Intellectual Abundance

[Matt R.]

Lestat,
I think another good way to look at it is as follows:

When choosing a mate, intelligence has always been a positive quality, as it gives a higher probability of success to your offspring. Both in terms of the genes they get and the resources your mate can provide with his/her higher than normal intelligance. Even if there is no environmental selection pressuring for a higher and higher IQ, there is still sexual selection going on. When the more highly intelligent members of the species are favored, this leads to selection of their "high IQ" genes.

I think this may be more along the lines of what you were looking for. Once you get past the ability to use tools, there isn't much environmental selection going on for smarter individuals (e.g. we're not getting any reproductive advantage by formulating quantum mechanics). But, there is still sexual selection occuring for the more fit members, and intelligence is highly correlated with "fitness".