A gateway substance is something that may be menial but could lead to something bigger, an example is weed in the drugs world, weed is not hard to get and is usually a gateway drug to stronger types of drugs and potentially more dangerous
Species selection. Species selection is the process by which intrinsic biological characteristics in organisms change depending on the type of environment they're interacting with. In this case, the subsequent generations of the fly lost the ability to fly because their environment didn't require them to use wings for survival, hence they stopped growing them and subsequently lost the ability to fly.
