How does the process of evolution through mutations compare to writing drafts of a book and why is this a useful analogy?

I assume that many people start to write a book with a good idea of the start, the amorphous middle with a few ideas and a vague ending. I have done this myself. The start is easy; you know it and nothing really interferes with it. Then you get deeper into the book. your characters become more (or less) than you expected. They are cleverer, stranger, more difficult. The book begins to change, evolve.

You have lost control. In an effort to regain it you accept that all has changed, you rewrite, you evolve once more and use the new adapted characters, and are now in the unholy depths of the middle. The focal point where your cool start has to meet your imagined end. Each and every evolution of your tale is what leads to the end. Planners have it all set out before them. Evolution is a great way to explain this process. I suppose each of my books have evolved. I hope that any writers reading this also metamorphose into wonderful tales.