Transcript 0.5 - UMK
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 The unit of selection and evolution Unicellular organisms Multicellular organisms Higher taxonomic level Species Classical population genetics (Fisher, Haldane, Sewall Wright) Population Higher taxonomic level Group Species Family Population Individual Cell Organelle Genome Gene Nucleotid Wynne Edwards (1962) to explain cooperation 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. The basic unit is the gene as the smallest essential carrier of information C. Richard Dawkins (1941- The game theory approach The classical hawk and dove game Assume two players: • a hawk that will always fight until injured or until the opponent retreats John F. Nash (1928- • a dove that will always retreat. John Maynard Smith (1920-2004) Contests are associated will potential benefits (B) and potential costs (C). 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 The pay-off matrix Hawk Dove Hawk ½(B-C) B Dove 0 ½B Hawk v. Dove: The hawk will always win Dove v. Dove: Each contest has a 50% chance to win. There are no costs The pay-off matrix Hawk Dove Hawk ½(B-C) B Dove 0 ½B 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 Is H an ESS? W(H) pW(H, H) (1 p)W(H, D) W0 BC (1 p)B W0 2 B pB / 2 pC / 2 W0 p p BC (1 p)B W0 (1 p)B / 2 W0 B pC 2 If B > C, H is always an ESS because per definition 0 ≤ p ≤ 1. W(D) pW(D, H) (1 p)W(D, D) W0 Is D an ESS? (1 p)B / 2 W0 B / 2 pB / 2 W0 For H to be an ESS W(H) > W(D) BC (1 p)B W0 (1 p)B / 2 W0 B pC 2 For D to be an ESS W(D) > W(H) If B > C, D is never an ESS p The pay-off matrix Hawk Dove Hawk ½(B-C) B Dove 0 ½B What is if costs are higher than benefits C > B? H : B pC D : B pC At equilibrium we have B pC p B C 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. The Retaliator game The Bourgeois game (fight when meeting a hawk and retreat when meeting a dove) (fight when owner, retreat when intruder) Hawk Dove Retaliator Hawk ½(B-C) B ½(B-C) Dove 0 ½B ½B-e Retaliator ½(B-C) ½B+d ½B-¼C+g Retaliator and a mixed strategy are the two ESS of this game. Realization depends on the initial frequencies of players. Hawk Dove Bourgeois Hawk ½(B-C) B ¾B-¼C Dove 0 ½B ¼B Bourgeois ¼(B-C) ¾B ½B The Bourgeois is the only ESS of this game. 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. pM rM CM pF rFCF p: probability to produce a son; r: expected reproductive success, C: cost of reproduction Which sex to produce? The probability that a son reproduces is high The probability that a daughter reproduces is low 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 Two examples of sex ratio allocation Figs and fig wasps Parasitic wasps Agaonidae are closely connected to figs. Depositing eggs into the ovaries they pollinate figs. Sex ratio ro 0.6 0.5 0.4 0.3 0.2 17 species of fig wasp species (Agaonidae) 0.1 0 0 0.2 0.4 0.6 0.8 z Sex ratio second female Males are wingless and mate only with the local clutch 1 Proportion of of fruits Proportion fruitparasitized parsitized Sex ratio is defined as the proportion of males Secondary parasitism of the parasitoid wasp Nasonia vitripennis parasitoid of blow and flesh flies 1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 y = 0.27x-0.43 0 1 2 3 4 Offspring second female / Offspring first female 5 Selective infanticide in man 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) 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 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) Reciprocal altruism Reciprocal altruism beween non-related individuals needs: Blood sharing in the vampire bat • Long term association of group members. • Donorship can be predicted from past helping. • Roles of donors and recipients reverse. z • Benefits of the recipients outweigh donor costs. Percentage of prefeeding weight • Donors can detect cheaters. 130 Exponential vampire bat weight loss function due to starvation 120 • Primary social groups contain 8 to 12 adults with depending young. 110 100 • 30% of the blood sharing events involve adults feeding young other than their own. Weight Donor lost 90 Recipient • Blood sharing intensity depends on the degree of relatedness. Weight gained 80 Time lost Time gained 70 0 20 40 60 Hours Benefits outweigh costs 80 • Blood sharing is often reciprocal. • Cheaters have not been observed. 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. Male helpers S Primary helpers are older sons that are yet unable to breed. Additional young fledged per helper P They increase their fitness via their younger sisters and due to additional experience. >5 Number of adults providing care 4 Secondary helping males are unrelated to the pair they help. 3 2 0 1 2 3 Young fledged 4 5 Secondary helpers increase their fitness due to the chance to become the widow’s mate if the breeding male dies. The evolution of cheating or the Prisoner’s dilemma Assume two prisoners have the alternative either of defect the other or to cooperate. Defection means shorter imprisoning. B>C The pay-off matrix Defect Cooperate 0 Defect 0 Cooperate 0 B(A) B(B) 0 C(B) C(A) 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. 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). Defect CooperateTit for Tat Defect 0 e 0 Cooperate d g g Tit for Tat 0 g g The prisoners dilemma cannot fully be resolved analytically. The first software solution was provided by Rapoport in 1980. The program played Tit for Tat or reciprocal altruism. The other EES of this game is always defect. Inbreeding What is the probability for a children to get a certain allele from their grandparents? Grandparents GM1 A,B Parents P(C)=0.25 GF1 GM2 GF2 C,D E,F M F Ch Childrens G,H P(C)=0 P(C)=0.125 The probability that Ch gets allele C is 0.125. GF1 is already inbred GM1 GF1 GM2 GF1 A,B P(C)=0.5 C,C M E,F F GM1 GF1 GM2 GF1 A,B P(C)=0.25 C,D E,F M F Ch C,D P(C)=0.25 P(C)=0.25 The probability that Ch gets allele C is 0.25. The mean probability to get an allele X from one of the members of a lineage is called the coefficient of inbreeding. C,C P(C)=0.5 Sewall Wright defined this coefficient as n Ch P(C)=0.5 The probability that Ch gets allele C is 0.5. rlm 21 Li (1 rl ) i 1 rl→m is the path from l to descendent m and L the length of path i. Inclusive fitness In the Hawk - Dove game the EES for C > B was B<pC → pB>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. William D. Hamilton (1936-2000) Hamilton’s rule of inclusive fitness rB C A simple example Assume a new gene A that promotes parental care. 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 Hamilton’s rule. 0.5 = 1 / 2 Parental care should therefore be widespread in animals. In cockroaches (Phoraspis and Thorax) the young bite wholes in the mothers thorax to feed from their haemolymph. Kin selection and the evolution of sociality Members cooperate Part of the members loose but retain reproductive own reproductivity in favour ability of other group members Individualistic life → Sociality → Eusociality (superorganisms) Joined parental care → Cooperative breeding and defence Most bacteria and True multicellular → organisms (Metazoa, Colonies → single cell eucaryotes Fungi, Plantae Most ‘primitive’ animals and plants → Social spiders, isopods, many insects, many fishes Higher birds and mammals Often intensive common parental care, aunt behaviour, playing groups, and group defence → 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) All termites (Isoptera). They have male and female workers and different casts. Some Aphidae and Thripidae (Homoptera) have sterile soldiers. Sometimes rudimentary parental care. All ants (Hymenoptera). They have female workers only and highly differentiated cast systems. Two species of mole rats have non-reproducing workers and a queen. Colonies have up to 300 members. 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. What favours Hymenoptera to become eusocial? Hymenoptera are haplo-diploid organisms The haplo-diploid system Queen King Daughter Son Brother Fertilized eggs become females Unfertilized eggs become males Queen A,B King C Daughter 0.5 0.5 0.75 0.25 0.25 King 0 1.0 1.0 0 0.5 Queen 1.0 0 0.5 0.5 0.25 The diploid-diploid system Son A Daughter A,C Son B Daughter B,C Hamilton’s rule of inclusive fitness rB C Queen King Daughter Son Brother Daughter 0.5 0.5 0.5 0. 5 0.5 King 0 1.0 0.5 0.5 0.5 Queen 1.0 0 0.5 0.5 0.5 Queen - daughter 0.5 C B Queen - sister C 0.75 B 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. 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 0.75 2 0.67 2 1 0.5 2 0.67 2 1 Even for one additional offspring of the sister it pays to resign of own offspring 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 Eusociality and monogamy 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. From Hughes et al. 2008 Today’s reading The game theory site: http://www.holycross.edu/departments/biology/kprestwi/behavior/ESS/ESS_index_frms et.html Selfish gene theory: http://en.wikipedia.org/wiki/Gene-centered_view_of_evolution The evolution of eusociality: http://www.thornelab.umd.edu/Termite_PDFS/EvolutionEusocialityTermites.pdf Biology and sexual orientation: http://en.wikipedia.org/wiki/Biology_and_sexual_orientation http://www.newscientist.com/article/mg20427370.800-homosexual-selection-thepower-of-samesex-liaisons.html Biased sex ratios in man: http://huli.group.shef.ac.uk/lummaaproceedins1998.pdf and http://www.jstor.org/cgibin/jstor/printpage/00664162/di975349/97p0109i/0.pdf?backcontext=page&dowhat=Ac robat&config=jstor&[email protected]/01cce4405a00501c7b1f1&0.pdf and http://en.wikipedia.org/wiki/Gender_imbalance Figs and fig wasps: http://www.figweb.org/Interaction/index.htm