Fig. 1.

Darwinian gradualism (left) predicts that genomes acquire new mutations (arrowheads) sequentially and independently of one another (top) at linear and constant rates (bottom). The hypermutation model (center) predicts that following acquisition of a destabilizing “mutator” mutation, a genome may become chronically unstable and acquire new mutations independently at an elevated rate. The punctuated bursts model (right) predicts that genomes can experience discrete and transient episodes of systemic genomic instability (SGI) and acquire multiple new mutations rapidly but will resume stable genome propagation for extended periods.