Number of words: 445

From the lofty summit of this mathematical achievement, we had a commanding view of evolutionary dynamics. We could reveal the precise conditions for natural selection to favor cooperation over defection in populations that are structured around sets. One simple conclusion that we could draw from Corina’s work was that the more sets there are, the better it is for cooperation. The reason is that when there are more sets, cooperators have more opportunities to escape. It is easier for them to find sets that are free of the troublesome defectors that could exploit them.

Another twist of the model that provides a powerful engine for the evolution of cooperation is that individuals might only begin to interact with each other if they have several sets in common. When I realize that the person who has joined my tennis club also studies theoretical biology, for example, then I will be more likely to begin collaborating with her. By the same token, it is not enough that we are both Democrats, or that we both shop at the same supermarket, or that we live in the same neighborhood. To have a reasonable chance of cooperating, we have to be Democrats who are neighbors and shop at the same supermarket. This “choosiness” of cooperators dramatically enhances their probability of success. As a result of this feature, sets are the structures with the greatest potential to promote the evolution of cooperation. Corina’s equation makes a fascinating prediction. It suggests that there is an optimum level of mobility (meaning the rate at which people move between different sets and explore new sets). If the mobility is too low, then the population is too static and cooperators can be exploited by defectors. But the converse is bad for cooperation too. If the mobility is too high, then any “fellowship of cooperators” that fosters mutual help does not persist for very long. The most fertile ground for cooperation comes between these two extremes.
With intermediate mobility, cooperators have a chance to hang around for long enough to benefit each other, but they can also escape from defection by colonizing new sets. The process is guided by natural selection: if a few cooperators find a new set without any defectors then they do well and attract more cooperators. Only after some time one of them might switch to defection and, by doing this, destroy the happy situation that had once thrived there. Then the resident cooperators have to find a new set. Because it is harder for sets with defectors to attract new members, over time they dwindle and, eventually, become empty.

Excerpted from page 262-263 of ‘Super co-operators ’ by Martin Nowak