During mutation how is it decided that a function is used rather than a terminal (here again is the question of selecting a symbol based on weightings of functions and non-weighting of terminals)?
During mutation, the same rules that are used to create the initial population apply, that is, when the number of functions in the function set is smaller than the number of terminals, for each mutation point in the heads, the probability of it mutating into a function is 1/2. When functions outnumber terminals though, all elements (functions and terminals) are equally weighted.