The Evolution of Cooperation

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The Evolution of Cooperation
E3 Lecture 6
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Clicker Question

• In the Rainey Rainey study, which of the
  following would NOT be consistent with the idea
  that smooth-morph cells functioned as cheats,
  while wrinkly spreader cells functioned as
  cooperators?
• Wrinkly spreader mats infiltrated by
  smooth-morph cells were weaker (that is, they
  collapsed under a lower weight of glass beads)
  than mats with solely wrinkly spreader cells.
• Smooth-morph cells reap benefits (in terms of
  numbers) in the presence of wrinkly spreader
  cells.
• Wrinkly-spreader cells suffer costs (in terms
  of numbers) in the presence of smooth-morph
  cells
• Wrinkly spreader mats infiltrated by smooth
  morph cells took longer to collapse than mats
  consisting of solely wrinkly spreader cells.

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Another exercise We get the point

• At this point in Biol. 481, everyone has earned
  at least a few points towards their final grade.
• Were going to play a game where you may be able
  to increase the number of points you have.
• The game is simple you have to make a decision
  on what to do with one of the points you have
  earned. You have two choices you can donate 1
  point youve earned to a common pot or you can
  keep your point.
• A donated point is lost to you. However, after
  all the donations are counted, 1.5 times that
  amount of points is split evenly between every
  student in the class.
• So, despite whether you donate or not, you get
  the same return from the common pot.
• Consider a few scenarios
• Note, Josh, Toby, your TAs, and I (as well as
  your fellow students) will never see your
  decision. Only a recorder will know if youve
  donated a point which will be subtracted from
  your final total at the quarters end.
• Click C to donate your point and D to keep it

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The Tragedy of the Commons

• In Wealth of Nations, Adam Smith (1776) suggested
  that a collection of rational agents, each acting
  in their own self-interest, would work for the
  common good.
• William Lloyd (1883) discussed how
  self-interested parties might over-exploit a
  common resource.
• In a seminal paper, Garrett Hardin (1968)
  extended Lloyds idea, suggesting that many
  shared resources are utilized in ways that are
  good for the individual in the short term, but
  bad for the social group in the long term.
• Harvesting of natural resources (fisheries,
  lumber, etc.)
• Generation of pollution (air, water, scenery)
• The population bomb

?
B
C
A
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Selfishness and Cooperation

• The traditional evolutionary perspective is that
  selfishness trumps cooperativity.
• However, biological systems are characterized by
  substantial cooperativity.
• Animals work together in social groups,
  participate in interspecific mutualisms, and
  sacrifice future reproduction for kin.
• Humans too will share critical resources (e.g.,
  food, shelter, etc.), will work with one another,
  and will contribute resources towards the smooth
  operation of their group

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Biological Altruism
Altruism is the central theoretical problem of
sociobiology (E. O. Wilson, 1975)

• An altruist improves the fitness of a recipient
  at a fitness cost to itself.
• Altruism has fueled many debates in evolutionary
  biology (e.g., group selection versus individual
  selection)
• From an orthodox evolutionary perspective
  altruistic behavior is confusing
• Altruism is the very opposite of survival of
  the fittest (Sober Wilson, 1998)

-

Altruist
Other
Honeybees
spore head
somatic stalk
Slime mold cells
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Cooperation and the Major Transitions

• Evolutionary biologists are interested in the
  subject of the major transitions
• Genes to chromosomes
• Prokaryotes to Eukaryotes
• Single cells to multi-cellular
• Solitary individuals to societies
• What are the common themes for these
  transitions?
• Lower level units cooperate to ensure the
  functioning of the higher level unit
• There is often division of labor between the
  lower level units
• There is a sense of common fate the lower
  level units go down together with the higher
  level ship

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The Evolution of Cooperation

• Lecture Outline
• Introduction to cooperation theory
• Examples of cooperation
• Cooperators in a sticky situation
• Slime mold with greenbeards
• Summary

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The Evolution of Cooperation

• Lecture Outline
• Introduction to cooperation theory
• Examples of cooperation
• Cooperators in a sticky situation
• Slime mold with greenbeards
• Summary

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The Prisoners DilemmaA Mini Tragedy of the
Commons
Bob did it!
Bill did it!
D
D
I aint talking!
I aint talking!
C
C
Put yourself in Bobs shoes, what should you do?
The payoff matrix
Bill does C
Bill does D
If you think Bill is going to cooperate, then you
should
defect.
Bob does C
If you think Bill is going to defect, then you
should
defect.
Of course Bill is thinking the same so you will
end up with mutual defection, whereas mutual
cooperation would have been much better
Bob does D
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Evolution of Strategies
The payoff matrix (payoff to the PLAYER)
PARTNER
C
D
Conditions for the PD
C
PLAYER
D
Defection is an Evolutionarily Stable Strategy
(ESS) A strategy that, once dominant, cannot be
invaded by alternatives
The Prisoners Dilemma game is an elegant
embodiment of the problem of achieving mutual
cooperation.
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How Can Cooperation Evolve?

• A major hurdle to the emergence of cooperation
  is that, when rare, cooperators face defectors.
• Given random pair formation, cooperators get the
  suckers payoff (S0) when rare, whereas the
  defector gets the punishment (P1) and
  occasionally the temptation (T5).
• However, if cooperators have a way to
  preferentially pair with each other, then the
  cooperator gets the reward (R3), whereas the
  defector gets the punishment (P1).

PARTNER
C
D
C
PLAYER
D

• Cooperative acts are directed towards relatives
  (kin selection)
• Cooperation is given only if received previously
  (reciprocal altruism)
• Cooperation is distributed to others based on
  their reputation (indirect reciprocity)

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Hamiltons Rule
?
?
?
W. D. Hamilton
c
1/4
1/4
1/4
1/4

• Between any two individuals, a coefficient of
  relatedness (r) can be computed.

r
r 1/2
b
The ultimate criterion which determines whether
a gene will spread is not whether the behaviour
is to a benefit of the behaver but whether it is
to the benefit of the gene.

• This coefficient is basically the probability
  that the recipient has the same allele as the
  donor (by descent).

HAMILTONS RULE
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Evolution of Cooperation by Kin Selection
I'd gladly give my life for three of my
brothers, five of my nephews, nine of my cousins.
A simple asexual example
B
A

• The A type asexually splits into
• two offspring where one helps
• the other at a cost to itself.

J. B. S. Haldane

• The B type asexually splits into
• two offspring.

• There is a base number of
• second generation offspring.

• The condition that allows A
• to increase in frequency is

c
b
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An Example The Brain Worm
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The Evolution of Cooperation

• Lecture Outline
• Introduction to cooperation theory
• Examples of cooperation
• Cooperators in a sticky situation
• Slime mold with greenbeards
• Summary

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Cooperative Helping in Turkeys

• In wild turkey, males will form coalitions to
  court females and defend the females from other
  males. Only one male (the dominant) in the
  coalition actually mates why does the other male
  (the subordinate) agree to help?
• Alan Krakauer (2005) suspected that helping had
  evolved through kin selection.
• He estimated
• Relatedness r0.42 (from neutral genetic loci)
• Benefit to dominant male b6.1
• ( offspring per dominant - offspring per
  solo)
• Cost to subordinate male c0.9
• ( offspring per solo - offspring per
  subordinate)
• Hamiltons rule (b/cgt1/r) is satisfied!

Alan Krakauer
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Microbial Cooperation

• Microbes display various forms of cooperativity
• Reproductive sacrifice (e.g., as slime molds and
  social bacteria form fruiting bodies, stalk cells
  display reproductive sacrifice to hold up spores)
  
• Production of public goods (e.g., yeast and
  bacteria exude extracellular proteins that break
  down complex sugars, degrade antibiotics, and
  gather critical resources)
• Competitive restraint (e.g., phage may display
  restraint in its use of a common host bacteria)
• Turner and Chao (1998) evolved a viral strain
  under well-mixed conditions where the virus
  outnumbered its bacterial host they found that
  mean fitness eventually decreased!

• These authors suspected that the evolved virus
  was a defector in a Prisoners Dilemma.
• By mixing the ancestral (Anc) and evolved (Evol)
  phage strains together at different frequencies,
  they estimated the payoff matrix and confirmed
  the PD.

Viral Payoff Matrix
Anc
Evol
Anc
Evol
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The Evolution of Cooperation

• Lecture Outline
• Introduction to cooperation theory
• Examples of cooperation
• Cooperators in a sticky situation
• Slime mold with greenbeards
• Summary

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A Model Organism for Diversity Cooperation

• Pseudomonas fluorescens is a soil dwelling
  microbe often associated with plants
• When a single genotype is placed into an
  unshaken microcosm, several different colony
  morphs evolve (i.e. an adaptive radiation see
  Rainey Travisano, 1998)
• The same morphs repeatedly appear across
  replicates, where morphology is based on the type
  of colony formed (smooth, wrinkly or fuzzy).
• Shaking the microcosm destroys the diversity.
  But letting the microcosm sit unshaken restores
  the diversity.
• One strain of this organism also produces a type
  of public good important for a group trait (mat
  formation).

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Specialists in the Adaptive Radiation
WS
SM
Static microcosm (an unshaken flask)
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Conditions to Demonstrate

• In order to demonstrate that the Wrinkly
  Spreader is a cooperator and the Smooth type is a
  defector, the authors needed to confirm the
  following
• 1) When together, the Wrinkly Spreader (WS) is a
  costly morph relative to the Smooth Morph (SM)
• 2) The WS group is susceptible to invasion by
  defectors
• 3) The WS have a positive effect on the SM
  types, while the SM types have a negative effect
  on the WS type.

PARTNER
D
WS
C

SM

WS
C

PLAYER
Cooperation is not an ESS
D
SM

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Being a Wrinkly Spreader is Costly

• Rainey Rainey placed a the bacteria producing
  the SM and WS at equal density in a flask
  competing for common resources.
• By measuring the density of WS cells and SM
  cells at the beginning and end of a growth cycle,
  the authors could calculate relative fitness
• The authors find that the WS type loses in
  pairwise competition to the SM type

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A WS Population is Susceptible to Defection

• The authors initialized a culture with only the
  WS type.
• By Day 5, colonies that resemble the SM appear,
  a de novo SM type.
• The de novo SM type does not significantly
  differ in fitness in pairwise competition with
  original SM type.
• The de novo SM type does not form mats or any
  type of aggregative structure

WS
WS
WS
WS
SM
SM
SM
WS
de novo defectors (SM)
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Facilitation and Debilitation

• The authors compared the performance of each
  strain alone and in combination with the other.
• The presence of WS increases the density of SM
  (mat hitch-hiking defectors)
• The presence of SM decreases the density of WS
  as the mat collapses early (a realization of the
  tragedy of the commons)
• Defecting SM types appear to weaken the
  integrity of the mat
• -They checked the integrity with a glass bead
  technique (a mat without SM defectors holds about
  5 times as much weight as a mixed mat).

SM
with
WS
without
WS
WS
without
SM
with
SM
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Cooperation in a Microbial World

• They claim the de novo generation of the Wrinkly
  Spreader morph is an evolutionary transition
  (from individual cells to a biofilm group)
• Production of the polymer is costly and thus
  susceptible to defection from within, however,
  it may evolve over and over if individuals tend
  to similar to those they stick to (i.e., kin
  selection argument)
• This is an example of cooperative behavior
  (e.g., as laid out by the Prisoners Dilemma) and
  may be an ideal system to test theories about the
  evolution and maintenance of cooperation.

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The Evolution of Cooperation

• Lecture Outline
• Introduction to cooperation theory
• Examples of cooperation
• Cooperators in a sticky situation
• Slime mold with greenbeards
• Summary

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Greenbeard Theory

• Consider a (haploid) population with two alleles
  at a given locus, G and N.
• Individuals with genotype G have a green beard,
  while individuals with genotype N have no beard.
• G does the following
• 1) The allele produces a perceptible trait (a
  green beard)
• 2) The allele allows for recognition of the
  trait in others
• 3) The allele allows for preferential
  treatment based on phenotype

Take 5 minutes to talk to your neighbor about the
following 1. Does such an allele sound
plausible to you? 2. If so, how frequently
would you expect find such an allele? 3. Would
greenbeards be immune or susceptible to cheaters?
4. Assume you have found an allele that you
suspect is a greenbeard how would you
experimentally demonstrate it?
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Cooperation in Slime Mold

• Dictyostelium discoideum is a protist that
  forages in the soil as single cells
• The life cycle can be broken down into three
  stages
• Aggregation when cells starve they come together
• Migration The collection of cells move as a
  slug
• Culmination A fruiting body is formed in which
  non-reproducing cells form a stalk to hold up
  reproductive spores.
• Stalk cells are altruists, sacrificing future
  reproduction to help disperse the spores in their
  collection.
• Why dont cheats (cells that preferentially get
  into the spore head) invade the system?

cheat
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Discovery of Greenbeard

• Queller, Strassman colleagues studied the
  effect of different alleles in the csA locus.
• This gene codes for a cell adhesion factor
  important in homophilic binding.
• Cells with this gene knocked out lack this
  adhesion protein.
• In foraging mixtures with equal proportions of
  the wildtype and the knockout, slugs form with
  82 wildtype due to the homophilic binding.
• csA is a greenbeard!
• 1. The allele produces a perceptible trait (an
  
• adhesion protein)
• 2. The allele allows for recognition of the
  trait in
• others (through homophilic binding)
• 3. The allele allows for preferential
  treatment
• based on phenotype (only cells that enter
  
• the slug have a chance to become spores)

wildtype
knockout
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The Evolution of Cooperation

• Lecture Outline
• Introduction to cooperation theory
• Examples of cooperation
• Cooperators in a sticky situation
• Slime mold with greenbeards
• Summary

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Summary

• The standard Darwinian picture is that selfish
  variants should always displace their cooperative
  competitors (individuals that provide a benefit,
  b, at a personal cost, c). This expectation is
  belied by many instances of cooperation and
  altruism in biological systems.
• Such cooperation can be favored if interactors
  are related. Hamiltons rule formalizes this
  relationship cooperation evolves if b/cgt1/r.
• More generally, cooperation is favored if
  cooperators have ways to distinguish cooperators
  from defectors and preferentially interact with
  cooperators.
• Cases of cooperation have been discovered in
  organisms ranging from bacteria to turkeys to
  primates.
• Experiments have been designed to explore the
  susceptibility of cooperators to cheats (mat
  formation in Pseudomonas) and the exclusion of
  cheats by cooperators (greenbeards in
  Dictyostelium).