So, we’ve always known that some things that are pretty distinct from each other can interbreed and produce viable offspring occasionally, but they don’t do it commonly, so I don’t think anyone has trouble calling those different species.

So basically what happened was, people proposed alternative definitions. Ernst Mayr, for example, he was one of the main architects of this modern synthesis, and probably the one who wrote the most about species and species concepts. And for him, they had to be intrinsically reproductively isolated. Once the lineages reached that point where they c