Charles Darwin and Stanford biologist Joan Roughgarden have three things in common: a fascination with barnacles, a passionate desire to understand evolution, and a knack for controversy. While writing Evolution's Rainbow a few years ago, Roughgarden concluded that the astonishing diversity of sexual types and interactions meant that Darwin's theory of sexual selection (based on competitive males and choosy females) was not just wrong, but unfixable. She wants to replace it with social selection, in which a wide variety of social interactions, say, same-sex bonding and group membership, determine an animal's reproductive success and therefore shape bodies and behaviours. Her cooperation-based theory would, she argues, explain not just the peacock's tail, but the female spotted hyena's clitoral penis, the bonobo's use of sex for bartering and bonding, and the side-blotched lizard's five sexual types. Not surprisingly, most biologists are unwilling to jettison sexual selection. Some see her critique as a political statement reflecting her experience of metamorphosing from John to Joan in her early 50s. But many others take her ideas seriously, even if they strongly disagree. Roughgarden, a practising Christian, has now ventured into the face-off between evolution and religion. Robert Adler talked to her at her San Francisco flat.