Figure 1. Mechanism by which the number of cells (n(t)) increases or decreases in the absence of natural death (δ = 0).

The situation before and after a birth event is shown in the left and right columns, respectively. A parent cell immediately dies after giving rise to exactly two daughters. Replication is semi-conservative: Each daughter independently acquires a Poisson distributed number of lethal mutations, with mean Uλ. If both daughters are free of lethal mutations (check marks), then n(t) increases by one (top row). If both daughters acquire lethal mutations and die (“x” marks), then n(t) decreases by one (bottom row). The probability of each transition is also shown in the left column. In addition to these mechanisms, cells may experience natural death with a fixed rate (δ).