I haven't read your book yet, so forgive me if you discuss this there. But I’ve been wondering:

Simple traits (such as an organism's height) are probably relatively easy to alter via genetic mutations, without needing to combine many different genes chosen from huge populations. So, e.g., dog breeding altered dogs’ size relatively easily.

Complex adaptations aren’t nearly so easy to come by.

If intelligence is a conceptually simple thing, there might be simple mutations that create “more intelligence” -- it might be possible to make smarter people/mice/etc. by tuning a setting on an adaptation we already have. (E.g., “make more brain cells”).

If intelligence is instead something that requires many information-theoretic bits to specify, e.g. because “intelligence” is a matter of fit between an organism’s biases and the details of its environment, it shouldn’t be easy to create much more intelligence from a single mutation. (Just as if the target was a long arbitrary string in binary, and the genetic code specified that string digit by digit, simple mutations would increase fit by at most one digit.)

From the manner in which modern human intelligence evolved, what’s your guess at how simple human (or animal) intelligence is?