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13. Altruism and sociality Primitive animals are all the same. There is no individualistic behaviour. Higher animals evolved individualism. The highest.

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Presentation on theme: "13. Altruism and sociality Primitive animals are all the same. There is no individualistic behaviour. Higher animals evolved individualism. The highest."— Presentation transcript:

1 13. Altruism and sociality Primitive animals are all the same. There is no individualistic behaviour. Higher animals evolved individualism. The highest birds and mammals evolved individualistic characters (moods), motions and fears. Classical population genetic does not predict individualism because it focuses on optimisation and equilibrium states that are the same for all members of a population. Evolutionary theory has to explain: Altruism (the help of others despite of own costs) Cooperation of related and unrelated individuals The evolution of cheating Sexual selection (the existence of differentiated sexual behaviour and mating rituals) Biased sex ratios (the prevalence of either males or females in a population) The existence of highly altruistic insect societies (eusociality) The existence of infanticide in many mammals and birds The existence of homosexuality in many mammals and birds The appearance of common beliefs and religion in man

2 C. Richard Dawkins (1941- The unit of selection and evolution Nucleotid Genome Gene Cell Organelle Species Population Individual Species Population Higher taxonomic level Family Group Higher taxonomic level Unicellular organismsMulticellular organisms Classical population genetics (Fisher, Haldane, Sewall Wright) Wynne Edwards (1962) to explain cooperation The basic unit is the gene as the smallest essential carrier of information A more liberal view sees any trait inducing carrier of information as a potential unit of evolution. These include genes, individuals, and even groups but not species.

3 John F. Nash (1928- The game theory approach The classical hawk and dove game ½(B-C) 0½B B Dove Hawk Dove John Maynard Smith (1920-2004) The pay-off matrix Hawk v. Hawk: Each contest has a 50% chance to win. The net gain is the difference between benefits and costs of the contest Dove v. Hawk: The dove will always loose Hawk v. Dove: The hawk will always win Dove v. Dove: Each contest has a 50% chance to win. There are no costs Assume two players: a hawk that will always fight until injured or until the opponent retreats a dove that will always retreat. Contests are associated will potential benefits (B) and potential costs (C).

4 ½(B-C) 0½B B Dove Hawk Dove The pay-off matrix The idea behind game theory is now to define equilibrium conditions that define which game (strategy = behavioural phenotype) will have the highest payoff in the long run. Maynard Smith defined such equilibria that cannot be beaten by other strategies as evolutionary stable strategies (ESS). Populations of individuals playing an ESS cannot be invaded by immigrating individuals or by mutants playing other strategies. The fitness For H to be an ESS W(H) > W(D) For D to be an ESS W(D) > W(H) Is H an ESS? If B > C, H is always an ESS because per definition 0 p 1. Is D an ESS? If B > C, D is never an ESS

5 ½(B-C) 0½B B Dove Hawk Dove The pay-off matrix What is if costs are higher than benefits C > B? At equilibrium we have For C > B an ESS is to play hawk with probability p and dove with probability 1-p. Even simple games favour mixed strategies. This is the start of individualistic behaviour. ½(B-C) 0½B B Dove Hawk Dove ½B- ½(B-C) Retaliator ½(B-C) ½B+ Retaliator ½B-¼C+ The Retaliator game (fight when meeting a hawk and retreat when meeting a dove) ½(B-C) 0½B B Dove Hawk Dove ¼B ¾B-¼C Bourgeois ¼(B-C)¾BBourgeois½B The Bourgeois game (fight when owner, retreat when intruder) The Bourgeois is the only ESS of this game.Retaliator and a mixed strategy are the two ESS of this game. Realization depends on the initial frequencies of players.

6 Local mate competition In 1967 W. D. Hamilton proposes that in the long run organisms should preferentially invested in the cheaper sex. The cheaper sex is the one that promises more offspring at equal costs. Which sex to produce? The probability that a son reproduces is high The probability that a daughter reproduces is low p: probability to produce a son; r: expected reproductive success, C: cost of reproduction For a proper choice a female needs knowledge about the actual sex ratio and must have the ability to control which sex she produces Many Hymenoptera and some other insects have these abilities Mammals and birds perform selective infanticide

7 Sex ratio is defined as the proportion of males Two examples of sex ratio allocation Secondary parasitism of the parasitoid wasp Nasonia vitripennis parasitoid of blow and flesh flies Figs and fig wasps Agaonidae are closely connected to figs. Depositing eggs into the ovaries they pollinate figs. Males are wingless and mate only with the local clutch Parasitic wasps

8 Selective infanticide in man Selective infanticide in man is found in nearly all cultures. Often it serves to stabilize population size to adjust sex ratios to marriage probabilities in cases of highly unequal reproductive success to adjust to a culturally preferred gender (frequently the male gender) Some reported sex ratios in childhood of preindustrial societies: Inuit Eskimos: 0.67 Yanomamö Indians: 0.56 Cashinahua, Peru: 0.60 Rajput caste, India: > 0.9 Upper class medieval Florence: 0.57 The sex ratio is the proportion of males: SR = males / (males + females) The normal cross cultural sex ratio at birth is 105 males to 100 females = 0.512 (range 101 to 107: 100)


10 Reciprocal altruism Weight lost Time lost Donor Recipient Weight gained Time gained Exponential vampire bat weight loss function due to starvation Long term association of group members. Donorship can be predicted from past helping. Roles of donors and recipients reverse. Benefits of the recipients outweigh donor costs. Donors can detect cheaters. Reciprocal altruism beween non-related individuals needs: Blood sharing in the vampire bat Primary social groups contain 8 to 12 adults with depending young. 30% of the blood sharing events involve adults feeding young other than their own. Blood sharing intensity depends on the degree of relatedness. Blood sharing is often reciprocal. Cheaters have not been observed. Benefits outweigh costs

11 Primary helpers are older sons that are yet unable to breed. They increase their fitness via their younger sisters and due to additional experience. Secondary helping males are unrelated to the pair they help. Secondary helpers increase their fitness due to the chance to become the widows mate if the breeding male dies. Cooperative breeding and helpers at the nest In the pied kingfisher Ceryle rudis primary and secondary helpers at the nest occur. Helpers occur in many higher bird species and help adults to raise the offspring.

12 The evolution of cheating or the Prisoners dilemma Assume two prisoners have the alternative either of defect the other or to cooperate. Defection means shorter imprisoning. Cooperate Defect Cooperate 0 0 B(A) 0 0 B(B) C(A) C(B) The pay-off matrix 0 Cooperate Defect Cooperate 0 Tit for Tat 0 Tit for Tat Now assume an iterative game where the players play many times. What would be the best strategy? In the long run there are several possible strategies One EES is Tit for Tat (defect if prior being defected and cooperate if the other prior also cooperated). The program played Tit for Tat or reciprocal altruism. The other EES of this game is always defect. B>C If both prisoners defect they do worse than if both cooperate. However cheating the other is superior irrespective of what the other makes. Hence pure cooperation can never evolve. The prisoners dilemma cannot fully be resolved analytically. The first software solution was provided by Rapoport in 1980.

13 Inbreeding GM1GF1GF2 A,BC,DG,H Grandparents Parents Childrens GM2 E,F MF Ch The probability that Ch gets allele C is 0.125. What is the probability for a children to get a certain allele from their grandparents? GM1GF1 A,B C,D GM2 E,F MF Ch The probability that Ch gets allele C is 0.25. P(C)=0.25 P(C)=0.125 P(C)=0.25 P(C)=0 P(C)=0.25 GM1GF1 A,B C,C GM2 E,F MF Ch The probability that Ch gets allele C is 0.5. P(C)=0.5 GF1 is already inbred The mean probability to get an allele X from one of the members of a lineage is called the coefficient of inbreeding. Sewall Wright defined this coefficient as r lm is the path from l to descendent m and L the length of path i.

14 William D. Hamilton (1936-2000) Inclusive fitness In the Hawk - Dove game the EES for C > B was B C P was the probability of a trait to occur. This is formally identical with the probability of a gene to occur via descent, it is identical to the coefficient of inbreeding. Hamiltons rule of inclusive fitness A simple example Assume a new gene A that promotes parental care. In cockroaches (Phoraspis and Thorax) the young bite wholes in the mothers thorax to feed from their haemolymph. The probability of transmitting A from mother to daughter is 0.5. Even if the mother would die due to parental care (cost = 1) two additional raised offspring (B = 2) satisfy Hamiltons rule. 0.5 = 1 / 2 Parental care should therefore be widespread in animals.

15 Kin selection and the evolution of sociality Individualistic life Sociality Eusociality (superorganisms) Members cooperate but retain reproductive ability Part of the members loose own reproductivity in favour of other group members Most primitive animals and plants Most bacteria and single cell eucaryotes Colonies True multicellular organisms (Metazoa, Fungi, Plantae Social spiders, isopods, many insects, many fishes Higher birds and mammals Joined parental care and defence Cooperative breeding Isoptera (autapomorphy) Some Aphidae and Thripidae At least 14 independent lineages of Hymenoptera Eucalyptus ambrosia beetles (Australoplatypus incompertus) Sponge shrimp (Synalpheus regalis) Naked mole rats (Heterocephalus glaber and Cryptomys damarensis) Often intensive common parental care, aunt behaviour, playing groups, and group defence

16 All termites (Isoptera). They have male and female workers and different casts. All ants (Hymenoptera). They have female workers only and highly differentiated cast systems. Some eusocial Apidae and Vespidae (Hymenoptera). They have female workers only. Some bumble bees and other Apidae species may be either solitary or eusocial depending on environmental conditions. Two species of mole rats have non-reproducing workers and a queen. Colonies have up to 300 members. Some Aphidae and Thripidae (Homoptera) have sterile soldiers. Sometimes rudimentary parental care.

17 What favours Hymenoptera to become eusocial? Hymenoptera are haplo-diploid organisms Fertilized eggs become females Unfertilized eggs become males Queen A,B King C Son A Son B Daughter A,C Daughter B,C Daughter King Queen QueenKingDaughterSonBrother Hamiltons rule of inclusive fitness Queen - daughterQueen - sister Daughter King Queen QueenKingDaughterSonBrother 50.5 01.0 The haplo-diploid system The diploid-diploid system Given that costs and benefits of reproducing are similar it pays for a hymenopteran female more to invest in her sisters than in her own brood. This explains why eusocial Hymenoptera all have sterile female workers and never sterile males.

18 But be careful Most of the haplo-diploid Hymenoptera are solitary. The theory requires that queens a priori invest more in daughters than in sons. Interestingly, many Hymenoptera are able to decide whether to lay male or female eggs. They are able to control sex ratios Termites are diplo-diploid For instance a hymenopteran female helps her sister at the cost of no reproduction. At equlilibrum the number of surviving offspring should be 2. Hence C = 2 The sister raises one additional offspring Even for one additional offspring of the sister it pays to resign of own offspring

19 Eusociality and monogamy From Hughes et al. 2008 Phylogenetic analysis shows that all ancestral eusocial hymenopteran species were monogame. Polygamy has derived after the transition to eusociality. Polygamy never occurs in species with totipotent workers.

20 Todays reading The game theory site: et.html et.html Selfish gene theory: The evolution of eusociality: Biology and sexual orientation: power-of-samesex-liaisons.html Biased sex ratios in man: and bin/jstor/printpage/00664162/di975349/97p0109i/0.pdf?backcontext=page&dowhat=Ac robat&config=jstor& and bin/jstor/printpage/00664162/di975349/97p0109i/0.pdf?backcontext=page&dowhat=Ac robat&config=jstor& Figs and fig wasps:

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