A ring species is an example of a species complex, and subpopulations with different but not mutually exclusive breeding compatibility are known as microspecies. If populations with distinct morphological features are isolated in space or time but could still interbreed with eachother if they were to be brought into contact, they are subspecies.

This is really just an instance of Sorites paradox. But the inability to draw a precise line between two (or more) states does not mean there aren't two (or more) states.