Paul E. Ehrlich received his doctorate in biology from the University of Kansas. He is a co-founder of the field of co-evolution, and has carried out long-term studies on the structure, dynamics, and genetics of butterfly populations. Gerardo Ceballos González is a researcher at the Institute of Ecology at UNAM. His research focuses on issues of ecology and conservation, as well as macroecology, biogeography, population and community ecology, and species and ecosystem management. Anne H. Ehrlich is the co-author of several technical papers on population biology. She has also written extensively on public interest issues such as population control, environmental protection, and the environmental consequences of nuclear war, as well as co-authoring ten books. She is currently a board member of the Pacific Institute for Environmental and the New-Land Foundation.