EEB214H1 Lecture 16: Cooperation and Altruism

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Ecology & Evolutionary Biology
Jennifer Carpenter

EEB214: Lecture 16 Cooperation and Altruism • Cooperation is costly, and seems like something should be favored by selection o Ground squirrels: alarm callers 2x more likely to get eaten ▪ live in groups, sit on rocks and call to warn others in the group ▪ also attracts attention and gets eaten first • The prisoner’s dilemma is one way of thinking about cooperation o Two people are arrested and cops need one of the two thieves to confess, which will get a smaller jail time o Cost: Both don’t talk and cooperate, police don’t have evidence so they’re in jail for 1 year o If both confess, police have strong evidence so they’re in jail for 5 years o if one confesses and other doesn’t, one guy is free and other is 20 years in jail ▪ 0 years—benefit of cheating o it’s always better to not cooperate, and just cheat • Things that cheat can evolve and spread through population which increase their own fitness • Kin selection depends on inclusive fitness o An individual’s total fitness can be viewed as the sum of its direct fitness, which is the number of viable offspring that it produces, and its indirect fitness which is the incremental effect • So how can allele increase its fitness under this framework? o Increase own fitness by increasing number of babies you have o or increasing fitness of your close relatives • Hamilton’s rule defines when an altruistic behaviour will be beneficial o r = relatedness to an individual o B = benefit to individual being helped o C = cost to helper o rB-C>0 • Coefficient of relatedness (r) o r is the probability that a gene in one individual • Half gene get from parents • r = 1 (0.5)2 • r = 0.5 • Relatedness between two full siblings o also 0.5 o half the alleles one has is the same sibling has • Visualizing relatedness o if Grandmother is orange, half the genome of all her kids but .25 with grandkids o further generation you are, the lower the number o 0.5 –parent to offspring (on average) o identical twin, r = 1 and non-identical is the same as normal siblings 0.5 • but because recombination is random, it’s not always 0.5 o due to randomness, siblings can look more different or more similar to each other than we expect • variation in how related we are to members of family • more cooperative to members of family who’s closer • JBS Haldane famously said: o “lay down life for his 2 brothers or 8 cousins” • Brothers o rB-C > 0 o Let’s say that: B = 1 (to each brother) o C = 1 o r for a brother = 0.5 o 0.5 * (1*2) – 1 = 0 o if he dies from saving two brothers, the benefit is outweigh • Cousin = 0. 125 o 0.125 * (1*8) – 1 = 0 • Example: Ground squirrels are more likely to call when surrounded by relatives o if they had mother, daughter/granddaughter, 20 of them warned 0.5 related o cousin, nieces = half as likely to take the risk o residents = more likely to call than non-residence an
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