Transcript The Evolution of Cooperative Behavoir

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