If we don't achieve a technological singularity in the next couple decades, I doubt that an alien invasion will be a worry a couple centuries from now. I say this because of impending ecological collapse, which will end human civilization as we know it... if not humanity in total.
And I'll save for another time a rant about how expensive it is for anything to cross interstellar distances. At most we'll see a self-replicating probe that tries to subsume all matter in the solar system, the "invaders" won't physically travel here.
And even as software, I predict that new colonists would likely arrive by communications laser, well after the local infrastructure has been built. Why bother coming over during the boring building phase?
lol, that's a great line.
I'm thinking more along the lines of attenuating certain (hypothetical) genetic contributors to antisocial personality disorder, violent psychosis, etc.
Not engineering docile perfection, just less of the obviously really bad stuff.
Edit: I do share your concerns about stopping Darwinian selection, though. Choosing local optima for cooperation might come back to bite us in the coming TransNeptunian War of 2217, e.g., when psychopathic, ruthless strategy is all that saves the human race from the conflict of first alien contact.