Humans are special creatures, for we are the only organisms on the planet with the increasing capability to take charge of our own evolution.
Will the great undertaking of harnessing our own evolutionary destiny be our undoing? That is the real question.
"I don't want to be a product of my environment. I want my environment to be a product of me." - The Departed (2006)
I would say both and the percentage of each depends on the person and their life style. Then again at what stage would evolution be called devolving?
If someone does not have to work at survival then not only would their skills be lacking but their bodies would also change over time. What interests me is how many generations of couch potatoes would it take to change at an inherent physical level?
So in my opinion fully adapting to today's western world is a step away from actual survival and more like being pet animals. Back when all the syndromes were popping up I added one I called “electric blanket syndrome”. When a person relies on an electric blanket too much the body does not need to warm it's self, so over time it stops. So if a person becomes so reliant on an outside source that even their body forgets how to work with out it then this is devolving.
I think humans have an instinctual, cognitive will to survive, but some may lose this instinct if they have adapted to an environment that doesn't cultivate that drive for self-preservation (think of someone on life support). One can survive but not evolve, yet to evolve is oftentimes essential for a species' survival.
I kind of agree with Sonofject but see the life support analogy as wasting away or to devolve.