Surely once caste is established intelligence can't be selected for any more strongly than under any other conditions - the rules of the game are fixed at that

Surely once caste is established intelligence can't be selected for any more strongly than under any other conditions - the rules of the game are fixed at that point. Your final point about selection via coercion holds more water, but one has to believe that the subjects would be explicitly chosen for their intelligence, rather than for any other features, which seems difficult to support.