspecies develops when two populations have been isolated from each other for so long that their characteristics begin to drift apart to the point that they are no longer genetically compatible to produce fertile offspring.
That should read "Species" and not "Subspecies".
Subspecies look different but their genetic characteristics are such that they can breed and produce fertile offspring on a regular basis.
Here is an artificial example...... if Collies and Beagles were naturally found in the wild on two different islands, biologists would classify them as subspecies as they look quite different but the biologists would be classify them as the same species as they can produce fertile offspring on a regular basis.