INTRODUCTION e evolutionary transition from unicellular organisms to multicellular organisms with dierentiated tissues included a transition in the level of organization at which natural selection acts (Maynard  Smith and Szathmary 1995; Buss 1987). Mutations that cause an increase in the reproduction and survival rates in unicellular organisms give those organisms an advantage over their competitors. However, mutations that increase reproduction and survival rates in somatic cells of a multicellular organism can lead to a fatal cancer. Natural selection on multicellular organisms has led to mechanisms for suppressing somatic evolution on multiple levels, including tumor suppressor genes that regulate the growth of cells and the architecture of proliferative epithelia that limits the number of cells that are vulnerable to neoplastic evolution (Cairns 2002, 1975).