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Group selection, inclusive fitness, and ants

Group selection, inclusive fitness, and ants. ants (Hymenoptera: Formicidae) 10-12,000 species; 15-25% of animal biomass worldwide all are eusocial  colonies of fertile queens & sterile workers, 3 classes of offspring: gynes workers males how is eusociality produced

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Group selection, inclusive fitness, and ants

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  1. Group selection, inclusive fitness, and ants

  2. ants (Hymenoptera: Formicidae) • 10-12,000 species; 15-25% of animal biomass worldwide • all are eusocial  colonies of fertile queens & sterile workers, 3 classes of offspring: • gynes • workers • males • how is eusociality produced • and maintained?

  3. ants (Hymenoptera: Formicidae) • 10-12,000 species; 15-25% of animal biomass worldwide • all are eusocial  colonies of fertile queens & sterile workers, 3 classes of offspring: • gynes • workers • males • how is eusociality produced • and maintained?

  4. ants (Hymenoptera: Formicidae) • 10-12,000 species; 15-25% of animal biomass worldwide • all are eusocial  colonies of fertile queens & sterile workers, 3 classes of offspring: • gynes • workers • males • how is eusociality produced • and maintained?

  5. inclusive fitness, kin selection • W.D. Hamilton (1964, 1972), then Trivers & Hare (1976), then Boomsma & Grafen (1990, 1991) • fitness = direct reproduction + effects on others’ reproduction • aiding in reproduction of others is favored if: • fitness benefit to relative x relatedness > fitness cost to self

  6. inclusive fitness, kin selection • W.D. Hamilton (1964, 1972), then Trivers & Hare (1976), then Boomsma & Grafen (1990, 1991) • fitness = direct reproduction + effects on others’ reproduction • aiding in reproduction of others is favored if: • fitness benefit to relative x relatedness > fitness cost to self

  7. inclusive fitness, kin selection • W.D. Hamilton (1964, 1972), then Trivers & Hare (1976), then Boomsma & Grafen (1990, 1991) • fitness = direct reproduction + effects on others’ reproduction • aiding in reproduction of others is favored if: • fitness benefit to relative x relatedness > fitness cost to self

  8. ants: haplodiploidy

  9. haplodiploidy • due to haplodiploid reproduction, ant workers may be more closely related to a queen’s offspring than to their own • this supports the evolution of eusociality • however, conditions for rqueen’s offspring> rown offspring are limited, and origins of eusociality aren’t tractable

  10. haplodiploidy • due to haplodiploid reproduction, ant workers may be more closely related to a queen’s offspring than to their own • this supports the evolution of eusociality • however, conditions for rqueen’s offspring> rown offspring are limited, and origins of eusociality aren’t tractable

  11. haplodiploidy • due to haplodiploid reproduction, ant workers may be more closely related to a queen’s offspring than to their own • this supports the evolution of eusociality • however, conditions for rqueen’s offspring> rown offspring are limited • origins of eusociality aren’t tractable

  12. haplodiploidy • monogynous colonies & singly mated queens & 1:1 sex investment ratios  relatedness of workers to queen’s offspring = 0.5 • if sex investment ratio = relatedness skew (1:3 males:females) •  r = 0.625 • 1:3 sex investment ratio • is generally found in these • conditions

  13. haplodiploidy • monogynous colonies & singly mated queens & 1:1 sex investment ratios  relatedness of workers to queen’s offspring = 0.5 • if sex investment ratio = relatedness skew (1:3 males:females) •  r = 0.625 • 1:3 sex investment ratio • is generally found in these • conditions

  14. haplodiploidy • monogynous colonies & singly mated queens & 1:1 sex investment ratios  relatedness of workers to queen’s offspring = 0.5 • if sex investment ratio = relatedness skew (1:3 males:females) •  r = 0.625 • 1:3 sex investment ratio • is generally found in these • conditions

  15. problems • polygynous colonies? • multiple mating of queens? • < relatedness of workers to the queen’s offspring • but if you can’t explain eusociality, at least explain sex ratios...

  16. problems • polygynous colonies? • multiple mating of queens? • < relatedness of workers to the queen’s offspring • but if you can’t explain eusociality, at least explain sex ratios...

  17. problems • polygynous colonies? • multiple mating of queens? • < relatedness of workers to the queen’s offspring • but if you can’t explain eusociality, at least explain sex ratios...

  18. multiple mating • multiple mating  overall relatedness drops; but relatedness to male offspring is unchanged • more male-biased sex investment ratio

  19. multiple mating • multiple mating  overall relatedness drops; but relatedness to male offspring is unchanged • more male-biased sex investment ratio

  20. multiple mating • effective mating frequency is generally low in ants • facultatively polyandrous ants: • predicted changes occur (Leptothorax) or not (Lasius) • obligate polyandry (rare): • predicted changes occur (Attini) or not (Pogonomyrmex)

  21. multiple mating • effective mating frequency is generally low in ants • facultatively polyandrous ants: • predicted changes occur (Leptothorax) or not (Lasius) • obligate polyandry (rare): • predicted changes occur (Attini) or not (Pogonomyrmex)

  22. multiple mating • effective mating frequency is generally low in ants • facultatively polyandrous ants: • predicted changes occur (Leptothorax) or not (Lasius) • obligate polyandry (rare): • predicted changes occur (Attini) or not (Pogonomyrmex)

  23. polygyny • polygyny  relatedness drops • related queens  relatedness asymmetry changes • unrelated queens  relatedness asymmetry unchanged • polygyny  greater cost of gynes?

  24. polygyny • polygyny  relatedness drops • related queens  relatedness asymmetry changes • unrelated queens  relatedness asymmetry unchanged • polygyny  greater cost of gynes?

  25. polygyny • polygyny  relatedness drops • related queens  relatedness asymmetry changes • unrelated queens  relatedness asymmetry unchanged • polygyny  greater cost of gynes?

  26. polygyny • gynes cost more  more male-biased sex ratio (or investment ratio) • related queens  more male-biased sex investment ratio • support mixed; polygyne ants generally more male-biased, but: • competitive benefit from neighboring related nests (Linepithema) • polygyny shifts sex investment ratios without relatedness asymmetry changes (e.g. Pheidole, Formica)

  27. polygyny • gynes cost more  more male-biased sex ratio (or investment ratio) • related queens  more male-biased sex investment ratio • support mixed; polygyne ants generally more male-biased, but: • competitive benefit from neighboring related nests (Linepithema) • polygyny shifts sex investment ratios without relatedness asymmetry changes (e.g. Pheidole, Formica)

  28. polygyny • gynes cost more  more male-biased sex ratio (or investment ratio) • related queens  more male-biased sex investment ratio • support mixed; polygyne ants generally more male-biased, but: • competitive benefit from neighboring related nests (Linepithema) • polygyny shifts sex investment ratios without relatedness asymmetry changes (e.g. Pheidole, Formica)

  29. split sex ratios • colonies often specialize in production of one sex • Boomsma & Grafen (1990, 1991)  colonies specialize in sex to which workers are more related than average • supporting evidence in some taxa, but: • split sex ratios without any relatedness changes (Solenopsis) • or in the opposite direction (e.g., Pheidole)

  30. split sex ratios • colonies often specialize in production of one sex • Boomsma & Grafen (1990, 1991)  colonies specialize in sex to which workers are more related than average • supporting evidence in some taxa, but: • split sex ratios without any relatedness changes (Solenopsis) • or in the opposite direction (e.g., Pheidole)

  31. split sex ratios • colonies often specialize in production of one sex • Boomsma & Grafen (1990, 1991)  colonies specialize in sex to which workers are more related than average • supporting evidence in some taxa, but: • split sex ratios without any relatedness changes (Solenopsis) • or in the opposite direction (e.g., Pheidole)

  32. nepotism? • polyandry and polygyny  selection for nepotism in workers • nepotism is rare or absent!

  33. nepotism? • polyandry and polygyny  selection for nepotism in workers • nepotism is rare or absent!

  34. males? • males generally ignored in inclusive fitness explanations • male fitness increases with female-biased investment • if sex-ratio arguments are correct  strong selection in males against multiple mating

  35. males? • males generally ignored in inclusive fitness explanations • male fitness increases with female-biased investment • if sex-ratio arguments are correct  strong selection in males against multiple mating

  36. males? • males generally ignored in inclusive fitness explanations • male fitness increases with female-biased investment • if sex-ratio arguments are correct  strong selection in males against multiple mating

  37. phylogenetic inertia? • can this explain tolerance of polygyny? • workers may be stuck with sociality, but they are not stuck with polygyny, or even queens: • queen-killing occurs (Linepithema), but not nepotistically! • reproduction can (rarely) be coopted by workers (Rhytidoponera)

  38. phylogenetic inertia? • can this explain tolerance of polygyny? • workers may be stuck with sociality, but they are not stuck with polygyny, or even queens: • queen-killing occurs (Linepithema), but not nepotistically! • reproduction can (rarely) be coopted by workers (Rhytidoponera)

  39. so what’s the alternative? • reviews of kin selection in Hymenoptera omit competing hypotheses! • what about group selection?

  40. so what’s the alternative? • reviews of kin selection in Hymenoptera omit competing hypotheses! • what about group selection?

  41. group selection • formed by analogy to natural selection: • “This preservation of favourable variations and the rejection of injurious variations, I call Natural Selection.” Darwin, 1859. • Darwin does not specify the units of selection here

  42. group selection • formed by analogy to natural selection: • “This preservation of favourable variations and the rejection of injurious variations, I call Natural Selection.” Darwin, 1859. • Darwin does not specify the units of selection here

  43. group selection • the between-group component of natural selection • preconditions: heritable variation in fitness between groups • this is probably a general property: • random sampling error • assortative group membership

  44. group selection • the between-group component of natural selection • preconditions: heritable variation in fitness between groups • this is probably a general property: • random sampling error • assortative group membership

  45. group selection • the between-group component of natural selection • preconditions: heritable variation in fitness between groups • this is probably a general property: • random sampling error • assortative group membership

  46. group selection • increased recent prominence; E.O. Wilson has announced the demise of kin selection • need not be in conflict with individual-level selection • increases with partitioning of variance between vs. within groups

  47. group selection • increased recent prominence; E.O. Wilson has announced the demise of kin selection • need not be in conflict with individual-level selection • increases with partitioning of variance between vs. within groups

  48. group selection • increased recent prominence; E.O. Wilson has announced the demise of kin selection • need not be in conflict with individual-level selection • increases with partitioning of variance between vs. within groups

  49. what are groups? • all units are groups at a lower level of analysis • the question is: which is most explanatory? • genes of ultimate importance, but selected through phenotype: • which level of phenotype? • primary unit of selection  reproduction? • colonies reproduce

  50. what are groups? • all units are groups at a lower level of analysis • the question is: which is most explanatory? • genes of ultimate importance, but selected through phenotype: • which level of phenotype? • primary unit of selection  reproduction? • colonies reproduce

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