What do you guys think? How did Romulans lose their psychic abilities? And are Romulans and Vulcans separate species?
I dunno about speciation since the usual biological definition means "cannot interbreed to produce fertile offspring." We don't know about the Human-Vulcan hybrid Spock since he didn't marry or have any known children. However, this definition would indicate that Humans and Klingons aren't a separate species (as we now define the term) because K'Ehleyr, the half-Human half-Klingon mother of Alexander Rozhenko, was clearly fertile. The same goes for B'ellana Torres and her daughter Miral Paris; neither could have been anyone's mother if not. Vulcans and Romulans descended from a common ancestor so they would presumably be closer physiologically than any Human-Klingon pairing. So...let's just back away from that whole topic and wave it away with space magic time travel whatever blah. :P As for the loss of telepathy, let's try to retcon it.
During the early alliance with the Klingons, the Romulan Star Empire got its hands on not only Klingon ships, but it had the Tal Shiar extract Klingon research as well. This included data on the Augment virus, which the Tal Shiar immediately began testing. Due to the differences in physiology, the virus created cranial ridges in Romulans where it destroyed them in Klingons, and the virus' genetic damage caused only minor behavioral differences but did erase the limited telepathy that Romulans had possessed. Unfortunately, the Tal Shiar's quarantine protocols were inadequate and the virus swept through the population, being much more aggressively contagious than it had been for the Klingons. This pandemic caused the Romulans in 2311 to retreat into isolation behind the Neutral Zone until encountering Captain Picard and the USS Enterprise-D on Stardate 41986.0, when their now-permanent appearance change became known to the outside for the first time.
Or not. But hey, it's as plausible as any of the other retconning that happens in Trek...