1. Unit of Mathematical Eco-Evolutionary Biology, Laboratoire 'Ecologie, Ecole Normale Supeérieure, 46 rue 'Ulm, 75230 Paris cedex 05, France
2. Department of Ecology and Evolutionary Biology, University of Arizona, Tucson AZ 85721, USA
3. Adaptive Dynamics Network, International Institute for Applied Systems Analysis, 2361 Laxenburg, Austria
4. Dipartimento di Elettronica ed Informazion, Politecnico di Milano, 20133 Milano, Italy
5. Department of Biology, University of York, PO Box 373, York YO10 5YW, UK