Density of cooperators at equilibrium state as a function of the combination of and .

<p>These panels are related to different migration patterns: no migration, local migration with , local migration with , and global migration, under strong selection . Those ones are related to different selection intensities: , , under local migration pattern of . Whenever individuals are prohibited to move, the empty sites play the role of demarcating cooperators and defectors, leading to that a few small but not many cooperator clusters survive the evolutionary race. This does not alter the fact that cooperators are in a disadvantageous place in contending with defectors. Integrating the migration into the system qualitatively changes the evolutionary outcome. Under the migration pattern, a large area sees red (full cooperative), reversing cooperators' fate. Under global migration pattern, the area surviving cooperators is somewhat compressed, but is still much larger than the situation of no migration. Comparing panels and shows that increase in proves to be adverse for the evolution of cooperation. The red area tends to drop down as increases. Of interest is that for large , cooperation can always be stabilized at a markedly large tract, even for . For small the red area nonetheless plummets much more rapidly for increasing . In terms of strategy revision, adds the stochasticity, offering much longer time span for cooperators to agglomerate. This constitutes a key reason why cooperators absolutely outperform defectors for relatively weak selection ().