Phenotypic Variation?

After being switched, only offspring that grow to be larger survive. This can be either observed as a repressive trait or like a mutation. If guppies produced offspring that carried this trait, they would be more likely to survive (being larger), and therefore furthering the development of their population with this larger phenotype. Basically, say you had 50 fish. They all produced as much offspring as possible before either surviving or being eaten. 25 fish are small, 25 fish are a little larger. The larger fishes offspring is more likely to outlive the smaller fishes offspring bcause it has a better chance of growing larger. Over the generations, fish will mostly produce larger offspring.