[ncmncm]

"Species" is an organizational convenience for biologists. Nature doesn't have such a boundary. It just has varying degrees of reproductive compatibility, inclination, and opportunity.

"Subspecies" is a concession to what lumpers call splitters.

[derefr]

There is certainly a conservative definition for speciation, though: the point where something has zero reproductive compatibility—where there is no known example of viable offspring. At that point, inclination and opportunity cease to matter.

Why not just define “species” by that clear formal boundary, and then call everything that doesn’t manage to reach that line “subspecies”?

[ncmncm]

Because the line is very hard to discern, where it exists as a line at all, and it is nowhere sharp. Lions can be bred with tigers, in captivity. Are their offspring fertile? Well, sorta. Does it make sense to call lions and tigers subspecies? Hell, no. Say lions and tigers are one species and biologists will call you a lumper. You don't want that.

Sometimes the product of mating between species becomes, instantly, another species, if they prefer mating with one another over either progenitor. That just happened, with some birds, in the Galapagos.

Legally, there are no endangered subspecies, only endangered species. So, claiming some variety is "just a subspecies" may mean they get no legal protection against extermination. To me that's more than enough reason for a species.