Chapter 8 the evolution of social behavior
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Chapter 8: The Evolution of Social Behavior. What is social behavior? Types of social interactions The Conundrum of Altruism Kin Selection or Inclusive Fitness Reciprocal Altruism. Social Behavior. Group living requires tradeoffs of costs and benefits

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Chapter 8 the evolution of social behavior l.jpg
Chapter 8: The Evolution of Social Behavior

  • What is social behavior?

    • Types of social interactions

  • The Conundrum of Altruism

  • Kin Selection or Inclusive Fitness

  • Reciprocal Altruism

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Social Behavior

  • Group living requires tradeoffs of costs and benefits

  • Social interactions are behaviors that has a fitness consequences for two or more individuals (of the same species).

    • Excludes:

      • Parenting

      • Mating

  • In a social interaction there is an ACTOR and a RECIPIENT(S) of the action.

  • An action can be said to be beneficial (+) if it increases fitness, and costly or detrimental (-) if it decreases fitness.

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Types of Social Interactions

  • A taxonomy of pair wise, or dyadic, social interactions based on fitness outcomes:

    Type Actor Recipient

  • Selfish + -

  • Mutualistic + +

  • Altruistic - +

  • Spiteful- -

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The Conundrum of Altruism

  • Selfish and mutualistic acts increase the fitness of the actor. It is clear that these behaviors will be selected for by natural selection, because those who act selfishly or mutualisticly derive a direct/immediate benefit from their action.

  • Altruism is a problem to explain because by definition it decreases the fitness of the individual performing the behavior while increasing the fitness of a competitor (the recipient) and therefore reduces the contribution of the genes that underpin that behavior to the next generation.

  • Even spiteful interactions can be explained by natural selection as long at the recipient pays a greater fitness cost than the actor.

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Altruism and Warning Calls

Gives a warning call (ACTS ALTRUISTICALLY)

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Kin Selection is one answer to the puzzle: Hamilton’s (1964) theory of kin selection (inclusive fitness) predicts that altruistic behaviors will be favored by selection if the costs of performing the behavior are less than the benefits to the receiver discounted by the coefficient of relatedness between actor and recipient.



c= the fitness cost to the individual performing the behavior

b = the sum benefits to all individuals affected by the behavior

r = the average coefficient of relatedness between the actor and recipients

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Giving the warning call and accounting for kin selection where the cost of giving the call is .3 and the benefit .1 to each of the others and the actor is the sister of the others (r = .5)

c = .3

b = .1 x 8 = .8

r = .5

rb = .5 x .8 = .4

Give the warning call because c<rb (.3 < .4)

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  • Kin selection is a powerful motivation for cooperation in social interactions.

  • Kinship is an important principle for the organization social structures

  • In tribal and band societies kinship is the primary principle around which groups form and is primary in defining the relationships between groups

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Spheres of Interaction and Influence of Kin Selection social interactions.


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  • Among the Yanomamö the value r in Hamilton’s Rule: social interactions.

    • Is related to how large a village gets before fissioning

    • Predicts who will side with whom during conflicts

    • Predicts who will go with whom when a village fissions

  • Kinship is likely the most important principle underlying group structures in the EEA

  • What about chimp social structures?

  • Kinship is also used as a principal for organizing non-kinship based organizations like religion

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Reciprocal Altruism social interactions. (Trivers 1971)

c < bw

c = cost to the actor

b = benefit to the recipient

w = the likelihood that the actor will receive a benefit in the future as a result of paying the cost now.

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Proximity to the Central Hierarchy social interactions.















(After Hall and DeVore, 1965)

Baboons show signs of Reciprocal Altruism

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Game Theory: social interactions. Tit for tat and the Prisoner’s Dilemma



me: R = +3 me: S = -2 C = Cooperate

C D = Defect

you: R = +3 you: T = +5 R = Reward for mutual cooperation

T = Temptation to defect

Me S = Sucker’s payoff

P = Punishment for mutual defection

me: T = +5 me: P = 0


you: S = -2 you: P = 0

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  • In a one-time game, you should defect because the average payoff is greater.

  • If the game is to be repeated many times (as is the game of life), it is in both player’s long-term interest to cooperate.

  • In game theory the value w is defined as the number of times the game will be played

  • Tit-for-tat is an evolutionarily stable strategy, or solution, to a repeated Prisoner’s Dilemma game. The rule is: cooperate on the first play and then do what your opponent did in the last play.

  • Also known as the Golden Rule

  • All social interactions, like games, are competitive (winners and losers)

  • Assignment: go to and do the tutorial on the Evolution of Cooperation.