Confusion now that I have 2 races....

[Windborne]

I've found that the game decides whether or not your population needs domes by whichever race has the biggest population on the world, make sure the worlds major race can live without domes, the percentages seem to stay constant because both races grow at your worlds regular rate regardless of their supposedly different gestation cycles. This lets you have the benefits of a mixed planet an the benefits of not living in domes at the same time.

[Drake]

Any population of a type that can't breathe the planet's atmosphere will make the colony domed.