Isn't this just stating the obvious? Things evolve when there's a natural selection pressure, i.e. stress, to evolve? If variations within the species don't con

Isn't this just stating the obvious? Things evolve when there's a natural selection pressure, i.e. stress, to evolve? If variations within the species don't confer any special advantages then all the variations will continue. If the environment were to suddenly change such that only one or two of those variations would confer an actual competitive advantage then the species will rapidly evolve. I thought this has been known for quite some time? Am I missing something?