A specific population can't be sustained by just a single pair. That much is true. But I'm not interested in sustaining. What interests me is how the process starts.Moreover, population studies are very clear: you cannot sustain a population with just a single initial breeding pair. The population would have too many fatal defects.
This is all basic biology here.
It would seem that you would need two creatures that have reached sexual maturity at the same chronological point in time, in the same location, that they would successfully mate and produce offspring that could also repeat this process. That's what interests me. Where or how do these two biological creatures come into contact at the right place, at the right time, with the optimum health needed to reproduce and carry on?