1. A. P. Dobson is in the Department of Ecology and Evolutionary Biology, Eno Hall, Princeton University, Princeton, NJ 08544–1003, USA. A. D. Bradshaw is in the School of Biological Sciences, University of Liverpool, Liverpool, L69 3BX, UK. A. J. M. Baker is in the Department of Animal and Plant Sciences, Alfred Denny Building, Sheffield University, Western Bank, Sheffield, S10 2TN, UK.