Adaptive Evolution of Social Traits

1. Adaptive Evolution of Social Traits

2. Kinds of social structures • Solitary breeding • Colonial breeding • Communal breeding • Cooperative breeding • Helpers at the nest • Eusociality Increasing sociality

3. Altruism • Altruistic behaviors impart direct costs • Negative effect on actor’s reproductive success • … and indirect benefits • Positive effect on recipient’s reproductive success • The origin and persistence of altruism • How can an altruistic strategy spread in a selfish population? • How can an altruistic strategy resist selfish cheaters?

4. The genetic basis of altruism • Models assume genetic basis of variation • Genetic variability in stem-ness of slime molds

5. Plasticity & altruism • Status in eusocial insects • Nutrition, pheromones, age, and social environment • Habitat saturation promotes helping in the Seychelles warbler Smith et al. 2008 J Insect Behav 21:394-406

6. Direct costs and benefits • Costs • Present breeding • Future survival, fecundity • Benefits • Reciprocity • Inheritance • Breeding status • Learned skills • Territory • Coalitions • Prestige White-winged chough

7. Indirect costs and benefits • Costs • Increased competition • Brotherly fights in fig wasps • Benefits • Helpers at the nest actually do help • Food provisioning • Predator protection • Reduce work load on breeding pair Florida scrub jay Helpers removed Controls

8. Hamilton’s rule (revisited) • Consider a large, stationary population in which altruism is initially rare • ρ = mean encounter rate of altruists • Altruists growth rate = 1 – c + ρ × b • Selfish growth rate = 1 • Altruists grow if 1 – c + ρ × b > 1 • Rearrange to ρ > c / b • Hamilton’s rule states altruistic allele will grow if r > c / b • It turns out that r = ρ

9. Ecology of unconditional altruism • Spatial structure of kinship • Altruistic core, variable front • Kin competition • Solutions • Competition does not completely overlap cooperation in time /space • Mobility and altruism • High cost to mobility promotes altruism • Mobile cheaters prosper

10. Genetics of unconditional altruism • High relatedness promotes altruism • Haplo-diploidy in Hymenoptera • Haploid males • Diploid females • If monogamy… • r sisters = 0.75 • r mother-daughter = 0.50 • Relatedness decreases with multiple queens, fathers • Constraints, high benefits of altruism maintain sociality

11. Group augmentation • If a big group is good for you, and cooperation helps your group grow, cooperation may be worth the costs • Big group advantages • Foraging • Predator avoidance • Reproduction • Group-level competition

12. The prisoner’s dilemma • The setup • One defects • Freedom / 10 yrs • Both defect • 5 yrs each • Neither defects • 6 months each • Solution • Always defect! • Iterated PD’s has other solutions…

13. Conditional altruism: Donor sensitivity • Advantages of conditional altruism • Tit-for-tat (TFT) • Win-Stay-Lose-Shift (WSLS) • Memory helps • Individual recognition • Examples of reciprocal altruism in nature are rare • Continuous donation makes it hard to evolve

14. Conditional altruism: Recipient properties • “Nepotism” • Mechanisms • Direct recognition of genetic similarity • Familiarity • Preferentially donate to reputable individuals • “Image score” and indirect reciprocity • Requires strong cognitive abilities

15. Regulating social conflict • The tragedy of the commons • Volunteering • Participation is optional, costly • Specialization • Reproductive skew • Cheaters suffer • Social control • Breeding suppression • Usurpation stopped by queen & workers • Incentives / concessions to help