Here are some notes from the optional reading entitled "Five Rules for the Evolution of Cooperation" by Martin A. Nowak.
The author starts by noting the paradox that because of natural selection, one would expect that "every gene, every cell, and every organism should be designed to promote its own evolutionary success at the expense of its competitors," and yet we observe cooperation on many levels of biological organization. He uses game theory to describe five mechanisms that may explain the evolution of cooperation.
In this mechanism, natural selection can favor cooperation between relatives because of shared genes. The cooperator may sacrifice so as to benefit a relative who will pass on some portion of the same genes. In mathematical terms, cooperation can evolve when the probability of sharing a gene exceeds the cost-to-benefit ratio of the altruistic act. This is known as Hamilton's rule. It is important to note that cooperation also occurs between unrelated individuals, so kin selection cannot be the only explanation for the evolution of cooperation.
If same individuals expect to encounter each other many times, they may cooperate in anticipation of future cooperation (this scenario is known as the repeated Prisoner's Dilemma). Computer models of repeated Prisoner's Dilemma games have shown that strategies that include elements cooperation are most successful. Specifically, tit-for-tat (which always starts with a cooperation and then does whatever the other player has done in the previous round) will enable cooperation when nearly everybody is a defector and, once cooperation is established, win-stay, lose-shift strategy (in which one repeats their previous move whenever they are doing well and changes otherwise) will serve to maintain cooperation.
Because helping someone can improve one's reputation and improved reputation increases the likelihood receiving help from others, indirect reciprocity also provides a mechanism for the evolution of cooperation. Because such indirect reciprocity requires strong cogitative abilities – for tracking and communicating about one's own reputation and the reputation of others in the group – the author suggests that selection for indirect reciprocity played key roles in the evolution of human intelligence, morality, and social norms.
Because individuals are not mixed equally and some mix more often than others, cooperators may form network clusters within which they help each other. This may enable them to prevail over defectors.
Selection can act on groups as well as individuals. Groups made up only of cooperators may grow faster than groups of defectors and therefore outcompete them over time.
The author goes on to show how each of these mechanisms can be illustrated with a standard 2 × 2 payoff matrix.