"So if these "estimated 20 billion billion billion" Pelagibacters are spread out all across the oceans, that's one huuuuuge population. Neutral mutations would have no chance at all to spread across the population, let alone even a minisculely harmful mutation. So any genetic innovation, which might survive in a small founder population long enough to develop into something positive, would never get the chance to start the experiment."
IOW it's setting itself up for extinction?
IOW it's setting itself up for extinction?
I dunno about that. But natural selection is more dominant in large populations than in small ones, and so it overwhelms the action of genetic drift.
So I think they're not setting themselves up for extinction, but they are preventing themselves from ever making any major evolutionary changes. They'll always remain nothing more than the best Pelagibacters they can possibly be.