THEME 3 Flashcards
What is reciprocation?
Whenever the benefit of an altruistic act to the recipient is greater than the cost to the actor, then as long as the help is reciprocated at some later date, both participants will gain
What is the problem for its evolution in animal population?
The possibility of cheating
Because of the time delay between one individual gaining and the other doing so, one may accept help from another today but refuse to repay the favour tomorrow
Be able to briefly explain the “Prisoners Dilemma”
The problem is that while an individual can benefit from mutual co-operation, it can do even better y exploiting the co-operative efforts of others
What is the only stable strategy if two players meet only once?
Defect is the only stable strategy, defection will be optimal on the last encounter and therefore also on the next-to-last and so on, back to the first encounter.
Be able to explain the “tit for tat” strategy
Tit for tat is a rule that can be evolutionarily stable in the Prisoners dilemma game
Co-operated on the first move and thereafter did what the opponent did on the previous move, strategy of co-operation based in reciprocity
Why is tit for tat successful strategy
- It was retaliatory – which discouraged the other side from persisting whenever a defection was tried
- It was forgiving – after just one act of retaliation, which helped restore mutual co-operation and the greater rewards this gave
What two conditions must hold before co-operation based on reciprocity can spread to be stable?
- Cooperation could emerge at first between pairs of relatives, evolving by kin selection.
- Selection would favour recognition of cues which give indications of relatedness. One cue that could be used is simply is simply the fact of reciprocation of co-operation.
- Then individuals could use rules such as “co-operate if the other co-operates” and co-operation could now emerge - When most of the population is using “always defect” is clustering
- If tit for tat individuals occur in clusters then they can interact with each other more than expected from random encounters in the population
- Enjoy increased benefits of mutual co-operation
- Clustering often associated with kinship and the two mechanisms can enforce each other in promoting the initial spread of reciprocal co-operation
Be able to briefly describe one example from nature where individuals do play tit for tat co-operation
Alliances in Primates
Grooming – most common form of affinitive behaviour therefore common for two individuals to do
Most grooming and alliances involve close relatives but sometimes friendships form between non-relatives
Field experiment (Seyfarth and Cheney) which showed that grooming between unrelated Vervet moneys increased probability that they subsequently attended to each-others solicitations (act of asking for or trying to obtain something from someone) for aid
What is an ESS?
Strategy which when adopted by most members of the population cannot be beaten by any other strategy in the game
What is the ESS for a war of attrition?
War of attrition or “the waiting game” – winners are decided by a contest involving waiting or displaying for different lengths of time
The ESS for this game is to play a randomly chosen waiting time – be unpredictable
How could this stable solution come about?
- Every individual in the population could play the same variable strategy, or mixed ESS. Every individual would sometimes wait for a long time and sometimes for a short time, and the times would be selected from the appropriate random distributions
- There could be polymorphism in the population with every individual playing a fixed waiting time, or pure strategy, but with the frequencies of the strategies following the random distribution. There would be a mixture of pure strategies, with most individuals playing short waiting times and fewer playing longer times
How would “equality of success” come about at the ESS?
At the ESS, the payoffs for the different waiting times will be equal - Equality of success will come about through frequency-dependent selection. If short waiting time increases eventually there will be more and more competition at this waiting time therefore at this point the increase in proportion of the short stay times will come to stop when this strategies success equals that of the other strategies
Be able to briefly explain why a mixture of Hawks and Doves could be stable? When does each strategy do best?
The stable equilibrium will be when the average pay offs for a Hawk are equal to the average payoffs for a Dove. If the population moved away from the equilibrium then either Dove or hawk would be doing better so the population would not be stable.
Each strategy does best when it is relatively rare.
How will frequency-dependent selection drive the frequencies of Hawk and Dove in the population?
The tendency will be for frequency-dependant selection to drive the frequencies of Hawks and Dove in the population so they each enjoy the same success.
What are the 3 main conclusions of Maynard-Smiths model?
- The most important point is that the best fighting strategy for any one individual must depend on what other competitors are doing, because the payoffs for employing a strategy will be frequency dependant. Is Hawk a good strategy? Yes if the population consists mainly of Hawks.
- The ESS will depend on the strategies in the game. In the first example where there were just two strategies, the ESS was a mixture of Hawk and Dove. However, when we introduced another strategy, Bourgeois, the ESS solution changed, Bourgeois turned out to be a pure ESS.
- The ESS will also depend on the values of the payoffs in the game. If we changed the payoffs in the first table then the ESS mixture of Hawks and Doves would change. I this game it can be shown that as long as the cost of injury exceeds the value of winning then the ESS is a mixed one.
What are individuals fighting for in such species?
Mating with females or other resources – mainly fighting for genetic life or death
The value for resource probably exceeds the cost of injury. We would expect Hawk-like strategies because failure in a contest could mean failure to pass on genes to future generations.
Where the value for resource is less valuable or where cost f fight is high would expect Bourgeois (owner wins, intruder retreats)
Why do “owners” of resources often tend to win encounters?
Owners may win for three reasons (hypotheses)
- They are better fighters. This may be the reason why they got ownership in the first place
- They have more to gain from a fight and so are prepared to fight harder. Owners may value their resources more than intruders
- Arbitrary asymmetry of ownership settles the contest. Ownership is simply a conventional settlement
Which of the 3 hypotheses was supported by Kreb’s experiment with Great Tits?
Removed owners and kept them in a cage, newcomers then took over the territory. According to:
Hypothesis 1 = owner should win back its territory through superior fighting ability
Hypothesis 2 = gradual reversal of dominance in favour of newcomer with increased time on the territory, which allows newcomer to learn territory characteristics and so gain greater value from winning a contest
Hypothesis 3 = newcomer will win simply because it is now in the role of owner
Kreb’s experiment supported the
SECOND hypothesis
How do “real” contests in nature differ from the Hawk-Dove model?
Real contests in involve sequences of various displays in which individuals compete to assess each others fighting ability, something which is not incorporated in the Hawk-Dove model
What does Anisogamous mean?
fusion of gametes of unequal size
Evolved from isogamous sexual reproduction
How might Anisogamous gametes have evolved?
Parker 1927
- He assumes that the survival of the zygote is increased with size
- Once large gametes evolved, immediate selection for smaller gametes to seek out large ones to fuse with (cheat the system)
- Tiny amount of competition between sperm and rival male is enough to maintain anisogamy
- Once this situation is in place its almost impossible to get away from
