how does cooperation evolve?

1. how does cooperation evolve? cooperation => group evolution => natural selection => mechanism of evolution of cooperation is group selection

2. factors determining strength of group selection • local fitness effects genes which give the individual higher fitness are selected • genetic structure groups are defined by the sharing genetic structure, i.e. cooperation

3. evolution of altruism by group selection (Pepper & Smuts 2000) investigate the effects of • varying ecology • group selection + kin interaction VS group selection – kin interaction • alarm calling VS restrained feeding

4. agent-based model world • 2D wrap around lattice agents • plant • forager

5. model continued linear plant behaviour • grow • linear • logistic • be consumed logistic

6. model continued forager behaviour • movement same as sugarscape with vision = 1 and can move into any of 8 cells • death same as sugarscape with forager lifetime = infinity • reproduction reproduce asexually when energy >= fertility threshold parent energy -= child initial energy child born in cell closest to parent

7. model continued cooperation • alarm calling • feeding restraint

8. model continued targeted individual Range around it in which foragers will give alarm calls

9. model continued forager has 0.02 probability of being targeted alarm callers will respond if within 5 cells of targeted forager probability of kill = 1 / ( n + 1 ) where n is the number of alarm callers targeted forager can not make an alarm call kill population = alarm callers + targeted forager a random forager is chosen from the kill population

10. model continued Restrained feeders consumption = 0.5 * plant energy Unrestrained feeders consumption = 0.99 * plant energy 50% plant size 99%

11. model continued

12. model continued patch gap width patch width

13. uniform environment (one plant per cell) patch width = 529 gap width = 0 alarm-caller non-caller mixed population pure population

14. uniform environment (one plant per cell) patch width = 529 gap width = 0 restraint feeding non-restraint feeding mixed population pure population

15. discussion of results (pure population) who cares tells us nothing about between-group selection since there is only one group

16. discussion of results (mixed population) local fitness effects • group selection ignores suboptimisation problem within cooperative group (Heylighen 1997) fitness(non-cooperators) > fitness(cooperators) genetic structure • cooperative systems eroded from within by genetic competition (Campbell 1983) mixed population => non-cooperative genes selected => local fitness and genetic structure effects not strong enough for group selection to occur

17. variable environment (mixed population) population = 0.5 * alarm caller + 0.5 * non-alarm caller

18. variable environment (mixed population) population = 0.5 * restraint feeder + 0.5 * non-restraint feeder

19. discussion of results (mixed population) local fitness effects • population size must be small (Futuyma 1986) small patch width + high gap width => many small population groups groups a #(cooperators) >> #(non-cooperators) groups b all other groups fit into groups b

20. discussion of results (mixed population) local fitness effects continued • altruistic group has higher fitness due to synergy of cooperation (Heylighen 1997) fitness(groups a) > fitness(group b)

21. discussion of results continued (mixed population) genetic structure • there can not be significant gene flow (Futuyma 1986, Goldstein & Zimmerman 2000) • migration rates must be implausibly low (Ridley 1993) low patch size + high gap width + low vision => low probability of migration => gene flow => reduced probability of non-cooperator infiltration of groups a

22. discussion of results continued (mixed population) genetic variance continued • successful groups must be able to export their local productivity from the local area (Wilson et al 1992) patch full => steady emigration fitness(cooperator) > (non-cooperator) => higher probability of successful colonisation for cooperators than non-cooperations difficulty of migration => infiltration of non-cooperators low => local fitness and genetic structure effects are strong enough in some scenarios for group-selection => cooperation evolves

23. variable environment (mixed population + absence of kin assortment) alarm calling never evolved in any of the 100 runs BUT restraint feeding did

24. discussion of results (mixed population + absence of kin assortment) local fitness • alarm calling can only spread if foragers are heavily recompensated by others increasing their fitness relative to themselves (Wilson 1979, 1980) recompensation comes through spatial association to cooperators cooperators <=> kin spatial association was removed largely by randomising birth locations fitness(alarm callers) < fitness(population)

25. discussion of results (mixed population + absence of kin assortment) local fitness continued however, feeding restraint conferred benefits as well as costs on the bearer => fitness(restraint feeders) > fitness(alarm-callers)

26. discussion of results continued (mixed population + absence of kin assortment) genetic structure • kin selection increases genetic selection between-groups and decreases it within-groups (Smith 1964) spatial association <=> kin discrimination randomised birth starting location => kin selection was not operating => selection between-groups was reduced

27. discussion of results continued (mixed population + absence of kin assortment) genetic structure continued migration rates must be implausibly low (Ridley 1993) there can not be significant gene flow (Futuyma 1986, Goldstein & Zimmerman 2000) random birth locations => mixed population => gene flow => non-cooperators selected over cooperatots => local fitness effects and genetic structure are not enough for between-group selection to occur for alarm callers

28. discussion of results continued (mixed population + absence of kin assortment) genetic structure continued however restraint feeders were selected when patch width low and gap-width high small group size => restraint feeder becomes an increasing proportion of the acts recipients => kin selection was not needed => local fitness effects and genetic structure were strong enough for the evolution of feeding restraint

29. summary evolution of cooperation • favored by group-selection diminshed by within-group selection • evolution of cooperation is dependent on ecological patchiness small patches and large gaps stabilise degree of migration strong vs weak altruism

30. critique • kin selection there was no kin discrimination rule but the rule is defined in biology • reproduction reproduction was asexual and the offspring were the genetic clones of their parents whereas the rules of genetics are well established • movement movement rule had vision of 1 which made migration difficult if not impossible

31. critique continued • model parameters the starting population size was 40 which is small the size of the world was not given, the assumption is x = y = 527 which is small • death foragers lived forever, a more realistic life expectancy was given in sugarscape • simple not a very sophisticated model

32. references d. j. Futuyma, evolution biology, 1986 t. h. Goldsmith, w. f. Zimmerman, biology, evolution, and human nature, 2000 f. heylighen, http://pespmc1.vub.ac.be/COOPGEVO.html, genetic scenarios for evolving cooperation, 1997 j. w. Pepper, b. b. Smuts, the evolution of cooperation in an ecological context: an agent-based model, 2000 m. Ridley, evolution, 1993