I think the definition of “species” is not whether they can interbreed, or even if the offspring is viable, but whether the offspring can sustain it’s half and half traits over many generations. There are lots of instances of species interbreeding and producing fertile hybrids, but the hybrids only exhibit the “half and half” appearance for one generation. After that first generation they start to gradually assume the appearance of one or the other. It’s almost like a dominant-recessive thing going on. One litter from the mating of two hybrids will have individuals that appear to be all one species, or all the other species, and very few of the individuals will exhibit the hybrid appearance. The next generations of these “hybrids” that don’t look like hybrids anymore have even greater degree of separation. And then suddenly the individuals begin to exhibit a preference for littermates of “like” attributes.
There isn’t a spectrum of genetic attributes. THere is a definite clumping of grouping of two distinct types.