The authors want to thank the following individuals for comments on an earlier version of this manuscript: Leo Buss, Harald Freund, Terry Jones, and Sean Rice. GPW is has grateful for numerous discussions on the ideas motivating this study: Leo Buss, Jim Crutchfield, Terry Jones, Junhyong Kim, Manfred Laubichler, Sean Rice, Peter Stadler. The financial support by NSF grant BIR-9400642 and the Yale Institute for Biospheric Studies is gratefully acknowledged. This is contribution # 33 of the Yale Center for Computational Ecology.