Does natural selection act primarily on individual organisms, on groups, on genes, or on whole species? The question of levels of selection - on which biologists and philosophers have long disagreed - is central to evolutionary theory and to the philosophy of biology. Samir Okasha's comprehensive analysis gives a clear account of the philosophical issues at stake in the current debate. "The current volume provides an exceptionally lucid and analyitically rigorous review of the main conceptual challenges facing biologists and philosophers who have engaged in this work."--Mark E. Borrello, The Quarterly Review of Biology "Major contribution toward putting this controversial area on a coherent conceptual and philosophical footing. ... I can't imagine anyone working on multilevel selection-or attempting to dismiss it-without reading this book."-- Science "Every philosopher of biology interested in aspects of the levels of selection debates ought to confront this material, and should think seriously about how the positions he or she ahs staked out fits into the frameworks Okasha outlines. Okasha has written an extremely important book."--Jonathan Michael Kaplan, Notre Dame Philosophical Reivews This will be the key reference-point for future work on this crucial topic Samir Okasha is Professor of Philosophy at Bristol University. Before that he taught at the University of York for 3 years, and was a Jacobsen Research Fellow at the London School of Economics for 2 years. He was a Postdoctoral Research Fellow at the National Univeristy of Mexico for 1 year and received his doctorate in 1998 from the University of Oxford.