...and, because they are capable of producing offspring with each other (fertile offspring?), arguably aren't even separate species.
I don't think that's a standard we can really apply in the Trek universe...
Well, yes, in worlds where a Cardassian can find that his Bajoran mistress can conceive and successfully bring to full term a child of mixed heritage, yes.
Still, the Aenar do seem to have been pretty closely related to the Andorians, and inasmuch as neither species seems to have reproduced with non-Aenar/Andorians that definition would still hold. I think.
(The case can be made that different and apparently readily interfertile populations like Humans, Betazoidsvery human