1 Department of Biology, University College London, 4 Stephenson Way, London NW1 2HE, U.K., Department of Genetics, Downing Street, Cambridge CB2 3EH, U.K., 2 ‡Institute for Evolutionary and Ecological Sciences, Rijksuniversiteit Leiden, PO Box 9516 Leiden, NL-2300 RA, The Netherlands...