Oh, I think we've seen *evidence*, but not enough to establish *truth*. And that's perfectly fine. Wiggle room that a campaign can make use of without compromising the published baseline is an ideal state, IMO.
so the Vargr can't adapt, the Aslan can't adapt, the Hivers can't adapt - only the racially superior Humans can adapt.
The Vargr can and do. The evidence is all over the map.
The Aslan can and do, as documented in every version of them (except possibly Mongoose, haven't slogged through that one recently). The main body of the Aslan are portrayed as not liking that adaption AT ALL, but most of the time they put up with it if it occurs where they don't have to see it.
Hivers are weird. They engineer and maintain genetic homogeneity, and *expect* that to produce cultural homogeneity. While we value diversity internally, the Hivers see internal diversity as dangerous but have less issue with external diversity. Their polity is probably second only to Imperial space in terms of being species diverse. They adapt differently than we do, but they do adapt.
The K'kree are *violently* monocultural and more successful at it than even the Aslan. At least, that's what they want outsiders and their own citizens to think. I'm not convinced.
Only Terrans and the Vargr undertook spaceflight and empire building from a divided position. The other Major Races were all culturally unified when they developed Jump Drive and expanded. The Zhodani had been through a significant population bottleneck worse than the Black Plague in recent and recorded history when they attained space the second time, the Aslan had just backed away from an extinction level nuclear conflict, and the K'kree had just unified in what they thought was the face of extinction. They have reasons for being monocultural, while a thousand years of war and another 1800 years of diaspora and isolation has led the Vilani and Terrans to become localized. NONE of the other major races were affected by the Long Night that way because they didn't have the same Long Night, or one at all.
So while Terrans/Solomani and the Vargr are recognized as mavericks sociologically, that isn't necessarily seen as an advantage by the others. We're the *weird* ones.