Chapter 12: Family, Society, and Evolution

1. Chapter 12: Family, Society, and Evolution Robert E. Ricklefs The Economy of Nature, Fifth Edition (c) 2001 W.H. Freeman and Company

2. Background • The behavior and, indirectly, life histories and ecological relationships of an individual are under strong selective pressure from: • the social and family environment • relationship to members of both sexes • For example, fitnesses of the male morphs of the side-blotched lizard are dependent on frequencies of other male morphs in the population: • these morphs interact through complex social interactions that determine reproductive success (c) 2001 W.H. Freeman and Company

3. Background • Individuals interact with other members of the same species throughout their lives. • Each individual must perceive the behaviors of others and make appropriate responses: • some interactions pay benefits for cooperative behaviors because of a common interest: • interactions with kin (common evolutionary heritage) • interactions with mates (common interest in success of offspring) (c) 2001 W.H. Freeman and Company

4. Cooperation or Competition? • All interactions between members of the same species delicately微妙地 balance conflicting tendencies of cooperation and competition, altruism利他主义 and selfishness. • Such a balance is evident in humans, the most social of animals: • society is sustained by role specialization • social life balances cooperation and conflict (c) 2001 W.H. Freeman and Company

5. What is Social Behavior? • Social behavior includes all interactions among individuals of the same species. • These interactions range from cooperation to antagonism对抗. • Consequences of these interactions for individuals are substantial, with effects on individual fitness. (c) 2001 W.H. Freeman and Company

6. Territoriality领域性 • Any area defended by an individual against the intrusion入侵 of others may be regarded as a territory: • territories vary enormously in size and permanence • animals are likely to maintain territories if: • the resource is defensible • the rewards outweigh the cost of defense (c) 2001 W.H. Freeman and Company

7. Dominance Hierarchies优势等级 • Defense of territories may not always be practical. • In absence of territories, the outcome of conflict may be establishment of social rank. • When individuals order themselves by social rank or status, the result is a dominance hierarchy. • Social rank and occupancy of space may be directly related, as low-ranking individuals may be relegated to the periphery外围 of a flock集群. (c) 2001 W.H. Freeman and Company

8. To fight or not to fight? • Establishment of territories or social rank depends on the outcome of contests between individuals. • In any confrontation对抗, participants must weigh: • costs of fighting and benefits of winning • likely outcome of the contest • Determining optimal behavior is complicated by each individual’s lack of knowledge about the behavior of the other participant. (c) 2001 W.H. Freeman and Company

9. Optimal Behaviors and Game Theory博弈论 • Game theory analyzes the outcomes of behavioral decisions when these outcomes depend on the behavior of other players. • Game theory predicts the individual’s behavior based the best estimates of: • the other contestant’s response • the reward for winning (c) 2001 W.H. Freeman and Company

10. Advantages and Disadvantages of Living in Groups • True social groups result from a purposeful joining together of individuals. • Living in groups results in benefits and costs to flocking群集 birds, like the European goldfinch金翅雀: • benefit is less individual vigilance警戒 • cost is the more rapid depletion耗尽 of resources, forcing the flock to move more frequently (c) 2001 W.H. Freeman and Company

11. Natural selection balances the costs and benefits of behaviors. • Toward a classification of behaviors: • Most social interactions can be broken into acts performed by: • donors - individuals initiating behaviors • recipients - individuals toward whom behaviors are directed (c) 2001 W.H. Freeman and Company

12. A Classification of Behaviors • Four combinations of fitness increments增长 to donor and recipient lead to the following classification: • cooperation (benefits donor, selected for) • selfishness (benefits donor, selected for) • Spitefulness恶性 (benefits no one, selected against) • altruism (benefits recipient at cost to donor) • Altruism, among these, is most problematic: • selfish behaviors would be expected to prevail盛行 • yet altruistic acts are common in social species (c) 2001 W.H. Freeman and Company

13. Kin selection favors altruistic behaviors. • When an individual directs a behavior toward a sibling or other close relative, it influences the fitness of an individual with whom it shares more genes than it does with an individual drawn at random from the population. • This special outcome of social behavior among relatives is called kin selection. (c) 2001 W.H. Freeman and Company

14. Identity by Descent血统识别 • The likelihood that two individuals share copies of any particular gene is the probability of identity by descent, which varies by degree of relationship: • also called the coefficient of relationship • full sibs have a 50% probability of sharing any gene • parents and children also have 50% probability of sharing any gene, etc. (c) 2001 W.H. Freeman and Company

15. A Model for Assessing Altruistic Behavior • Total fitness of a gene responsible for a particular behavior is its inclusive fitness: • contribution to fitness of donor plus product of change in fitness to recipient X, weighted by coefficient of relationship • a gene promoting altruistic behavior will have a positive inclusive fitness if: C < Br where: C = cost to donor B = benefit to recipient r = coefficient of relationship (c) 2001 W.H. Freeman and Company

16. Implications of the Model • Genes for altruistic behaviors should increase in the population when: • behaviors have low cost to donor • behaviors are restricted to close relatives • Opportunities for evolution of altruistic behaviors do exist: • individuals often associate in family groups • individuals can often assess their relatedness (c) 2001 W.H. Freeman and Company

17. Cooperation among Individuals in Extended Families • Complex relationships among extended human families are familiar to us: • often such families include only one child-producing pair • a portion of the behavior of non-nuclear members of the extended family are directed toward well-being福祉 of these related children (c) 2001 W.H. Freeman and Company

18. Cooperation in Bee-Eaters • Extended families of bee-eaters exhibit cooperative and competitive behaviors: • selfish and selfless acts are directed toward others in direct accordance with the degree of relationship • inclusive fitness is the appropriate measure of selection on social behavior: • altruistic behaviors can evolve among close relatives by kin selection (c) 2001 W.H. Freeman and Company

19. Cooperation Among Unrelated Individuals • Social groups can form to promote mutual self-interest互利营私 of unrelated individuals. • Can groups of unrelated individuals move toward true cooperation? (c) 2001 W.H. Freeman and Company

20. Game Theory and Cooperation • The paradox悖论: • conflict can reduce the fitness of selfish individuals below that of cooperative individuals, so cooperative behaviors should evolve among unrelated individuals • but, when most of a social group consists of cooperative individuals, a selfish individual can achieve high fitness by cheating (c) 2001 W.H. Freeman and Company

21. The Hawk-Dove Game • The hawk-dove game (prisoner’s dilemma): • a hawk always competes over resources, taking all the rewards when it wins: • the hawk strategy is not the best overall because hawks incur招致 costs of conflict • a dove never competes over resources, sharing resources with other doves, yielding them to hawks: • the dove strategy is the best overall because resources are shared without costs of conflict (c) 2001 W.H. Freeman and Company

22. Hawks invade societies of doves. • Dove behavior is not an evolutionarily stable strategy: • a population of doves is easily invaded (from an evolutionary perspective) by hawkish behavior: • a hawk in a population of doves reaps得到 twice the rewards of doves • a population of hawks is resistant to invasion by dove behavior, however (c) 2001 W.H. Freeman and Company

23. Can hawks and doves coexist? • When the benefit is less than twice the cost of conflict, dove behavior can invade a population of hawks. • In this situation the proportion of hawks is one-half the ratio of the benefit to cost. • Persistence持续 of hawks and doves in this case is an evolutionarily stable mixed strategy. (c) 2001 W.H. Freeman and Company

24. Parents and offspring may come into conflict. • Offspring consume parental resources, but this is desirable from the perspective of the parents: when progeny thrive繁荣, so do the parents’ genes. • Parents and offspring come into conflict when accumulation of resources by one offspring reduces the overall fecundity生育力 of its parents. (c) 2001 W.H. Freeman and Company

25. Parents and offspring have different goals. • Offspring try to resolve conflicts over resources in favor of their own reproductive success. • For parents, a balanced approach to current and future reproduction is favored: • resources allocated to one offspring cannot be allocated to another • resources allocated to current offspring reduce those that can be allocated to future offspring (c) 2001 W.H. Freeman and Company

26. When does parent-offspring conflict occur? • As young mature, the benefit to them of parental care declines. • Because of coefficients of relationship among parents, an offspring, and that offspring’s sibs: • when the benefit to parent of providing additional care falls below the cost of this care for future reproduction, the parent should cease停止 providing care • offspring should continue to request additional care until the benefit to parent of providing that care falls below twice the cost of this care for future reproduction (c) 2001 W.H. Freeman and Company

27. Eusocial群居的 Insect Societies • Social insects exhibit the extreme of family living, in which most offspring forego放弃 reproduction and help their parents raise siblings. • This situation raises evolutionary questions: • how did such societies evolve? • how can natural selection produce individuals with no individual fitness? (c) 2001 W.H. Freeman and Company

28. What is eusociality真社会性? • Eusociality entails: • several adults living together in groups • overlapping generations • cooperation in nest building and brood care • reproductive dominance by one or a few individuals, including the presence of sterile castes • Eusociality is limited among insects to Isoptera 等翅目(termites白蚁) and Hymenoptera膜翅目 (ants, bees, wasps), and to one mammal, the naked mole rat裸耳鼠. (c) 2001 W.H. Freeman and Company

29. How did eusociality evolve? • Potential sequence of evolutionary events: • parents have a lengthened延长 period of care for developing brood 孵蛋(parents guard brood or provision larvae) • parents live and continue to produce eggs after first progeny emerge • offspring are in a position to help raise subsequent broods • when progeny remain with their mother after adulthood成年, the way is open to relinquishing放弃 reproductive function to support mother’s (c) 2001 W.H. Freeman and Company

30. Organization of Insect Societies • Insect societies are dominated by one or a few egg-laying females, queens: • queens of ants, bees, and wasps mate once and store sufficient sperm to produce a lifetime of offspring • Nonreproductive progeny of the queen: • gather food and care for their developing brothers and sisters, some of which become sexually mature and leave the nest to mate • Specific details vary somewhat for termite白蚁 colonies, which are headed by a king and queen. (c) 2001 W.H. Freeman and Company

31. Coefficients of Genetic Relationship in Hymenoptera膜翅目 • Hymenoptera have a haplodiploid sex-determining mechanism: • females (workers) develop from fertilized eggs • males (drones雄峰) develop from unfertilized eggs • Coefficients of genetic relationship are skewed: • female worker to female sibling is 0.75 • female worker to male sibling is 0.25 • queen to son or daughter is 0.5 • Sex ratios are female-biased, 3:1. (c) 2001 W.H. Freeman and Company

32. Summary • All behaviors have costs and benefits to the individual and to others affected by the behavior, with special consequences for close relatives. • Behavior is influenced by genetic factors and is thus subject to evolutionary modification by natural selection. • Interactions within a social setting lead to important evolutionary consequences when interests of individuals conflict or coincide一致. (c) 2001 W.H. Freeman and Company