but no proof of that happening has ever been found?
Of course it has, it's universal, and we can see the results in any zoo.
For example, if you put Indian and African elephants together, they not only won't breed, they make each other sick.
Another example is zebras -- over a dozen breeds & sub-species which eagerly interbreed, within three species that don't normally interbreed with each other in two genera which not only don't, but can't be forced to interbreed successfully.
That's what evolution-speciation is all about.