Lect. 17: Social behavior Altruism: Cooperative male display, alarm calling • Altruism & cooperation • Exam II: 1 Nov. Study guide • Inclusive fitness, kin selection Tuesday, 24 Oct – Hamilton’s rule • Seminar, 4:10, Today, 306 Abelson – Relatedness
• Eusociality and Hymenopterma – Bethany Marshall: “Forensic entomology” – Sex ratio bias
• Eusociality in diploids
A solution? Kin selection Hamilton’s rule JBS: “If I had a one in 10 Favors traits that increase rB - C > 0 chance of drowning but WD Hamilton 1964 saved the life of my child, I indirect fitness would save 5 copies of Natural Selection favors altrusistic acts genes for this when indirect fitness benefits to the behaviour….” JBS Haldane, 1953 receiver, reduced by the coefficient of Inclusive Direct Indirect relatedness, exceeds costs to the Fitness Fitness Fitness altruist
Hamilton’s Rule Who is the altruist?
• r, coefficient of Altruism altruism relatedness – Probability that homologous alleles in two individuals are IBD Cost to actor (C) Benefit to recipient (B)
Individual reproduction Kin’s additional C = 1 offspring B = 2 offspring reproduction rB-C > 0 r = coefficient of relatedness r = coefficient of relatedness r = coefficient of relatedness = probability that an allele found in one Relationship r individual will be found also in another due to inheritance from common ancestor (IBD). Parent - offspring 0.5 Full siblings 0.5 Relationship r Relationship r Half siblings 0.25
P - O 0.5 full sib 0.5*0.5 + 0.5*0.5 Cousins 0.125 = 0.50 0.5 0.5 half sib 0.5*0.5 = 0.25 A A
Belding’s Ground Squirrel
BGS: “I will alarm call Who alarm calls? and risk my life for 2 Females call for terrestrial predators siblings, 4 half-sibs, or • BGS dispersal: Male > 8 cousins” female
rB - C > 0?
rB - C > 0? Male Avg. RS Male Avg. RS Solo 0.9 young Solo 0.9 young Dom. 7.0 young Dom. 7.0 young Subord. 0.0 young Subord. 0.0 young
Cost = reduced # of offspring to subord. Benefit = # extra offspring to dominant = 0.9 - 0.0 = 0.9 young = 7.0 - 0.9 = 6.1 young
Krakauer (2005) Nature 434:69-72 Krakauer (2005) Nature 434:69-72 Krakauer (2005) Nature 434:69-72 Solve for relatedness? Cooperative mating: are male Inclusive fitness? coalitions related? B = 6.1 C = 0.9 B = 6.1 young C = 0.9 young rB - C > 0 r = 0.42
Ownbyphotography.com Altruism pays: Net gain: r > C/B rB-C > 0 > 0.9/6.1 1.7 > 0 > 0.15 Krakauer (2005) Nature 434:69-72 Krakauer (2005) Nature 434:69-72
Eusociality: reproductive How does eusociality evolve? Eusociality & hymenoptera altruism • In Hymenoptera? Sisters share 100% the genes they inherit from – Haplodiploid sex determination: • Specialized their father, which is 50% of their genome – fertilized eggs (diploid) -> females reproductive caste – unfertilized eggs (haploid) -> males. (Queen) Relationship r WD Hamilton 1972 • Non-reproductive Q Q castes (e.g. workers, fem - mom 0.5 drones, soldiers) • Common in the reproductive sterile worker soldier sis - bro 0.5*0.5 = 0.25 Hymenoptera: bees, Q W W W W W sis - sis wasps, ants 0.5*0.5 + 0.5*1 = 0.75
Kin selection & Hymenoptera? Fisher’s sex ratio theory Kin selection & Hymenoptera?
Males < females Males > females • Favors altruistic • Favors biased sex ratio female worker caste in haplodiploids females
– Helping to raise their • Frequency- equilibrium Worker-Worker r = 0.75 sisters (r = 0.75) dependent Fitness males Worker-Brother r = 0.25 • Favors biased sex fitness Predicted sex ratio 75:25
ratio reproductive sterile worker soldier • Predicts a 50:50 sex ratio – Workers raise offspring W 0 50 100 % males in population Haplodiploidy & Eusociality? Haplodiploidy & Eusociality? How does eusociality evolve? 0.8 siblings • But, multiple mating by queen erodes • Haplodiploidy? • Multiple mating by relatedness0.6 among sisters – Not all haplodiploids are Diploid sibs eusocial queen erodes 0.4 – Eusocial Diploids relatedness among females • Termites • A shrimp sisters half-sibs • Naked mole rats
Relatedness 0.2
0.0 Genetic & Ecological 1 3 5 7 9 factors favor eusociality Number of mates
Kin selection & Social behavior • Mechanisms to explain altruism • Hamilton’s rule: rB - C > 0 • Benefits of altruistic act discounted by r • Relatedness in haplodiploid systems helps explain eusociality and biased sex ratios • Reproductive altruism favored by – Genetic factors: Haplodiploidy, inbreeding – Ecological factors