Social evolution, and levels of selection Malte Andersson 30.10.2019 1 Social evolution, Malte Andersson, 30.10. 2019 Fitness: an individual’s expected genetic contribution to the next generation Direct fitness: numbers of surviving own offspring A behaviour leading to higher fitness is favored by selection and will spread over the generations 2 1 Mobbing a raptor is risky for a crow 3 A honey bee dies after stinging an enemy 4 2 Costly helping: how can it evolve and persist? Does it benefit the donor of help via: 1) direct fitness (individual selection) ? 2) delayed direct fitness, by reciprocity ? 3) donor’s relatives: kin selection ? 4) a larger community: group selection ? These questions are debated. They were first asked by Darwin (1859), about sterile workers in eusocial insects. Some individuals can not reproduce, but offer their lives for the colony (for instance honey bee workers). Can natural selection lead to such behaviour? 5 Altruism (meaning in ecology): Helpful behavior that raises the recipient’s but lowers the donor’s direct fitness 6 3 Alarm signals - selfish or altruistic? Black-throated Shrike-Tanager, Springbok stotting, Kalahari Belize Stotting tells a predator it is detected Warning flock members, also using by a gazelle in good condition. Better the signal for own feeding advantage to hunt another prey (Caro 1986) 7 Is sentinel behavior in meerkats altruistic? Probably not. Sentinels are safer and have a direct benefit from their behavior. (Clutton-Brock et al. 1999) 8 4 0.5 0.4 0.3 Females 0.2 Males 0.1 0 Without close With With offspring genetic relatives nondescendant relatives Black-tailed prairie dogs Individuals give alarm calls mainly with relatives (offspring and others). But they sometimes call even without related neighbors. (Hoogland 1983) 9 Reciprocity (reciprocal helping) ’You scratch my back, I scratch yours’ Vampire bat, sucking blood from a chicken Vampires cut through the skin of victims by razor-sharp teeth, and may take blood from the same victim for several nights. Females regurgitate blood to hungry neighbors, and future roles may be reversed: reciprocal helping (Carter and Wilkinson 2013). Kin selection can also favor feeding of other females. 10 5 Game theory analyses of reciprocity: Prisoner’s dilemma A version of the Prisoners dilemma game is tit-for-tat (TFT): repeating the other player’s move Long-term cooperation in repeated games can arise by relatively simple strategies (e.g. Nowak and Sigmund 2007) (see Wikipedia: Prisoners dilemma) 11 Reciprocity in V-formation flight ? 12 6 Bald ibises reciprocate in V-formation 13 Individuals often change place, taking turns at the energy-demanding lead position (Voelkl et al. 2015) 14 7 Group selection Some social behaviors are suggested to evolve by increasing the success of the group. Do some individuals produce fewer offspring than they can raise, to reduce the risk of starvation and group extinction? Individual selection may work against group selection, and dominates if life length is much shorter for individuals than groups. Individuals raising as many young as possible outcompete those with fewer young. (As a consequence, group extinction risk may increase.) 15 16 8 Are lemmings migrating and drowning themselves to save the species from overpopulation, starvation and extinction…..? 17 Group selection of myxoma virus? Group selection may sometimes dominate over individual selection. Myxoma virus attacks rabbits. After it spread in Australia, myxoma evolved lower virulence. The reason may be selection among different myxoma strains. Within a rabbit, the most virulent strain has highest fitness and increases rapidly. But it may then kill the host before spreading to a new one. Less virulent types let the host rabbit survive longer, increasing the chances to spread to other rabbits. 18 9 Altruism (in ecology): Helpful behaviour that raises the recipient’s but lowers the donor’s direct fitness 19 Evolution of eusociality Some eusocial animal societies have sterile workers that cannot breed. How does eusociality evolve? Can selection favor steriles? Darwin suggested that selection among colonies can favor worker sterility, if individuals in a colony are related. This is what we now think, even though Darwin did not know about genetics. (This is not group selection. Colonies are not long- term isolated groups. Individuals from different colonies mate and found new colonies, causing much genetic mixture each generation.) 20 10 Red wood ant (Formica rufa) mound North-facing side 21 Army ants Eciton burchelli Soldier mandibles are hooked and Army ant workers form a bridge sharp, for cutting enemies and prey. across a gap in their path. 22 11 Leafcutter ants (Atta sp.) The ant riding on the leaf carried by a worker is a smaller cast. It watches for enemies, such as flies smuggling eggs into the nest on leaves. The ants feed leaves to fungi in their ‘gardens’. 23 Leafcutter ant nest model, superimposed over the original site Atta texana leafcutter ant colony, translated from ground radar scan 24 12 • Djurens beteende, Youtube videos : • • Leafcutter ants: https://www.youtube.com/watch?v=RH3KYBMpxOU • • African driver ants: https://www.youtube.com/watch?v=77SyZn2W4-Y • • American Army soldiers: https://www.youtube.com/watch?v=YqLwc56lRFU • • Termites: https://www.youtube.com/watch?v=xGaT0B__2DM 25 Kin selection in social evolution W.D. Hamilton (1964) suggested how eusociality and altruism may evolve. Individuals can spread their genes by helping relatives. Hamilton’s rule says that a donor should help a relative if: B x r > C , where B is benefit for the recipient, C cost for the donor, and r is their relatedness. In diploid organisms, siblings have r = 0.5. Ants, wasps and bees (Hymenoptera) are haplo-diploid. Haploid males hatch from unfertilized eggs, diploid females from fertilized eggs (controlled by the mated mother). Sisters are therefore highly related, r = 3/4. This may be why eusociality is common in Hymenoptera: workers are females, highly related to reproductive sisters they help raise. But probably more important for the evolution of eusociality is lifetime monogamy, which makes all siblings closely related. Levels of selection are still debated, but evidence for kin selection increases, and in many cases it is probably crucial for social evolution. 26 13 Phylogenetic evidence suggests that monogamy and kin selection are essential for evolution of eusociality in Hymenoptera (Hughes et al. 2008, Science 320:1213- 1216) 27 Termites are also eusocial Termite cathedral mounds in the Northern Territory of Australia 28 14 Termites are diploid relatives of cockroaches Some termites have soldiers that Termite reproductives, ‘queen defend the colony, biting intruders or and king. She produces shooting toxic glue from their nasus hundreds or thousands of organ. Many soldiers die in battles eggs every day. with predatory ants. 29 • Djurens beteende, Youtube videos : • • Leafcutter ants: https://www.youtube.com/watch?v=RH3KYBMpxOU • • African driver ants: https://www.youtube.com/watch?v=77SyZn2W4-Y • • American Army soldiers: https://www.youtube.com/watch?v=YqLwc56lRFU • • Termites: https://www.youtube.com/watch?v=xGaT0B__2DM 30 15 A eusocial vertebrate, the Naked mole-rat Naked mole-rats live in underground colonies. A breeding female, one or a few breeding males and many workers and soldiers cooperate for the colony – similar to termites. 31 Queen of All She Surveys 32 16 High relatedness and kin selection helps explain eusociality in many animals. In termites, naked mole rats and other diploid animals, colony members are closely related (aphids, thrips, crustaceans). But selection at the colony level may also play a role. Models suggest that kin and multilevel selection can both lead to eusociality. Surprising new discoveries are still made, such as a sterile soldier caste in a clonally reproducing parasitic fluke (trematode). The last word has not been said in this active research field. 33 17.