Just a curiosity that it seems to be absent in human evolution. I don't think we would consider the various races to be adaptive radiation. Or would we? http://youtu.be/IooAS_DQzGo
I think it depends on how adaptive radiation is defined. Animals that radiate by adapting to new environments do so by diverging as different species. Humans are all the same species but have still adapted to new environments. There are some genetic differences, Eskimos survive on food sources that Europeans wouldn't--whale and seal blubber. Most Europeans can digest milk and most Asians can't. Humans have therefore "radiated" through adaptation, but this hasn't produced different species.
The show I was watching used cats as an example--there are so many various animals that fall into the same genus. I think you have an interesting point though.