On the Bahamian island of Andros, mosquitofish populations live in various, now-isolated, freshwater ponds that were once united. Currently, some predator-rich ponds have mosquitofish that can swim in short, fast bursts; other predator-poor ponds have mosquitofish that can swim continuously for a long time. When placed together in the same body of water, the two kinds of female mosquitofish exhibit exclusive breeding preferences. If one builds a canal linking a predator-rich pond to a predator-poor pond, then what type of selection should subsequently be most expected among the mosquitofish in the original predator-rich pond, and what type should be most expected among the mosquitofish in the formerly predator-poor pond?

When natural selection favors one end of a certain phenotypic range, selection is called directional. In this case, the frequency curve is shifted towards the most advantageous phenotype. An example of this mode of natural selection can be seen in the question above if someone built a channel that would link a predator-rich lake to a poor predator lake. This channel would favor predators, but directional selection would be less intense among mosquitoes in the original predator-rich lake; while directional selection would be more intense among mosquitoes in the old predator poor lake.