Damien G Walter in full-on chin-strokin’ literati ponder-mode (which is how I like him best): genres are the fossils left by movements.
Movements are conversations between writers, conducted through stories. During the period of movement, writers are talking to each other, exchanging ideas and generally discussing how to move the art of fiction forward. As these conversations develop, the movement develops identifiable motifs. Over time, these motifs solidify in to tropes, which become genres.
Some examples. William Gibson, Bruce Sterling et al shape a movement to reform Hard-SF, which results in the Cyberpunk genre. (And also the Steampunk genre) J.R.R Tolkien, C.S.Lewis and the other Inklings form a movement to bring mythic values back to modern stories, and some decades later the Epic Fantasy genre is the outcome. A motley crew of British and US writers have the ambition to write fantasy and horror with added literary value, and a decade later we have the squid obsessed New Weird.
It’s a workable theory. But what about po-mo genre revivalism, retrogenres, mash-ups?
Is there a movement in the other direction, where writers eat up the fossilised genres to fuel new movements?
Of course there is, because it’s easier than finding new alternatives. Fossil fuels… heh, timely metaphor. 🙂