I think the atheist experiment of the last century or so is starting to yield clear evidence of what is and isn't "fit" in the Darwinian sense. Exactly as you are saying.
Atheists have existed for basically forever. I think you're referring to the 'not murdering, ostracizing and repressing them' experiment. If, like some in this thread, you think that violence, ignorance and removal of freedoms are acceptable means to getting an outcome some find preferable (or "fit", or "natural"), please state so directly.
