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Conflict and Cooperation

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  1. Conflict and Cooperation Lecture 7

  2. Lecture Outline Evolutionary game theory Sexual selection Kin selection and the evolution of cooperation Selfish genes

  3. John Maynard Smith John Nash William Hamilton

  4. Evolutionary Game Theory Evolutionarily Stable Strategy (ESS) is a strategy which, if adopted by a population of players, cannot be invaded by any alternative strategy that is initially rare • Interactions between 2 individuals, each of which has 1 of 2 or more phenotypes (strategies) • {H,D} • For each possible pairwise combination of strategies, an individual has a different payoff • w(H,D) • The payoff depends not only on the individual’s own phenotype, but also on that of the individual it interacts with • w(H,H)≠w(H,D) Payoff Matrix

  5. Evolutionary Game Theory • The fitness of phenotypes H and D are: • w(H) = pw(H,H) + (1-p) w(H,D) • w(D) = pw(D,H) + (1-p) w(D,D) • where p is the frequency of Hin the population • H is an ESS if: • w(H)>w(D) • If D is a rare mutant (p≈1) this condition implies that: • either (a) w(H,H)>w(D,H) • or (b) w(H,H)=w(D,H) and w(H,D)>w(D,D)

  6. Evolutionary Game Theory Hawk-Dove Game One of the earliest problems to which evolutionary game theory was applied was conflict between males competing for resources • Hawk Escalate a conflict until it is injured (with cost C) or until it gains the contested resource (with gain V) • Dove Retreats when threatened neither gaining nor loosing fitness • When 2 phenotypes of the same kind coincide they have equal chances of wining or losing

  7. Evolutionary Game Theory Hawk-Dove Game • Pure Strategies • Play always the same strategy Is Dove an ESS? Is Hawk an ESS? • If V>C it pays to be a Hawk • If V<C no pure strategy is an ESS

  8. Evolutionary Game Theory Hawk-Dove Game • Mixed Strategies • Play sometimes one strategy and sometimes the other Is there a mixed ESS? • A mixed strategy p* requires that • w(H)=w(D) • p* ½(V-C)+(1–p*)V= p* 0+(1–p*) ½V • and p*=V/C (playing Hawk with probability V/C) is a mixed ESS The optimal behavior is variable and contingent on the values V and C

  9. Evolutionary Game Theory Hawk-Dove-Assessor Game Payoff Matrix Assessor Strategy Display first and escalate the conflict only if it judges the opponent to be smaller or weaker. If larger plays Hawk and wins if smaller play Dove avoiding the cost • Assumptions: • The probability of being larger is 50% • Assessment is costless and accurate

  10. Evolutionary Game Theory Hawk-Dove-Assessor Game Pure Strategies Is Hawk an ESS? Is Assessor an ESS? Many animals react aggressively or not depending on their opponents size or correlated features

  11. Evolutionary Game Theory Hawk-Dove-Assessor Game Many animals react aggressively or not depending on their opponents size or correlated features Male toads that clasps females before the eggs are laid and fertilized are often aggressively displaced by larger males but not by smaller ones. A male is unlikely to try to displace a mounted male that is larger than himself

  12. Evolutionary Game Theory Hawk-Dove-Assessor Game

  13. Evolutionary Game Theory Hawk-Dove-Assessor Game 14/50 approaches result into fight

  14. Evolutionary Game Theory Hawk-Dove-Assessor Game Honest signals of the of the individual’s fighting ability or resource-holding potential Deceptive signals indicating greater fighting ability than the individual actually has Deceptive signals should be unstable in evolutionary time because selection would favor genotypes that ignored the signals, which having lost their utility would be lost in subsequent evolution Dishonest signals however are not uncommon Male fiddle crab Uca annulipes

  15. Sexual Selection Concept Describes differences among individuals of a sex in the number of reproductive capacity of mates they obtain

  16. Sexual Selection Concept Sexual selection exists because females produce relatively few large gametes (eggs) and males produce many small gametes (sperm) • This difference creates an automatic conflict between the reproductive strategies of the sexes: • A male can mate with many females and he often suffers little reduction in fitness if he should mate with an inappropriate female • A female can be fertilized by a single male and her fitness can be significantly lowered by inappropriate mating

  17. Sexual Selection Concept The operational sex-ratio (relative number of males and females in the mating pool) is often male-biased because males mate more frequently Females are a limiting resource for males but males are not Variation in mating success is generally greater among males than females and is a measurement of the intensity of sexual selection Phalaropes: Sex-Role Reversal Phalaropus fulicarius

  18. Sexual Selection

  19. Sexual Selection Contests Between Males and Between Sperm Contest among males for mating opportunities Antlers of red deer Cervus elaphus Canine teeth of the babirusa Babyrousa babyrussa Directional selection for greater size, weaponry or display features can cause an arms race Such escalation becomes limited by opposing ecological selection if the cost of larger size or weaponry becomes sufficiently great

  20. Sexual Selection Contests Between Males and Between Sperm Males reduce the likelihood that other males’ sperm will fertilize a female’s eggs In many damselflies, the male’s genitalia are adapted to remove the sperm of previous mates from the female’s reproductive tract Lateral Horn Sperm competition Occurs when the sperm of two or more males have the opportunity to fertilize a female’s eggs Spinelike hairs and clump of rival’s sperm Penis of the black-winged damselfly Calopteryx

  21. Sexual Selection Sexual Selection by mate choice

  22. Sexual Selection Sexual Selection by mate choice Individuals of one sex (usually male) compete to be chosen by the other Females mate preferentially with males that have larger, more intense, or more exaggerated characters The preferred male characters are often ecologically disadvantageous Tungara Frogs Physalaemus pustulosus Frog-eating Bats Trachops cirrhosus Female choosiness may likewise have costs: the time spent searching for acceptable males has been shown to reduce reproductive output in several species

  23. Sexual Selection Sexual Selection by Mate Choice • Why should females have a preference for these traits, especially for features that seem arbitrary and dangerous the males bearing them? • Sensory bias • Direct benefit to choosy females • Indirect benefit to choosy females • Runaway sexual selection • Indicators of gene quality • Antagonistic coevolution

  24. Sexual Selection Sexual Selection by mate choice Sensory Bias Certain traits may be intrinsically stimulating and evoke a greater response simply because of the organization of the receptor’s sensory system In some species of the fish genus Xiphophorus part of the male tail is elongated into a “sword” Females preferred males with swords not only in Xiphophorus but also in the swordless genus Priapella

  25. Sexual Selection Sexual Selection by mate choice Direct Benefit to Choosy Females Male provides a direct benefit to the female or her offspring: nutrition, superior territory, parental care There is selection pressure on females to recognize males that are superior providers The preference selects for males with the distinctive correlated character

  26. Sexual Selection Sexual Selection by mate choice Indirect Benefits of Mate Choice Male provides no direct benefit to either the female or her offspring but contributes only his genes Alleles affecting female mate choice increase or decrease in frequency depending on the fitness of the females’ offspring. Thus females may benefit indirectly from their choice of mates

  27. Sexual Selection Sexual Selection by mate choice Runaway Sexual Selection Model (Sexy Son Hypothesis) The evolution of male trait and a female preference, once initiated, becomes a self-reinforcing or runaway process Consider haploid males of genotypes T1 (fr t1) and T2 (fr t2) T2 has an exaggerated trait (longer tail) that carries an ecological disadvantage (increasing the risk of predation) Females of genotype P2 (fr p2) prefer males of type P2, whereas P1 females exhibit little preference It is assumed that P1 and P2 are selectively neutral

  28. Sexual Selection Sexual Selection by mate choice Although the expression of genes P and T is sex-limited both sexes carry both genes and transmit them to offspring Because P2 females and T2 males tend to mate with each other linkage disequilibrium will develop Any increase in the frequency of the male trait is accompanied by an increase in the frequency of the female preference through hitchhiking

  29. Sexual Selection Sexual Selection by mate choice Because of the genetic correlation between loci, an increase in t2 is accompanied by an increase in p2: T2 males have more progeny and their daughters tend to inherit the P2 allele, so P2 also increases in frequency As P2 increases males have a still greater mating advantage because they are preferred by more females Many exaggerated sexually selected traits carry ecological costs for the males that bear them. Female choice may also carry an ecological cost (ex delayed reproduction) Such costs may prevent the runaway process from occurring, or they mat lead to an equilibrium at which the male trait and the female preference are less extreme than if there were no cost Runaway sexual selection of this kind could explain the extraordinary variety of male ornaments among different species of hummingbirds any many other animals

  30. Sexual Selection Sexual Selection by mate choice Good Genes Model (Handicap Hypothesis) Because females risk substantial losses of fitness if their offspring do not survive or reproduce, one could argue that females should evolve to choose males with high genetic quality, so that their offspring will inherit “good genes” and so have a superior prospect of survival and reproduction Any male trait that is correlated with genetic quality (an indicator of good genes) could be used by females as a guide to advantageous matings, so selection would favor a genetic propensity in females to choose mates on this basis Stickelback Fish

  31. Sexual Selection Sexual Selection by mate choice Antagonistic Coevolution Genes that govern male versus female characters may conflict resulting in antagonistic coevolution Such evolution may consist of a protracted “arms race” in which change in male character is parried or neutralized by evolution of female character, which in turn selects for change in the male character and so on in a chain reaction Abalones are externally fertilizing species. Sperm compete to fertilize eggs so selection on sperm always favors a greater ability to penetrate eggs rapidly Selection on eggs should favor features that slow sperm entry or else polyspermy may result (polyspermy disrupts development) Abalone Haliotis sorenseni Coevolution might be the cause of the extraordinarily rapid evolution of the amino acid sequence of the sperm lysin protein of abalones, a protein released by sperm that bores a hole through the vitelline envelope surrounding the egg

  32. Sexual Selection Sexual Selection by mate choice Chase-away Sexual Selection Females evolve resistance to male’s inducements to mate and their resistance selects for features that enable males to overcome the females’ reluctance Semen Toxicity Forced Copulation Ex Water Striders Males force females to copulate Traumatic Insemination Ex Bed Bug Males pierce his mate’s abdominal wall with his genital structure Water Strider Gerris sp

  33. Social Interactions and the Evolution of Cooperation Explaining how altruism –which by definition reduces personal fitness– can arise by natural selection is a major problem, and the central theoretical problem of sociobiology

  34. Social Interactions and the Evolution of Cooperation Darwin’s theory of natural selection is based on individual advantage. Cooperative interactions seem antithetical to evolution by natural selection and require explanation Until 1960’s it was common for biologists to assume that cooperation had evolved because it benefited the population or species (group selection). The modern study of cooperation focus on individual selection.

  35. Social Interactions and the Evolution of Cooperation Prisoner's Dilemma

  36. Social Interactions and the Evolution of Cooperation Prisoner's Dilemma • The Prisoner's Dilemma game takes its name from the following scenario: • You and a criminal associate have been busted. Fortunately for you, most of the evidence was shredded, so you are facing only a year in prison. But the prosecutor wants to nail someone, so he offers you a deal: if you squeal on your associate –which will result in his getting a five year stretch– the prosecutor will see that you are set free. Which sounds good, until you learn your associate is being offered the same deal – which would get you five years

  37. Social Interactions and the Evolution of Cooperation • To cooperate, or not cooperate? This simple question, expressed in an extremely simple game, is a crucial issue across a broad range of life • Why shouldn't a barracuda eat the little fish that has just cleaned it of parasites • Fig wasps collectively limit the eggs they lay in fig trees. But why shouldn't any one fig wasp cheat and leave a few more eggs than her rivals? • At the level of human society, why shouldn't each of the villagers that share a common but finite resource try to exploit it more than the others? • Yet barracudas, fig wasps, and villagers all cooperate. It has been a constant problem in evolutionary studies to explain how such cooperation should evolve, let alone persist, in a world of self-maximizing egoists

  38. Social Interactions and the Evolution of Cooperation Explaining how altruism –which by definition reduces personal fitness– can arise by natural selection is a major problem, and the central theoretical problem of sociobiology Theories of cooperation and altruism Cooperation Direct Indirect Benefits Benefits Non Kin Green Enforced Selection Enforced -beard Punishment Limited Kin Reciprocity Reward Dispersal Discrimination Direct Indirect

  39. Social Interactions and the Evolution of Cooperation Direct Benefits: Non-enforced Cooperative behavior often evolves simply because it is advantageous to the individual Ex Joining flock or herd Ex Unrelated helpers and delayed benefit Pied kingfisher Ceryle rudis

  40. Social Interactions and the Evolution of Cooperation Direct Benefits: Enforced: Manipulation & Punishment A donor may dispense aid to a recipient not because it is adaptive but simply because the donor is being manipulated or coerced Ex Cuckoo Common Cuckoo Cuculus canorus

  41. Social Interactions and the Evolution of Cooperation

  42. Social Interactions and the Evolution of Cooperation Direct Benefits: Enforced: Reciprocity It can be advantageous for an individual to help another if the recipient provides reciprocal aid in the future Vampire Bats Desmodus rotundus

  43. Social Interactions and the Evolution of Cooperation Indirect Benefits: Green-beard Genes Facultative harming greenbeards include the Gp-9 gene of the red fire ant Solenopsis invicta, which makes carrying workers execute non-carrying queens

  44. Social Interactions and the Evolution of Cooperation Indirect Benefits: Kin Selection Kin selection Selection based on the effect of an allele on both the fitness of the individual bearing it and the fitness of other individuals that carry copies of the same allele • For kin selection to operate individuals must dispense benefits more often to kin than non-kin. This can happen in two ways: • Many animals can distinguish kin from non-kin • The population may be structured so that interacting individuals are more likely to be related than unrelated

  45. Social Interactions and the Evolution of Cooperation Inclusive Fitness and Kin Selection Inclusive Fitness Is the effect of an allele on both the fitness of the individual bearing it (DIRECT FITNESS) and the fitness of other individuals that carry copies of the same allele (INDIRECT FITNESS)

  46. Social Interactions and the Evolution of Cooperation Inclusive Fitness and Kin Selection • Kin selection • Selection based on inclusive fitness • Hamilton’s Rule • An altruistic trait can increase in frequency if the benefit (B) received by the donor’s relatives weighted by their relatedness (r) to the donor, exceeds the cost (C) of this trait to the donor’s fitness: • rB>C • Coefficient of Relatedness • Is the fraction of the donor’s genes that are identical by descent with any of the recipient’s genes • Mother/Offspring r = ½ • Sibling/Full Sibling r = ½

  47. Social Interactions and the Evolution of Cooperation INDIRECT BENEFITS: KIN SELECTION: KIN DISCRIMINATION AND CANNIBALISM Many species of animals are cannibalistic preying on smaller individuals of the same species. Many such species discriminate kin from non-kin and are less likely to eat related individuals • Tadpoles develop into: • Detritus and plant omnivores • Associate more with their siblings • Cannibalistic carnivores • Associate more with non-relatives • Carnivores eat siblings much less frequently than unrelated individuals Spadefoot Toad Scaphiopus bombifrons

  48. Social Interactions and the Evolution of Cooperation INDIRECT BENEFITS: KIN SELECTION: COOPERATIVE BREEDING In many species of animals, young are reared not only by their parents, but also by other individuals that are physiologically able to reproduce but do not. Several factors may explain cooperative breeding, including kin selection, since in many species, the helpers aid their parents in rearing their children Primary are related Secondary are unrelated Pied kingfisher Ceryle rudis

  49. Meerkats

  50. Social Interactions and the Evolution of Cooperation Social Insects Eusocial animals Animals in which nearly or completely sterile individuals (workers) rear the offspring of reproductive individuals , usually their parents Known in Naked mole rat, termites, many hymenoptera Naked Mole Rat Heterocephalus glaber