From the Gamespot preview of Dark Avatar: "One of the surprises in the game will be to find out which of the existing races is going to be wiped out and will not be in a future sequel (they'll still be in GalCiv II and its expansions, though, as a playable race)."
Unless there's been an update I missed, or I completely misread this, they are referring to only a single race being annihilated. A lot of people are talking about "what two races" will he destroyed, but last I checked it was only one.
If anyone can prove I'm wrong and two races really are being destroyed, please do so with a preview link or news story; something like that.