Linear versus branched evolution. In linear evolution, a clone evolves by accumulating sequential biologically relevant mutations. This would be assumed to create a large cancer clone of a similar phenotype—either resistant or sensitive, depending on a patient's luck. It is harder to understand in this model why patients would respond and then relapse, unless treatment was not given long (or hard enough) to a sensitive tumor. Below shows branching evolution, with related clones that may have similar or dissimilar biological properties. This is the foundation of a Darwinian landscape where clones compete, cooperate, and are differentially selected by therapy.