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The Evolution of Cooperative Behavoir An Evolutionary Puzzle….. If Darwin’s Theory is true…… • “Our very nature is based on competition . . . . There is no reason why human should live together civilly… anarchy will ultimately rule” • Is this true???? Examples of Cooperative Behavior Observed in Nature: • Hyenas cooperating to hunt prey • Lioness caring for another’s cub • Birds cleaning parasites from hippo’s teeth • others? Challenge: Can natural selection account for cooperative behavior??? Possible Explanations • Kin Theory – • Reciprocity - Game Theory • Developed by Nobel Laureate and mathematical genius Dr. John Nash • Featured in award-winning movie, “A Beautiful Mind” • Can be used to test Reciprocity explanation for Cooperative Beh. Prisoner’s Dilemna Scenario You and a partner have been caught with stolen goods; a minor crime punishable with 2 years in jail. Police, however, think you did the robbery (a major crime with a prison term of another 8 years ), but they have no evidence it was either of you. They bring you in separately for questioning. (Get two volunteers) What “should” you do? Your partner Cooperate You Cooper. 2 years Defect 0 years (Why is this a “Dilemma”?) Defect 10 years 8 years Ready to play? Your partner Cooperate You Cooper. 2 years Defect 0 years Defect 10 years 8 years Get Index Cards. Make your move! Results # of • CC = • CD = • DD = Question In a single-interaction scenario of Prisoner’s Dilemma, what is the logical move? (Defect, better choice, regardless of what partner does) Multiple Move PD • But in nature, often two organisms will interact on a regular basis. • What does Game Theory tell us about this situation? Cost-Benefit Matrix Cooperat e Defect Cooperate b–c c Defect b 0 Cost-Benefit Matrix (Cost = 2; Benefit = 4) Cooperat e Defect Cooperate b–c 2 C -2 Defect b 4 0 Normalized Cost-Benefit Matrix Cooperat e Defect Cooperate 4 0 Defect 6 2 Your turn! • Get ready to play a multiple move version of PD • You will interact with your partner “chimp” some number between 20 – 30 times • Try to think of a strategy which will maximize your points! Question • Which strategies seemed most beneficial for you & your classmates? Some common strategies in the Prisoner's Dilemma Game • ALTERNATE: The player alternates between C and D, starting with a C. • ALWAYS COOPERATE: The player always plays C, no matter what their partner has played in the past. Also known as sucker. • ALWAYS DEFECT: The player always plays D. Also known as cheat. • GRUDGER: The player starts playing C and continues to do so until the other player plays D. After that it plays D for the rest of the game with that particular partner. • RANDOM: The player chooses either C or D with equal probability. • SNEAKER: The player starts with a C and then plays whatever its partner play in the previous move. However, at random intervals it plays D. TIT FOR TAT: The player starts playing C and then plays whatever its partner did in the previous move. TIT FOR TWO TATS: The player plays C in the first and second moves. After that, if its partner played D in the two previous moves they play D, otherwise they continue to play C. TWO TITS FOR TAT: The player starts with C, and then if its partner plays D, then plays D in the following two moves, otherwise plays C. • • • The Evolutionary Game • Competition between two “nice” and two “nasty” strategies • Tabulate and graph results for 4 generations Evolutionary PD • Try these computer simulations: • http://bio150.chass.utoronto.ca/pdga me/evolution.html Tit for Tat in Nature? • Predator inspection • Blood sharing • Trench warfare Some Featured Ideas • See Hand out