abhi398 had the right idea, but he expressed it in unconventional language so later commenters seemed to have misunderstood his point. I'll clarify.
The original question was, "How could
...extinction may have been caused... be correct, since
extinction is singular and
have is plural?"
The answer is that the so-called modal verbs used to express past possibility and probability sometimes take different forms than do other verbs. For instance, these are all correct,
He could have answered my text.
He must have decided to ignore it.
He might have fallen down a well.
We always say could/must/might have and we never say could/must/might has, whatever the subject.
Similarly, these sentences are correct, even though they are about counterfactuals rather than about probability or possibility,
He would have answered if he could.
He should have been more careful.
I have heard such sentence moods characterized as subjunctives, perhaps by analogy with a more robust set of subjunctives in other languages, but most people use the word subjunctive in English to mean either hypothetical subjunctive or command subjunctive. I'm not going to go into those here, except to say that you can learn about them in the
MGMAT SC guide, that they're not a big deal on the GMAT, and that daagh obviously took abhi398 to mean the command subjunctive.