Look into a tank of male guppies and something becomes immediately obvious: almost no two fish look alike. Orange spots, black bars, iridescent flecks of blue — the variation is extraordinary, and generation after generation it refuses to collapse into a single winning style. A peer-reviewed review published June 11 in Philosophical Transactions of the Royal Society B now presents the most comprehensive case yet for why that is so — and the answer may reach far beyond one tropical fish.