Number of cooperators as a function of time with preferential selection.

<p>The parameters are , , and . The simulation was carried out for time steps. Each data point is an average over time steps. The system spends most of the time in a state of all cooperators, interrupted by instabilities that last for a short duration when defectors suddenly appear. This is analogous to the coexistence of prosperity and instability as observed in the model of Cavaliere <i>et al. </i><a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0049663#pone.0049663-Cavaliere1" target="_blank">[35]</a>.