A single gene can turn a docile fly into a fighter.

Ralph Greenspan and Herman Dierick of The Neurosciences Institute in San Diego, California, bred fruit flies to be aggressive. After 21 generations, they were 30 times more aggressive than controls, as measured by an index of fighting ability. Not only did they start more fights, and ones that lasted longer, they were also fiercer, wrestling and flipping opponents rather than just chasing and hitting them.

This cannot result it anything good based on evidence from past works of fiction (X-men, etc.) and computer games (Doom, Quake *).