Although Darwinian selection driving the genetic diversity within tumors has been widely assumed by the field of cancer biology,recent studies that employ the rigorous tests of population genetics have increasingly suggested otherwise.In these new studies [1-5],the non-Darwinian,or neutral,model of evolution can adequately explain the documented diversities in tumors without invoking natural selection.Therefore,the half-century-old debate of selectionism vs.neutralism [6-8] on the role of natural selection in populations is being replayed at the cellular level.The debate will have clinical relevance because the level of genetic diversity in tumors depends strongly on whether and how natural selection operates [1-5].