Effect on Reproductive Success of Various Social Interactions
Both benefit, but benefits delayed Reciprocity Reciprocal altruism Problems with cheaters However, if each animal must perform a necessary minimum itself that may benefit another individual as a byproduct By-Product Mutualism Cheating is not a problem Different categories of “mutualism”
Altruistic Behaviors Do they exist? Possible Examples: helpers at the nest in birds? social insects – some individuals incapable of reproducing
Why??? Would a ground squirrel give an alarm call when it sees a predator??
Theory “Kinship Theory” explained several puzzles The earliest expressions of the basic concepts were by R.A. Fisher (1930), J. B. S. Haldane (1955) “I would risk my life to save two of my brothers and seven of my cousins” But it was W. D. Hamilton who truly formalized the concept (1964). 1936-2000
Hamilton’s Rule Hamilton (1964a, b) The genetic evolution of social behavior I, II J. Theor. Biol. 7, 1-52. Cooperative behavior will evolve when the average inclusive fitness benefits exceed the direct fitness costs to the actor Simple version of rule: rb - c > 0 where r is the relatedness of the altruist to the beneficiary b is the extra offspring altruist helps beneficiary have c is the cost in number of offspring to altruist
Helpers at the Nest Florida Scrub Jay Individuals other than parents feed young and protect them from predators True Altruistic Behavior? Helpers are related to parents!
Indirect versus Direct Selection Direct selection - acts on variation in individual reproductive success Indirect selection – (indirect benefits) acts on variation in the effects individuals have on their relatives’ reproductive success
Three areas of cooperation research - not mutually exclusive Reciprocity By-product mutualism Group selection
Example: Prisoners Dilemma A two person game, imperfect information, one time or repeated, normal form The dilemma: the outcome is not efficient. Efficient = there is no other outcome that pays all players more. Payoffs for Player 1 are listed first T (temptation) > R (reward) > P (punishment) > S (sucker) RS TP P
Use arrows to determine ESS 1. Arrow along left side…. what player 1 should do if player 2 remains silent. 2. Arrow along right side….what player 1 should do if player 2 confesses. 3. Arrow along top…what player 2 should do if player 1 remains silent? 4. Arrow along bottom…what player 2 should do if player 1 confesses? RS TP
Prisoner’s Dilemma The dominant strategy for both players is to CONFESS. But both could do better if they cooperated and stayed silent. *
Axelrod and Hamilton (1981. Science 211,1390) Ran computer tournaments, had 14 entries, played in round robin Winner was Tit-for-Tat nice (starts by cooperating) retaliates (defects in response to defections) forgives (does not hold grudges…only remembers one play back) Modeled Tit-for-Tat – Cooperation through Reciprocity stays dominate if chances of running into same player are high
Evidence for TFT (Reciprocity) in nature?? Egg swapping in hermaphroditic fish (Nowak and Sigmund 1992 Nature 355, 250) Chalk bass retaliate against cheaters - wait longer to parcel out eggs Reciprocal grooming in Impala (Hart and Hart 1992 Anim. Behav. 44, 1073-1083) Blood sharing in Vampire Bats (Desmodus rotundus, Wilkinson 1988. Etho. Sociobiology 9:85-100)
Blood-sharing in Vampire Bats Who do vampire bats share blood with?? More often with relatives as well as individuals who are frequent roostmates (not random). Experiment - Held unrelated bats for nine days (hungry) and released a different one each night More likely to be fed by individuals they had fed before.
Prisoner’s Dilemma as model of non-kin cooperation Limitations to model can’t switch partners iterated and don’t know when it will end Many extensions have been suggested ---Parcelling model (Connor 1995 Animal Behavior 49, 528) Claims of model too general (Clements and Stephens 1995)
Conditions for Reciprocity (P Matrix) T > R, P > S Conditions for By-product Mutualism (M Matrix) R > T and S > P Each animal must perform a necessary minimum itself that may benefit another individual as a by-product. Payoffs such that it pays little to nothing to cheat…lowers the value of temptation (T). R = 4S = 1 T = 1P = 0 CD C D R = 3S = 1 T = 4P = 2 CD C D
Examples of By-Product Mutualism Territorial defense in Pied Wagtails (Davies and Houston 1981 J. Anim. Ecol. 50, 157-180) Blue jays in a “skinner box” (Clements and Stephens 1995 Anim. Behav. 50, 527) Blue jays had two keys of different colors to peck that would deliver food Payoffs (food pellets) were either as a By-product Mutualism or Prisoner’s Dilemma (depended on key other bird in pair pecked). Blue jays only cooperated with the By-product Mutualism payoffs.
Difficulties in Telling Reciprocity from Mutualism (R > T or T > R) Cooperation in African Lions Group Hunting Hunting together better than hunting alone, so R > P Success alone already high, second hunter does not add much, then T > R Two much better than one, then R > T R = 4S = 1 T = 5P = 0 CD C D R = 4S = 1 T = 3P = 0 CD C D
Group Selection - History Wynne-Edwards (1962) Idea that populations self regulate. Realized would not work under individual selection. Suggested group selection. Critique by George Williams (1966). Biotic (for Group) versus Organic (for Individual) Adaptations Argued biotic adaptations could not exist. Either it is not an adaptation, or it evolved by individual selection.
Group Selection versus Individual Selection Individuals are more numerous than the populations and they turn over much more rapidly than populations. The rate of replacement of less fit by more fit individuals is potentially much greater than the rate of replacement of less fit by more fit populations. Therefore, if individual selection is opposed to group selection, individual selection will prevail.
NEW View on Group Selection Spearheaded by D.S. Wilson (1975, 1976, 1977) and the empirical and theoretical work of Wade (1977, 1978, 1979). Differs from Wynne-Edwards (1962) genetic models that partition variance into within- and between-group components definition of group no longer limited to reproductively isolated deme (trait group) cooperation evolves when within group cost is offset by between group benefit
Altruistic allele (black) increases in frequency overall, even though its frequency decreases within each group, as group with higher frequency of altruistic alleles produces more offspring.
Evidence Many theoretical studies since have demonstrated that the conditions for group selection to cause the evolution of altruistic behaviors are realistic (Slatkin and Wade 1978, Crow and Aoki 1982, Leigh 1983, Wilson 1983). ‘Rethinking the Theoretical Foundation of Sociobiology’ (Wilson and Wilson 2007) and ‘Evolution “For the Good of the Group”’ (Wilson and Wilson 2008)
Conditions for Group Selection Altruistic allele must not be too deleterious to individuals, but very advantageous to population Very little gene flow (5%) Population small
Maynard Smith 1975. The Theory of Evolution, Cambridge U. Press Clearly an entire group of altruists will do better than a group consisting of all selfish individuals. Problem - explain how a group comes to consist wholly of altruists in the first place (since altruism will be eliminated in a mixed group). Answers - 1) Cultural Transmission 2) Group Selection
Black = full sibs, Yellow = half sibs Larvae benefit from cannibalizing eggs Altruism (refraining from cannibalism) evolved only in those populations that were divided in to inbreeding subpopulations. Within-group matingRandom Mating Difference in cannibalism rates (%) Generations Evidence: Wade (1977 Evolution 33, 749) showed group selection could override individual in Tribolium in the lab
Evidence Rainey and Rainey 2003. Nature 425, 72-74 The “wrinkly spreader (WS)” strain of Pseudomonas fluorescens evolves in response to anoxic conditions in unmixed liquid medium, by producing a cellulosic polymer that forms a mat on the surface. The polymer is expensive to produce, which means that non- producing “cheaters” have the highest relative fitness within the group. However, as the cheaters spread, the mat deteriorates and eventually sinks to the bottom. WS is maintained in the total population by between-group selection, despite its selective disadvantage within groups, exactly as envisioned by multilevel selection theory.
Evidence Packer and Heinsohn 1996. Science, 271, 1215–1216 Female lions share a common resource, the territory; but only a proportion of females pay the full costs of territorial defense. If too few females accept the responsibilities of leadership, the territory will be lost. If enough females cooperate to defend the range, their territory is maintained. Collective effort is vulnerable to abuse by their companions. Leaders do not gain `additional benefits' from leading. By failing to find a within-group advantage for territorial defense, between-group selection left as the most likely— and fully plausible—alternative.