Evolution of altruism Evolution of altruism Altruism Mutualism vs. altruism - behavior that benefits a Cooperation : receiver at a cost to the actor Displaying a behavior that benefits Examples: another individual. (If both benefit that's • Honey bee sting mutualism .) • Alarm calls Altrusim : • Blood sharing by Displaying a behavior that benefits bats another individual at a cost to oneself.
‘Social behavior’ is NOT cooperative behavior Evolution of altruism
Group living vs. cooperation How can altruism evolve? Sociality-no- • If the recipient of the cooperative/altruistic act benefits, it is going to leave more offspring . cooperation • The actor however is not going to leave and more offspring, or even fewer offspring – cooperation- fewer altruists in the next generation . no-sociality If such behavior is heritable, and it goes on over many generations, it will ultimately I define ‘sociality’ as living with other individuals of the same species at least semi-permanently. die out.
Evolution of altruism Evolution of altruism Selfish altruism? Evolution of altruism Helping somebody at a cost to yourself - If altruism was ultimately costly to where are the hidden benefits? reproduction, it would disappear in • Group selection – rare , only if long-term evolution. assortment maintained - Altruism can occur at the level of • Kin-selection – yes , if helping relative individuals, but if we see it today, we • Sexual selection – yes , if mating benefits have to assume that it benefits • Reciprocal altruism – yes , if reciprocation likely reproduction at some level in the long and enforced run (of genes, individual, or group). • Status – yes , if indirect benefits
1 Kin selection Kin selection Kin-selection Kin-selection Helping relatives increases your Helping relatives increases your ‘inclusive fitness’: ‘inclusive fitness’ therefore means: The more of your genes are in a Inclusive fitness: your own relative, the more interest you offspring (‘fitness’) plus your have in helping them. genes reproduced in others.
Kin selection ‘r’ Kin-selection Relatedness ‘r’ (also called coefficient of relationship) Helping relatives increases your Usually defined as: ‘inclusive fitness’ therefore means: The average proportion of alleles The more of your genes are in a of an individual A that are identical relative, the more interest you by descent to those in individual B. have in helping them. Or, the probability that A and B carry the This is measured by r (‘relatedness’) same allele, derived from the same ancestor, at a particular locus.
‘r’ ‘r’ Computing relatedness Computing relatedness If you have four nieces, on Relatedness: 0.5 average one copy of each of your genes If you have two children, on survives. average one copy of each of your genes survives. Relatedness: 0.25
2 ‘r’ Evolution of altruism Kin-selection Kin-selection
This means, if Hamilton’s rule: you sacrifice yourself for four An individual can be altruistic if nieces, ‘your c b * r genes’ have lost The cost should be smaller than the benefit nothing. multiplied by relatedness. E.g. an individual may not reproduce in a given year (c=1) to help its sibling (r=0.5) if this helps the sibling raise at least two additional offspring (b=2).
‘r’ ‘r’ Computing relatedness Computing relatedness
Values for a diploid, Trace back to common sexually reproducing ancestor and then individual: forward to target individual parent 0.5 sibling 0.5 uncle/aunt 0.25 Me My cousin Me My cousin cousin 0.125 grandparent 0.25 0.5*0.5*0.5*0.5 = 0.0625
‘r’ ‘r’ Computing relatedness Computing relatedness
Trace back to common ancestor and then forward to target Relatedness can individual – then repeat with other common thus be computed ancestors and add using a family tree together. Me My cousin (‘pedigree’).
0.0625 + 0.5*0.5*0.5*0.5 = 0.125
3 ‘r’ ‘r’ Relatedness ‘r’ Relatedness as measure of genetic similarity However, the definition that really Essentially ‘r’ is similar to measures of population reflects the ‘r’ in Hamilton’s rule is: structure (such as the inbreeding coefficient F). r is a measure stating how genetically F = (expected – observed)/expected similar the two individuals are relative to frequency of heterozygotes in a population two random members of the population. This is on average the same as r calcutated by r = (expected – observed)/expected pedigree only in a large, randomly mating, outbred number of differing alleles between two population. (Essentially, when inbreeding=0) individuals
Evolution of altruism Evolution of altruism Kin-selection Kin-selection
When it is demonstrated that Examples for kin-selection: individuals preferentially help kin - social insects rather than non-kin, this is taken - prairie dog alarm calls as evidence for kin-selection. to offspring & other relatives - generally parental care - cells in our body
Mechanisms of kin selection Mechanisms of kin selection Kin-recognition Kin-recognition Do individuals have to be able to NO – kin selection can operate, recognize relatives for and cause the evolution kin selection to work? of altruism, as long as altruists are more likely to help kin than non-kin - for whatever reason.
4 Mechanisms of kin selection Mechanisms of kin selection Inclusive fitness theory Kin-recognition vs. kin selection In fact, that’s why some argue that it should be called ‘inclusive fitness theory’ rather than ‘kin selection’ – Altruism can evolve as long as altruists are more likely than chance to dispense help to other altruists . (see John’s recent model on segregation)
Mechanisms of kin selection Kin selection in eusocial insects Kin-recognition The case of social insects • By smell (rodents, • Eusociality: some individuals sterile humans, insects) • Evolved > 10 times in • By song (some birds) Hymenoptera • By learning/familiarity (haplodiploid) (mice, humans) • All members of a • By visual similarity colony are usually (chimpanzees, humans) highly related
Kin selection in eusocial insects Kin selection in eusocial insects Does haplodiploidy cause Does haplodiploidy cause eusociality? eusociality?
• In complete monogamy, • However, workers are only related to + workers are more related + males by r=0.25 (less than to to the queen’s daughters daughters) – thus average 0.5 QM (r=0.75) than to their own QM relatedness to reproductive offspring 0.5 1 (r=0.5) is still 0.5 (depending on sex ratio) 0.5 0.5 0.5 • This would explain why so • Actual relatednesses measured in insect colonies are almost never 0.75 many Hymenoptera are (multiple queens, polygamy) eusocial • Recently more eusocial species W WW W W • and why workers are always W MM without haplodiploidy have been 0.25 + 0.5 = 0.75 females 0.25 discovered; and many haplodiploid species are not social
5 Kin selection in eusocial insects Kin selection in eusocial insects Alternative hypotheses for Wilson & Hölldobler 2005 the origin of eusociality
• Parental manipulation • Predisposition to sociality because of high b/c ratio (underground nests, extended brood care) • Group selection
Kin selection in eusocial insects Kin selection in eusocial insects Conclusions from the Wilson & Hölldobler 2005 controversy • Superiority of colony life over solitary life (b may • Haplodiploidy is not crucial to evolution of eusociality be much greater than c) • Ecological factors (high b/c) explain most of the • Euociality arose among unrelated individuals variation between species in sociality first; then relatedness increased • Controversy arises over the definition of ‘r’ – • In many species nests are founded by unrelated relatedness by pedigree or measure of genetic individuals similarity? • Real-existing relatedness low and • Complete worker sterility can only arise with positive r, counterproductive (?) whether by kinship or other segregation mechanisms • Eusociality rare even in highly related groups • However, many social insects do not actually have complete worker sterility
What you should remember (I) What you should remember (II) Kin-selection Kin-selection Hamilton’ s rule: c b * r Hamilton equation: c b * r • In a sense, kin selection is selection at the • If studying the evolution of altruism, c and level of genes b are important! • A behavior that is altruistic at the level of • r should ideally be calculated as similarity an individual could increase the relative to population variance representation of those genes in the next • If this is done, maybe it should better be generation (increase inclusive fitness) called ‘inclusive fitness theory’ • Only works if altruism dispensed to genetically similar individuals
6 Parental care & kin selection Kin selection or not? Parental care – Human altruism cooperation & conflict • Is altruism in humans explained by kin • How is kin selection relevant to parental selection? care? • What predictions does kin selection make about parental care?
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