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TAMPA, Fla. Evolution within groups of tumor cells follows the principles of natural selection, as evolution in pathogenic microbes. That is, the diversity of cellular characteristics within a group leads to differences in the ability of cells to survive and divide, which leads to selection for cells that bear characteristics that are most fit to the malignant environment. The ability to continuously create a diverse set of new cellular features enables cancers to develop the ability to grow in new tissue environments and to acquire resistance to anti-cancer drugs. The diversity of cell characteristics within groups of cancer cells can be created by a number of well-characterized mechanisms, including small-scale mutations, large-scale genomic changes involving losses, gains and reshuffling of large pieces of DNA, as well as by nongenetic mechanisms that create lasting changes in cellular features. At the same time, scientists generally believe that cancers lack a powerfu