Fig 1. Schematic representation of the model.

(a) During each cell division, a mutation might arise which changes the fitness of a daughter cell according to a fitness distribution. (b) Tumor development consists of two phases: the pre-initiation phase, in which the population consists of a constant number of N cells (blue), and the post-initiation phase, in which more and more aggressive cell types emerge (shades of purple and red). The tumor is diagnosed when the total population size reaches M. A neutral hitchhiker mutation (green) might arise during the pre-initiation phase (c) or during the post-initiation phase (d).