HINT 22K: You can assume that changes are slow, so that the effects of gene flow and selection can be added together. It is most convenient to write down a pair of differential equations for *dp*_{A}/*dt*, *dp*_{B}/*dt* (Box 28.2), but you can avoid calculus by working with the changes across discrete generations, Δ*p*_{A}, Δ*p*_{B}, if you prefer.