There is also the concept of ring species. And similarly, dog breeds. While Saint Bernards and chihuahuas aren’t likely to directly breed with each other and produce Saint Bewawas, St.Bs can breed with slightly smaller dogs and chihuahuas can breed with slightly larger dogs, and those viable offspring can breed with similar sized dogs and so on, so theoretically in a mixed population of dog breeds there can be gene flow between Saint Bernards and chihuahuas and both breeds still count as being the same species. But if you eliminated every dog breed except for St.Bs and chihuahuas while making no change to St.Bs and chihuahuas themselves, suddenly it becomes completely reasonable to call them separate species because they would be likely to remain reproductively isolated from each other in nature.