The canon has explicitly indicated they ARE a different species...
When has it ever done that? I just searched Chakoteya's transcript site for episodes that included the words "Romulan," "Vulcan," and "species," and I couldn't find a single definitive statement that the two are biologically distinct species. The closest thing was Troi's line in "
The Neutral Zone": "They are related to the Vulcans, but as each race developed, their differences grew wider." But she said "race," not "species," and "race" can mean an ethnic or phenotypic subgroup within a species. And Troi's statement is in the context of a discussion about the Romulans' behavior and mentality, implying that she was speaking of differences in their culture and value systems as opposed to their biology.
Of course, I could point out that the definition of distinct species is two populations that can't produce fertile offspring together, but by that definition, pretty much
all Trek humanoids would be the same species. So it's hard to define what the word "species" even means in the Trek universe.
I would normally agree but All Our Yesterdays seems to say otherwise and that it's at least partially nature, with mostly of nurture "tuning."
What happened to Spock in AOY is pretty much nonsense as presented. Why would just going back in time cause his behavior to change? Riker didn't suddenly become racist when he went back in time to 1890s San Francisco. There's no sense or sanity to the notion that a time-traveling individual's personality would be magically transformed to match what most of his people were like at the particular time he's visiting. So there has to be another, less stupid explanation. There's a remote chance that Spock was somehow being telepathically influenced by the collective consciousness of the Vulcans of the era, their own savagery overriding his control, but that seems unlikely given the distance involved. Perhaps the atavachron simply altered his neurochemistry in a way that undermined his control and his intellect, and he and McCoy jumped to the wrong conclusion about the reason for it because Spock wasn't thinking clearly.