"FF, I'm not sure we can accurately say A <=> B and B <=> C if there is even a slight mutation. I think "closely related to" is more accurate than "is the same as," na?"

You might be right, except the relation is not "is the same as," but "is the same species as." If A and B might naturally produce viable offspring, then they are the same species (but, again, the notion of species isn't perfect, imo).

k

p.s. I consider this a word post. It's about the meaning of the word 'species'
and possible limitations. The word was developed before the theory of evolution by a man who believed largely in fixed, discretized populations.