>>20879772Dominant alleles are the biggest reason.
If a world can't have redheads... Everybody with a foreskin can see the issue. So we need at least some ancestral homelands untainted by immigration.
We can also evolve to become different species and races if we are kept separate, but have an impossible time doing so if we mingle and mix together.
Mixing itself is fine, just not on genocidal levels. Ideally every mixture would have its own territory, and it would just go on and on. Perhaps flying cities, spaceships etc. will allow for this.
