Science 314(5805): 1560-1563

Five rules for the evolution of cooperation

Martin A. Nowak, Program for Evolutionary Dynamics, Department of Organismic and Evolutionary Biology, Department of Mathematics, Harvard University, Cambridge, MA 02138, USA;

Abstract

Cooperation is needed for evolution to construct new levels of organization. The emergence of genomes, cells, multi-cellular organisms, social insects and human society are all based on cooperation. Cooperation means that selfish replicators forgo some of their reproductive potential to help one another. But natural selection implies competition and therefore opposes cooperation unless a specific mechanism is at work. Here I discuss five mechanisms for the evolution of cooperation: kin selection, direct reciprocity, indirect reciprocity, network reciprocity and group selection. For each mechanism, a simple rule is derived which specifies whether natural selection can lead to cooperation.

Abstract

Evolution is based on a fierce competition between individuals and should therefore only reward selfish behavior. Every gene, every cell and every organism should be designed to promote its own evolutionary success at the expense of its competitors. Yet we observe cooperation on many levels of biological organization. Genes cooperate in genomes. Chromosomes cooperate in eukaryotic cells. Cells cooperate in multi-cellular organisms. There are many examples for cooperation among animals. Humans are the champions of cooperation: from hunter gatherer societies to nation states, cooperation is the decisive organizing principle of human society. No other life form on earth is engaged in the same complex games of cooperation and defection. The question how natural selection can lead to cooperative behavior has fascinated evolutionary biologists for several decades.

A cooperator is someone who pays a cost, c, for another individual to receive a benefit, b. A defector has no cost and does not deal out benefits. Cost and benefit are measured in terms of fitness. Reproduction can be genetic or cultural. In any mixed population, defectors have a higher average fitness than cooperators (Fig 1). Therefore, selection acts to increase the relative abundance of defectors. After some time cooperators vanish from the population. Remarkably, however, a population of only cooperators has the highest average fitness, while a population of only defectors has the lowest. Thus, natural selection constantly reduces the average fitness of the population. Fisher's fundamental theorem, which states that average fitness increases under constant selection, does not apply here because selection is frequency dependent: the fitness of individuals depends on the frequency (=relative abundance) of cooperators in the population. We see that natural selection in well-mixed populations needs help for establishing cooperation.

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Figure 1

Without any mechanism for the evolution of cooperation, natural selection favors defectors. In a mixed population defectors, D, have a higher payoff (=fitness) than cooperators, C. Therefore, natural selection continuously reduces the abundance, i, of cooperators until they are extinct. The average fitness of the population also declines under natural selection. The total population size is given by N. There are i cooperators and N − i defectors. The fitness of cooperators and defectors is respectively given by f_C = b(i − 1)/(N − 1) − c and f_D = bi/(N − 1). The average fitness of the population is given by $\overset{̅}{f}$ = (b − c)i/N.

References and Notes

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