Does your book need to fit into a defined genre to succeed?

I’m throwing this out as a question, as I’m not sure of the answer yet myself. As all those who’ve published on Amazon or Smashwords or Apple ibooks will tell you, the category in which your book is placed is central to whether or not your target readership will be able to discover it. So, it’s very important to get it right. Having said that, it is possible to switch genres at any time, so it’s worth keeping a close eye on your sales to see if your book is shelved in the correct place.
My novels are mystery thrillers, but like all decent pieces of fiction, there is plenty other stuff going on in them too, like History, psychology, family drama and the odd dash of romance. My first novel, Aoife’s Chariot, does very well in the Scottish fiction section and is regularly in the top 100 kindle bestsellers…