It must be a "Let that be your last battlefield" thing where ridged romulans were forced to live north (or unridged romulans forced to live south?)Assuming they were referring to the northern hemisphere of Romulus, how would such radical speciation occur to demonstrate such physical characteristic changes on the same world, when they are relative newcomers? Cross-breeding with Remans, perhaps?
Further, how did Remans come to exist? Were they the original native inhabitants of these worlds and got conquered when the "Raptor Vulcans" arrived and subjugated them as second-class citizens of their new empire?