Flying fish may have taken to the air when evolution tweaked electrical signals that control the size of their fins. This discovery suggests the existence of a previously unknown mechanism by which animals can change the relative size of specific body parts.

“How organs and tissues know when to stop growing at a certain size and stay there is a major mystery,” says Jake Daane at Northeastern University in Massachusetts. This scaling, known as allometry, is also a key driver of evolutionary change. The stunning variation …