How does one develop a new scientific theory?

On the bahamian island of andros, mosquitofish populations live in isolated freshwater ponds that were once united. currently, some predator-rich ponds have mosquitofish that can swim in short, fast bursts; 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(s) of selection should subsequently be most expected among the mosquitofish in the original predator-rich pond, and what type(s) should be most expected among the mosquitofish in the original predator-poor pond?