I would define the Theory of Evolution as, "the explanation of how evolution works."
First, it is THE explanation. There is no other.
Second, even creationists agree that evolution DOES work, so there's no need to argue that. They just have an unsupported claim that there's an unevidenced limit to how far evolution can go.
Third, note that there is no mention of how many original ancestors, nor any need to mention it. The explanation works whether there is one or forty.