Terry,
I'll offer another intelligent but somewhat uninformed opinion:
Imagine a chimp that thru some happy mutation is slightly more intelligent than his peers. He has the ability to reason out ways to get to food that others can't. So he's able to survive, and produce offspring. Among his peers' children, his will likely prosper, since they can always find food that others must leave behind. As the population grows, it splits geographically, with the more intelligent chimps migrating towards the food they don't have to compete for. But now they are all on an equal footing, they ALL have the intelligence to get the more-complicated-to-obtain food. The most intelligent among the group now has the best possibility for survival/reproduction.
Each migration (or other form of environmental split, including extinction of less-intelligent sections of the population) leads to a further refining of this characteristic of intelligence. Meanwhile, their unintelligent peers are still enjoying their easy to obtain food and are instead evolving greater strength or climbing ability. A gulf is growing between the original population and the one that split off.
I could carry this out, but you see the idea. Each step along the way allows for another level of increased survival/reproduction in comparison to the other members of the population in the environment. As all members of the population begin to have the advantage, it isn't an advantage anymore. As the environment changes, the group naturally decides to go where their skills allow them to survive most easily. The gulfs grow and grow so long as there is an advantage to be had in doing so.
Does all of this address your question, or am I just singing the same old song and not really breaking out of the "it's true because it's true and if it were false then it wouldn't be, but it is" circle?
Dave