Honestly there probably should be very few purebreds of any sort left on Federation planets.
Really? I'd say that the ability to colonize space would only keep ethnic groups together and if anything slow the intermixing of races.
When you can just up and go into space and colonize a random planet no one else is on, people are going to. Trek even supports this. You have the colony of Irish farmers. You have the colony of Native Americans(who are no longer native or in America... Indians then?), and so forth.
I wouldn't see why groups of other ethnic groups wouldn't form their own colonies too.