In that case, once everyone has the step, it ceases to be an advantage. The second category is when a step helps a creature to be competitive against some other species. The result may be an arms race . The third category is when a step just makes a creature better against raw nature. For example, a bird which flies better can travel over a larger desert.
Any useful effect is liable to involve several interacting factors.
For example, mimicry often involves changes in both color and shape. Even worse, an arms race involves two different species. So, these scenarios can involve an ensemble of factors. Any one of those factors could involve complex interactions at the genetic level. It's legitimate to think that the genotype involved is sometimes complicated.