Well, other bacteria have more room to breed, and thus natural selection has more to work with. The mitochondria in my cells get worse off over time - in fact, mitochondrial aging is implicated in actual aging. There's only room for a couple hundred mitochondria in a cell, even in a gamete. So there's only room for a few hundred to undergo natural selection to maintain their health. Since the female gametes degrade over time, why aren't the degredations cummulative? You'll note that the Bible has a good answer to the question (mitos were better then, and they are degrading). Gradual mt degredation fits the bible theory. Yeah, it's rare. But I included the information up front, so that I didn't look like an idiot.