This is a study of the evolution of cooperation in a small, heterogeneous population. Individuals can either be of type COOP (cooperator, depicted as blue vertices) or DEF (defector, depicted as red vertices). When two cooperating individuals interact, each individual gains a benefit
from the mutual cooperative act but also has to pay some costs
. If a cooperator interacts with a defector the cooperator has to pay the costs
but the defector gets the benefit
without paying any costs. If two defectors meet they get nothing but they also have no costs. (Reciprocal grooming in primates is a useful example for such a cooperative act: the time invested in grooming is the cost of the behavior and the benefit is increased health due to the decreased parasite load.) Individuals interact at different frequencies as indicated by the thicknesses of the connecting edges: a thicker line indicates that individuals are more likely to interact. Individuals update their strategy by copying the strategy of their neighbors with a probability proportional to the fitness
of the strategies in their neighborhood. Individual fitness is determined by the payoffs that they receive from their interactions and is indicated by the size of the disks. This Demonstration starts with a random number of initial cooperators and a cost:benefit ratio of 1:40.