The basic principles of the Moran process. The Moran process assumes that, over short periods of time, the total population of cells is constant. (a) To start with, a cell is selected for reproduction. Selection is dependent on the frequency of the cell in the population and its reproductive fitness (r). (b) The cell divides, and the number of cells increases by one. (c,d) Therefore, another cell is selected for export (c), which returns the population to its normal level (d).