1. Department of Ecology and Evolutionary Biology, Princeton University, Princeton, New Jersey, 08 544, USA
2. Institute of Evolutionary Biology, School of Biological Sciences, University of Edinburgh, EH9 3JT, UK
3. Centre for Immunity, Infection, and Evolution; School of Biological Sciences, University of Edinburgh, EH9 3JT, UK
4. School of Biology, University of St. Andrews, KY16 9TH, UK