Evolution is possible for oral stories, not replicable with the static written word. A storyteller adjusts his telling based on audience feedback. As they are transmitted, they mutate within the minds of those who will later pass them on, making their alterations. Over generations, the stories grow more engaging and meaningful - people stop telling those that lose relevance. The best that we have managed to achieve in print is with an editor making suggestions until they publish the final draft in perpetuity. In non-fiction, an author will occasionally update a book with a new revision. However, I know of no cases where a fictional story has been revised based on improvements the author wanted to make. There is an expectancy of immutability in media more generally - I would point out the changes that George Lucas made to the Star Wars movies as an example of people’s reluctance to have their stories altered.