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Evolutionary Dynamics Study how populations of reproducing individuals change over time. Prelife

Evolutionary Dynamics Study how populations of reproducing individuals change over time. Prelife Protocells Viruses Cells Cancer cells Multi-cellular organisms Insect societies People – genetic or cultural reproduction Evolution by learning, imitation, communication. Time line

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Evolutionary Dynamics Study how populations of reproducing individuals change over time. Prelife

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  1. Evolutionary Dynamics Study how populations of reproducing individuals change over time. Prelife Protocells Viruses Cells Cancer cells Multi-cellular organisms Insect societies People – genetic or cultural reproduction Evolution by learning, imitation, communication

  2. Time line 13700 Universe 4567 Sun 3500 chemical evidence of life on Earth 2100 bacterial fossils (simple multi-cellular) 1800 eucaria 600 complex multi-cellularity 120 insect societies 1 human language million years ago cooperation is needed for construction

  3. What makes evolution constructive? Why is evolution “open ended”? What is the difference between genetic and cultural evolution? Evolution is a search process. What generates the space that is being searched? Is i-life stable?

  4. Can you eradicate HIV infection? Can you cure cancer with the simultaneous use of 2 or more targeted drugs? Can you make people cooperate with the next generation (and thereby save the world)?

  5. What is it that evolves?

  6. What is it that evolves? Populations of reproducing individuals

  7. Mutation

  8. Selection Time

  9. Cooperation Time

  10. Evolution of cooperation Martin Nowak, Harvard University

  11. What is cooperation? Donor pays a cost, c Recipient gets a benefit, b Cost and benefit are measured in terms of fitness. Reproduction can be genetic or cultural.

  12. Prisoner’s Dilemma I cooperate I defect you cooperate b - c - c you defect b 0 you get b > c > 0

  13. What is the dilemma ? Two rational players defect and end up with a low payoff, 0. Two irrational players might cooperate and receive a higher payoff, b – c .

  14. Natural selection chooses defection C D D C D C C C D D C D D D D In any mixed population, defectors have a higher payoff than cooperators.

  15. Natural selection needs help to favorcooperators over defectors.

  16. Five mechanisms for the evolution of cooperation: Direct reciprocity Indirect reciprocity Spatial selection Group selection Kin selection

  17. Direct reciprocity ‘I help you, you help me.’

  18. Repeated Prisoner’s Dilemma Player 1 : C D C D C C C …. Player 2 : D C D D C C C …. The Folk theorem

  19. Repeated Prisoner’s Dilemma Player 1 : C D C D C C C …. Player 2 : D C D D C C C …. What is a good strategy for playing this game? Robert Axelrod

  20. Tit-for-tat • I start with cooperation. • If you cooperate, then I will cooperate. • If you defect, then I will defect. Anatol Rapaport

  21. Tit-for-tat is unforgiving Errorsdestroy cooperation Tit-for-tat : CCCCDCDCDCDDDDDD…. Tit-for-tat : CCCDCDCDCDDDDDDD….

  22. Let natural selection design a strategy Random

  23. Let natural selection design a strategy Always defect Random

  24. Let natural selection design a strategy Tit-for-tat Always defect Random

  25. Let natural selection design a strategy Tit-for-tat Generous Tit-for-tat Always defect Random

  26. Generous Tit-for-tat • I start with cooperation. • If you cooperate, then I will cooperate. • If you defect, then I will cooperate with a certain probability (q = 1- c / b). Evolution of forgiveness

  27. Let natural selection design a strategy Tit-for-tat Generous Tit-for-tat Always defect Random

  28. Let natural selection design a strategy Tit-for-tat Generous Tit-for-tat Always defect Always cooperate Random

  29. Let natural selection design a strategy Tit-for-tat Generous Tit-for-tat Always defect Always cooperate War and peace

  30. Let natural selection design a strategy Tit-for-tat Generous Tit-for-tat Always defect Always cooperate Win-stay, lose-shift

  31. Win-stay, lose-shift • If I am doing well (payoff b or b-c) then I will repeat my move. • If I am doing badly (payoff 0 or -c) then I will change my move. If b/c<2 then a stochastic variant of WSLS does well (where you return to C after DD only with a certain probability).

  32. Direct reciprocity … allows the evolution of cooperation if w > c / b b…benefit c…cost w…probability of another round

  33. Indirect reciprocity ‘I help you. Somebody helps me.’

  34. Indirect reciprocity works via reputation A helps B A does not help B A B A B The reputation of A increases. The reputation of A decreases. donor recipient donor’s reputation cooperate -c +b +1 defect 0 0 -1 Nowak & Sigmund, Nature 1998

  35. Experimental confirmation: People help those who help others. Helpful people have a higher payoff in the end. Wedekind & Milinski, Science 2000

  36. A Gossip spreads reputation A helps B B Observers Reputation of A is updated. gossip Rest of the population

  37. Games of indirect reciprocity are cognitively demanding; individuals need to monitor the social network of a group. => evolution of social intelligence Individuals must be able to talk to each other about others. => evolution of human language

  38. David Haig: “For direct reciprocity you need a face. For indirect reciprocity you need a name.”

  39. A rule for indirect reciprocity q > c / b q… probability to know someone’s reputation c… cost of cooperation b… benefit of cooperation

  40. Spatial selection Spatial games Games on graphs Games in phenotype space Games on sets

  41. Spatial games Cooperators Defectors Nowak & May, Nature 1992

  42. Games on graphs D D D D D C C C C C Cooperators Defectors The graph describes a spatial structure or a social network. ‘Evolutionary graph theory’ (Lieberman et al, Nature 2005)

  43. D D D D D C C C C C b b 2b-2c b 2b-5c b 2b-3c Cooperators pay a cost c for each neighbor to receive benefit b.

  44. Spatial selection on graphs favors cooperation if b / c > k k…(average) number of neighbors weak selection Ohtsuki et al, Nature 2006

  45. Evolutionary set theory People belong to sets. People interact with others in the same sets. People adopt strategy and set membership of successful individuals. Tarnita et al, PNAS 2009

  46. Group selection ‘There can be no doubt that a tribe including many members who [...] are always ready to give aid to each other and to sacrifice themselves for the common good, would be victorious over other tribes; and this would be natural selection.’ Charles Darwin, The Descent of Man, 1871

  47. D D D Group selection Play the game with others in your group. Offspring are added to the group. Groups divide when reaching a certain size. Groups die. C D D C C C C C C D C

  48. Group selection favors cooperators if b / c > 1 + n / m n … group size m … number of groups Traulsen & Nowak, PNAS 2006

  49. Kin selection The interaction occurs between genetic relatives. ‘I will jump into the river to save 2 brothers or 8 cousins’ • J.B.S Haldane

  50. Hamilton’s rule • r > c / b • r … coefficient of relatedness • c … cost of cooperation • b … benefit of cooperation • Inclusive fitness theory

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