Correct, and possibly not even then. The mitochondrial genome is highly conserved — functional defects would be catastrophic for an organism.

EDIT: That is, there’s enough variation for it to serve as a “molecular clock,” but there’s not a lot of tolerance for error. It would be more accurate for me to say most regions of the genome are conserved.

(Also, not a geneticist, so I’d be happy to be corrected by experts.)