Ecology & evolution - cooperation

How is there cooperation built into biology

Genes cooperate into a genome → into a cell → into multicellular organism → into society → into interspecies mutualism

What threatens cooperation, and how does evolution address the problem

Organisms can ‘cheat’ - average fitness of a population declines if one individual can easily take over

So there’s natural selection of social behaviour to avoid cheating

Different interactions between recipient and host

Mutual benefit, selfishness, altruism, spite

What factor is key to determining levels of cooperation

Relatedness → can be influential in cooperative behaviour

Relatedness factor to various familial members

And how is this different in natural scenarios

Parents/offspring → ½
Siblings → ¼
In nature there is a lot more variation in relatedness e.g. meerkat colonies with new dominant males/females, juveniles less related to them

What is Hamilton’s rule, and what does it represent

An action will be favoured by natural selection if: r.b - c > 0

r → relatedness factor

b → benefit (increase in offspring) to recipient

c → cost (reduction in offspring) to actor

What is direct fitness

Offspring of the actor (not including offspring that social effects have helped to have)

What is indirect fitness

Offspring of recipient that actor’s actions have helped have

What is inclusive fitness

Direct fitness + indirect fitness x relatedness factor

What is neighbour-modulated fitness

All an individual’s actual offspring - but some may have been had not through their own effort, but by neighbour’s help

Why is is necessary for cooperation to have developed without relatedness

Needed for egalitarian scenarios - where different cooperators come together with no relatedness

E.g. species in mutualistic relationships

What are the ways that cooperation exists in egalitarian scenarios

Unenforced, reciprocity, policing, imposed sanctions (rewards/punishments)

How can cooperation be unenforced (give example)

If the gain from cooperation is so large there is no alternative (or the loss from no cooperation is so large)

E.g. queen ants will cooperate and build nests together after mating. Cannot fly away and only one queen inherits all the eggs, so fight to the death. But favoured over having to build a nest alone.

How can cooperation be enforced through reciprocity (give example)

Repeated actions between same partners with direct benefits, and consequences if the action is not completed

E.g. cleaner fish for larger fish - larger fish have their parasitism controlled and cleaner fish get food.

If larger fish ate the cleaner fish, wouldn’t be able to find any other cleaner fish.

If cleaner fish took a bit out of the larger fish, wouldn’t be able to find any other clients.

Link between cooperation/reciprocity and game theory

Interactions can be modelled with game theory - like the ‘tit-for-tat’ model, or grim strategy

How can cooperation be enforced through policing (give example)

Systematic policy that prevents cheating

E.g. female worker insects collaborate running a nest looking after the queen’s offspring (more related to offspring of the queen than offspring of other workers as the queen usually has lots of mates). Possible cheat would be workers laying own eggs, but prevented due to systematic removal of the worker-laid eggs.

What is the maths behind the cooperation enforced by policing of female worker insects

Female workers’ decision on whose offspring to rear comes from relatedness. If p is chance the workers are siblings, high p means more likely to help worker-laid males.

Regression relatedness from workers to queen-laid males is always ½ (workers are female offspring of the queen so 1/2, and the males have all genetic material from queen).

Regression relatedness from workers to worker-laid males depends on relatedness - (1+2p)/4

Critical value of p is 1/2, if p is less than ½ than queen-laid males are favoured over worker-laid males for rearing.

How can cooperation be enforced through imposed incentives (give example)

Cheating doesn’t pay off due to rewards and punishments.

E.g. soybeans and Rhizobium bacteria. Soybeans will withhold oxygen from Rhizobium that do not fix nitrogen (attempted cheating)

In what scenarios is relatedness powerless and in what scenarios can it be expected

Powerless in egalitarian scenarios

Expected when observing fraternal cooperation

Why do you expect relatedness when observing fraternal cooperation

Because individuals can have indirect benefits through each other’s offspring

r.b - c > 0 → if r=0 (recipient is a non-relative) then only the effect on the actor matters.

What is kin discrimination (give examples) and why is it important

Ability to tell apart related individuals

Effects cooperation as organisms can choose who to help

E.g. long-tailed tit, if failed to breed on its own, will go and help others - 96% of the time being a relative

Slime molds preferentially form fruiting bodies with members of the same lineage

What is population viscosity and what is its impact

Offspring and kin do not travel far → have limited dispersal

Makes it more likely for those in a population to be closely related, reduces the need for kin selection

How often does population viscosity lead to altruism/cooperation

Could make it more likely for cooperation to occur as individuals more likely to be closely related, BUT most of the time it doesn’t lead to altriusm because of density-dependence

Impact of density-dependence

If the population stays stable and there’s the same number of offspring each year, any net benefit to an individual in the population of ‘b-c’ must come with a loss of ‘b-c’ spread across the population

So the actor/individual’s relatedness to the general population also has an impact, not just relatedness to the recipient of cooperative behaviour.

Adaptation of Hamilton’s rule to account for density-dependence. Extreme example of relatedness of actor to recipient = relatedness to random individual

r.b - c - r_e.(b-c) > 0 for the action to be favoured

If relatedness to recipient and random individual is equal, then r = r_e → equation becomes -(1-r)c > 0 and b is irrelevant

Relatedness no longer matters, actor should be selfish - population viscosity does not automatically lead to cooperation/altriusm

Field evidence for power of relatedness and cooperation

Field turkeys - some males act as wingmen and help others find a mate, dependent on relatedness. Hamilton’s rule was used to weigh benefits (offspring with some shared genetic elements) against cost (no. of successful courtships if he wasn’t a wingman) and helping was favoured.

Even greater positive outcome if the wingman was a weedier/weaker turkey and wouldn’t have had as many successful matings.

Experimental evidence for power of relatedness and cooperation

Pseudomonas aeruginosa evolved siderophore production which was an altruistic behaviour as they are excreted into the medium to grab ions and be absorbed back, but other organisms can use those siderophores.

Relatedness determined willingness to produce siderophores. When r_e was positive (around sibling level of 1/2) siderophore production was highest, whereas when r_e was 0 siderophore production was lowest

Comparative evidence for power of relatedness and cooperation

Can look at the tree of life and see where eusocial societies evolved (heavily rely on cooperation)

Shown that kin selection (relatedness) is key to eusocial evolution, as all the eusocial groups had monogamous ancestors which creates high-relatedness family groups.

What is the green beard idea

Specific genes can be recognised, specific phenotypic characteristic looked for.

Genes theoretically could identify themselves in another, relative relatedness would be 1.

Alleged examples of green beards

Solenopsis invicta - worker’s kill queens with different genotypes.

But many argue this is reductive, and the killing of the specific genotypes is to maintain ecological balance - not to help others with the same genotype.

How does relatedness impact the ‘strategies’

Siblings more likely to carry out the ‘tit-for-tat’ strategy