No, what you need is a population that has slowly changed characteristics over time. From one generation to the next, the changes will be small enough that reproduction is not an issue, but over a thousand generations (minimal), large enough changes can happen that a new species appears.<quoted text>
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.
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?
As for how sexuality started, you do realize there are many species with individuals that are *both* male and female, right? And species where the gender of an individual can change over a lifetime (sometimes several times). And species where sexual reproduction isn't the main form of reproduction, but it exists as an alternative?