So, I assume that if you put 100 people on a spaceship and sent them to wherever, they’d get very inbred in a few generations. How many people would you need for this to not happen, accounting for the fact that there will eventually be people who are infertile or die before having children?
Of course once they arrive and population expands we can expect random mutations to build up over the next 100k years or so. If you can last that long.
And presuming a species that built a generation ship doesn’t have the ability to handle these environmental changes (either through “fixing” the environment or the genes). And there are two migrations here. I’m not seeing much targeting the earth to generation ship migration directly here. But, they’d all die in free space so the ship is an environmental “fix” and they may need a genetic “fix” to handle things like long term exposure to lower gravity, or some quirks of centripetal gravity.