Suppose that humanity has spread across the universe, colonizing various habitable worlds, until their empire spans the entire galaxy (or near enough) - but there is no sign of alien life. There are worlds with water and an earth-like atmosphere, but no flora or fauna besides those the humans brought with them.
Then fast forward a few thousand years or so. Assuming inter-planetary migration is not too common, so that planets (or more likely systems) exist in relative isolation, how varied could these humans become? Living in survivable, but varied environments, is there a limit on how differently two groups of humans could evolve? Could they reach the point where they are no longer the same species?