They are not separate species, they are simply incorrectly classified as separate species. All felines are of the same species, simply variations of that same species. Scientists like to play the name game so they can get their names in the books for discovering new species. You can't get your name in the books for discovering a Tiger if it is classified the same species as a Lion. Just as all dogs are the same species, merely variations thereof.
Just as bacteria are not several species, but merely variations of the same species.
This is why the Genus is always capitalized, while the species name is never capitalized. because in reality genus and species is the exact same thing.
Science has a species problem because they can't settle on just what a species is. Their entire taxonomic ranks are in disarray and complete confussion.
Species problem - Wikipedia, the free encyclopedia
Intentional I believe so no one can pin them down when it comes to evolution. Even their famous evolutionary paleontologist Jack Horner admits they have a problem with the name game.
Jack Horner: Shape-shifting dinosaurs | Video on TED.com
They have even classified the babies of the same species as different species, because once again you can't get your name in the books for discovering a species someone else already has. And a correct classification of species would show how futile is their attempts at transitional species.
But you are correct, as science currently classifies them no change is ever observed above the Genus rank, which again precludes one Genus evolving into another, a necessary step in evolution from single cell organisms to human beings. Although as it currently stands I believe the Family taxonomic rank may be more relevant to kind, being it is a stable grouping and contains all the variations capable within a kind, including species and Genus.
This would place all cats in the felidae kind, all dogs within the canidae kind, all bears within the ursidae kind. Even then they can't be consistent as have whales divided into several families, subfamilies, etc.
All because in reality they have no idea how to classify anything except by whimsy and spur of the moment. Whatever gets their names written in the books.