Fig. 1.

a A general branching process. I indicates a budding or simple birth process, where the parental individual produces a single offspring (a ‘singlet’) without death. II indicates binary fission, where a parent dies at the same moment two newborn twins occur (a ‘doublet’). III indicates a more general fission event with four offspring (a ‘quadruplet’). IV indicates death, which can be viewed as fission with zero offspring. b A binary fission process such as cell division. At time $t_1$ we have four individuals; two sets of twins. At time $t_2$ we have six individuals; two pairs of twins and two singlets