But mutation isn't part of that definition. Evolution isn't happening if there's no mutation. Otherwise the ability to interbreed would never go away no matter how much evolution happens.

Evolution can happen without mutation. Evolution can happen within species. Species is a pool of genes, and changing environment can change the selection of those genes. Things like sexual selection or distance can make separate species.

Think about dogs or other domesticated animals. They have the same genes as wild ones but they have been selected. Mutation happens, like the cat orange coat, but many are original genes. The domesticated foxes someone mentioned are interesting cause of how fast the breeding and how features similar to dog appeared.