But didn't those race *have* such a single, shared biologically-based culture, prior to initial alien contact?

 

I will grant you that it *appears* as though RL 21st century humans does not have such a culture. But in the context of this discussion, they actually pretty much *do.* While there certainly exist isolated sub-cultures which remain largely cut off from the mainstream of global society, these are few, steadily becoming fewer and are of little consequence in pragmatic terms.

 

Mind you, I'm *not* saying that the cultures of (for example) the Middle America and Central China match up, point to point to point. But they definitely share more in common with each other than either would share with an actual, biologically alien culture.

 

What I do not see but what you seem to is that - by some unexplained mechanism - these various distinct biological cultures will somehow both merge into one common culture and then split off into distinct regional cultures.

 

I simply don't see that occuring. Instead, when face with actual alien cultures, I see each original biological culture turning inward upon itself, with the population emphasizing its common uniqueness (for want of a better term). This is what happened with the American Indians. It is what is happening now in the Middle East. I see no reason to suppose that will change in the future or that it will occur markedly differently in the case of actual aliens.

 

While this may sound like a human-centric viewpoint (e.g. assuming aliens will react like humans), I don't think this is the actual case. It is simply logic. Races which are pre-disposed to give up there own uniqueness aren't the type of race that will rise to the top of a galactic society. All other factors being equal, over time the races with the strongest self-identities will be the ones to take and hold power. So in the case of intergalactic states with populations formed largely of a single race, you *are* likely to get a single common culture. Or - more accurately and more importantly - a culture which is percieved to be common by the members of the race themselves.

 

I do grant that mono-racial-culture intergalactic states will *tend* to be relatively small, in galactic terms. This is because above a certain size, such a culture's uniqueness will no longer be constantly reinforced by the fact that it's surrounded by opposing cultures of the same size or larger. A large mono-racial-culture state has the opportunity to accept change and diversity, confident in the belief that doing so will no longer endanger it. But it is only an opportunity, not a requirement. And to experience it, each race must first attain and hold significant power over a large area; I don't believe in a spontaneously-arising, unified realm composed of equal-power sub-cultures. That's Ancient Greece, redux.

 

IMTU, the Third Imperium is a multi-cultural state. While there is an overall "Imperial" culture - an amalgam of elements from Solomani, Sylean and Vilani culture that evolved on Sylea over the course of the Long Night - this is not imposed beyond the required acceptance of the Imperial nobility. Even then, my Empire has no problem enobling local potentates or even local elective offices (as opposed to individuals or families) . . . whatever it takes to keep the taxes rolling in. 

 

As has been pointed out by Phil and Jeff, the Vargr and the Aslan are *not* biologically unified, due to their alien biology: the former has marked trouble acknowledging distant leaders, while the latter has a biological imperative to acquire new territory that is far beyond actual need. Actually, I see only three large mono-cultural states in canon and all of them are special cases: the Zhodani (who attain cultural rigidity through "magic" means), the Hive Federation (where the ruling race has spent centuries manipulating themselves and all other races into a common culture) and the Thousand Worlds (where the biology of the ruling race strongly reinforces a requirement for cultural consensus and rigidity).

 

All the other mono-racial-culture realms are cases of minor races expanding off their homeworlds until they run up against their neighbors. This puts them smack into the "Ancient Greece" model, so I see no problem with them remaining culturally unified inside their own area of control and also distinct from outside cultures.