Given that mutations happen, the change need not be a change in the environment. Any change in an individual that renders that individual more fit for the niche will lead to increased offspring for that individual, ergo change in the population.
How is what you write any different than what I have been stating over these many posts? When you use the words "more fit" you use them in relation to a point of stability. That drives them closer to the stable point not farther away. Now the consequence of having a stable point means that it has to be described in some way. Without a limiting factor, for instance, a longer jaw benefit will drive everything towards ever larger jaws. If the "best" jaw length is 1 meter, then everything will be driven towards 1 meter from every direction.