Artists assemble! How collectives took over the art world

They’re principled, they’re powerful and they make the art world jumpy. As the Turner prize is split four ways, we look at how collectives are shaking things upLast week, the four artists shortlisted for the Turner prize turned themselves into a four-strong collective in order to win as a group – a move that caused controversy in the press, consternation over social media and a bust-up on Radio 4 between the Guardian’s art critic Adrian Searle and his Sunday Times counter