A simple conception of mutational events leading to increased size of operons (solid rectangles represent genes within an operon). Translocation of monocistrons (open rectangles; a–g) or fusion via deletion of transcription termination and promoter elements (triangles and circles, respectively; h and i) between adjacent genes on the same strand could create or increase the size of an operon. Solid horizontal line represents noncoding DNA. For simplicity, we diagram only two genes in an operon; events of type a, b, d, e, or h involving only monocistrons would lead to the creation of a polycistronic operon. A dashed line represents DNA sequence between the first and last gene in an operon, in which additional gene members of the operon might reside. Also for simplicity, we diagram the enlargement of operons, although translocations f and g would result in the operon retaining the same number of genes or possibly decreasing in size. Conceivably, monocistrons in the diagram could be replaced by operons to increase operon size by more than one gene at a time; segmental duplication of operons also could generate changes in the abundance of an operon size class. Models 2 and 3 do not formally distinguish among these alternative mutational ways in which operon size could change. However, the models assume either that a single gene moves per time step or that genes move independently of one another. Consequently, these models do not capture fission–fusion events (paths h and i).