A genre responds to an audience.
Writers don't start genres. Audiences do.
A genre is born as a response to an audience's questions, needs, wishes, fantasies.
A niche audience starts the conversation by asking a certain kind of question over and over in the same general context. For instance, newspaper editors tend to ask, "What's the news item here? When can I release this? Who can I call for more info?"
In response, publicists came up with the generic press release, a fairly standard approach to answering those questions in a methodical way, with a heading, release date, and contact phone number. Through the ongoing virtual conversation between editors and flacks, a peculiar type of text developed--a genre.
A genre starts with thousands of texts created by individual writers in response to real or imagined requests from a group, attempting to resolve some of their anticipated problems, to address their needs, or to appeal to some of their passions.
As more people write the same type of text, the audience and the writers begin to recognize a pattern emerging--a new genre.
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Writing in a genre (Full chapter from Hot Text, in PDF, 770K, or about 13 minutes at 56K)