1. Theoretical Division, T-8, Mail Stop B285, Los Alamos National Laboratory, Los Alamos, NM 87545, and The Santa Fe Institute, 1399 Hyde Park Road, Santa Fe, NM 87501, USA.
2. Department of Biology, University of New Mexico, Albuquerque, NM 87131, and The Santa Fe Institute, 1399 Hyde Park Road, Santa Fe, NM 87501, USA.