Primarily a biological question, but I'd also love to dig into social implications.
Here's the story -
Humans have colonized at least one other planet and trade / communication remained open for some time before a massive and catastrophic event took place that effectively isolated the planets completely. Note: haven't nailed down an exact cause yet, though I'm leaning towards hostile alien occupation that cripples the systems and regresses technological advancement to pre-historic times, or simultaneous world wars that lead to a similar outcome.
Here are the questions -
(1) How many generations would the isolated populations have to go through before their genomes were incompatible for reproduction? Is there any evidence for a timeline of separation of species like this? What kind of time frame am I looking at? I'm thinking Darwin's island birds, though I don't know of any generation estimates. Clearly something that can happen within a relatively short time-span (millions of years or less).
(2) Assuming both populations were able to recover and re-advance technologically, what would the social implications be once "first" contact was made? If one system was able to win the space race in a land-slide, would they look down on the other as inferior (animals / barbarians) and be left alone as some sort of nature preserve? What if advancement was at a similar pace and apparent evolution was minimal, with the exception of the inability to procreate?