1. Department of Organismal Biology and Anatomy, University of ChicagoChicago, IL 60637, USA
2. Department of Biology, Dalhousie UniversityHalifax, NS B3H 4J1 Canada
3. Committee on Evolutionary Biology, University of ChicagoIL 60637, USA
4. Department of Biology, University College LondonGower Street, London WC1E 6BT, UK