Chapter 7: The Evolution of Cooperation
Chapter Review
Altruism and Cooperation: A Selective Puzzle
Most of the anthropoid primates live in social groups that create many and diverse opportunities for interaction. Why would an actor engage in a potentially disadvantageous interaction—such as one denoted by altruism—since Darwinian theory places individual fitness at a premium for evolutionary success? Surely, an individual engaging in altruistic acts is squandering time better spent mating or gathering resources. Plus, altruism can result in severe injury (or even death) without the promise of any tangible reward. How, then, did altruistic behavior evolve, given the apparent conflict with individual fitness?
Kin Selection
The problem of altruism as described above stumped evolutionary biologists until W. D. Hamilton (and J. B. S. Haldane, who foreshadowed the logic behind kin selection in the 1930s), addressed the question of altruism in quantitative terms in the 1960s. These findings introduced a new term—inclusive fitness—and described the logic of genetics as maintained over evolutionary time.
If there is a genetic basis for altruistic tendencies, Mendelian genetics should produce siblings with a specific probability for sharing genes. Logically, siblings share approximately 50% of their total genetic makeup, a parent and offspring share about 50%, and first cousins share about 12.5%; the amount of genes shared by other relatives depends on the degree of relatedness. Hamilton took these assumptions about the degree of relatedness among kin into account when addressing altruistic social encounters, constructing a formula that we now call Hamilton's rule:
rb > c
Where (in terms of fitness) c is the cost to the actor, b symbolizes the sum benefits to all individuals affected by the behavior, and r is the coefficient of relatedness between actor and recipient. Here, actors gain fitness by assisting relatives, provided that the cumulative benefit to the recipients is greater than the cost to the actor. This logic, coupled with the fact that interaction between individuals is likely to be biased toward kin rather than nonkin (particularly those of an altruistic nature), provides a theory in which altruism and evolutionary success no longer seem contradictory.
Example
Imagine a situation in which the threat of predation can evoke one of two possible responses in a vigilant monkey:
- Give an alarm call: The caller incurs a cost (being eaten) for the benefit of the others.
- Remain silent: Some of the other members of the group are eaten while the actor escapes unharmed.
If the beneficiaries of this act of ultimate altruism are close relatives of the actor, it actually benefits the inclusive fitness of the acting individual to give the call and save its kin. Because relatives have a higher tendency to share similar genetic material, the total genetic benefit for this behavioral strategy exceeds a purely selfish strategy. Natural selection therefore provides for the enhancement of group fitness by individuals acting in their own self-interest.
Reciprocal Altruism
Kin selection is not the only reason cooperative and seemingly altruistic behavior may have evolved. Reciprocal altruism—the continued, mutually beneficial interaction between individuals over time—is another useful behavioral strategy. For reciprocal altruism to work as a strategy, several conditions are required:
- Frequent interaction.
- The recognition of individuals.
- Remembering past interactions with individuals.
- Assisting only those who provided past assistance.
Given these conditions, the strategy of reciprocal altruistic behavior has proven to be both successful and evolutionarily stable.
Example
Researchers have observed reciprocal altruism in action in field studies with monkeys and apes, such as situations where monkeys endeavor to groom each other for roughly the same amount of time after hearing recordings of primate calls. More specific experiments to test the precepts of reciprocal altruism, though, were first devised by Robert Seyfarth and Dorothy Cheney who played recorded “recruitment calls” (invitations for grooming or support) to vervet monkeys in various situations. First, Vervet A’s call was played to Vervet B after A had groomed B. Cheney and Seyfarth then played the same call, but after a period when no grooming had occurred. They hypothesized that Vervet B would respond with more interest after having been groomed by Vervet A and with less interest after no grooming had occurred. Their hypothesis was affirmed: Vervet B responded more strongly after having been groomed, suggesting that monkeys remember and maintain reciprocal altruistic relationships.