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The Evolutionary Games We Play

The Evolutionary Games We Play. Psychology 3107. Introduction. Animals tend to behave in ways that maximize their inclusive fitness Usually pretty straightforward But, sometimes we must know what others are doing before we adopt a strategy

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The Evolutionary Games We Play

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  1. The Evolutionary Games We Play Psychology 3107

  2. Introduction • Animals tend to behave in ways that maximize their inclusive fitness • Usually pretty straightforward • But, sometimes we must know what others are doing before we adopt a strategy • What if your mating call is drowned out by others’ calls, what to do, ahh what to do…

  3. Fitness and Strategies • In certain cases payoffs, and hence fitness maximization, depend on what other populations are doing • When the payoff to one individual depends on the behaviour of others we cannot use the principle of fitness maximization until we know: • What the alternatives are • P(encountering alternatives) • Consequences of encounter

  4. Game Theory • Think of it like a game • Each individual’s behaviour is its strategy, payoffs are in units of fitness • Players produce more players (offspring) • Changes in fitness are directly proportional to payoffs • An evolutionary Stable Strategy is one that, when adopted by enough individuals, maximizes payoff

  5. Pure Strategy • One that cannot be replaced • Food storing • Recover your own seeds (Anderssen and Krebs, 1978) • If they recovered communally, a selfish hoarder would replace the communals damned quckly

  6. Mixed Strategies • Hawks and Doves • Not real hawks or doves, strategies • Always fight, or always give up • Look at the payoffs • Look at the costs • Determine what proportion should be hawks and should be doves

  7. Hawks and Doves • Say its all Doves • Hawk shows up, wins resource • Spreads genes • Now more hawks • Oh oh, now you are fighting, P(injury) = .5 • Now being a dove pays • Either strategy good when rare, bad when common

  8. Doves and Hawks • V =V alue of resource for winner • W = cost of a wound • T = cost of display (no fighting) • (John Maynard Smith, 1978)

  9. Whoa, I know Kung Fu • Set up a payoff Matrix Opponent in the contest Hawk Dove Payoff Hawk ½(V-W) V Received By Dove 0 ½V-T

  10. ESS as easy as 123 • If W > V then there can be no pure ESS • In a population of hawks, a small number of doves do better than hawks • E(dove,hawk) > E(hawk, hawk) • E(dove, hawk) = 0 • E(hawk, hawk) = ½(V-W) • W > V, therefore ½(V-W) < 0

  11. Pure Doves don’t do it either • Payoff to Hawk is V • Payoff to doves is less than that • (½W – T) • Hmmm • So, what proportion of hawks and doves balances it out?

  12. What is theoretical population biologist to do? • Find the proportion (p) of hawks of hawks such that the following equation balances: • p ½(V-W) = (1-p) V = p (0) + (1-p) (½V– T) • Simply (?) solve for p • p = (V+2T) / (W+ 2T)

  13. Apply it, sort of • Say V = 10 • W = 20 • T = 3 Opponent in the contest Hawk Dove Payoff Hawk -5 10 Received By Dove 0 2

  14. Now, sub that back into the formula • P = 16/26 or 8/13 • 8/13ths of the population, with these payoff values, must be hawks • The values are not that important really, the point is that you can determine the point at which a strategy can coexist with another strategy as an ESS • Could be percentage of population, or percentage of time each animal adopts a given strategy

  15. So? • It is actually applicable that’s so • Toads looking for breeding grounds (Davies and Hallaway, 1979) • Payoffs determined

  16. Another so • Dungflies • Should a male hang around poo as it gets older?

  17. Conclusions • This is a very brief intro to game theory • This stuff is way powerful • You have to sit and think some about the payoffs and costs • Dynamic programming models are becoming more popular

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