Group selection, inclusive fitness, and ants
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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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Group selection inclusive fitness and ants

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

    • and maintained?


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

    • and maintained?


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

    • and maintained?


Group selection inclusive fitness and ants

  • 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


Group selection inclusive fitness and ants

  • 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


Group selection inclusive fitness and ants

  • 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


Group selection inclusive fitness and ants

ants: haplodiploidy


Group selection inclusive fitness and ants

  • 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


Group selection inclusive fitness and ants

  • 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


Group selection inclusive fitness and ants

  • 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


Group selection inclusive fitness and ants

  • 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


Group selection inclusive fitness and ants

  • 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


Group selection inclusive fitness and ants

  • 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


Group selection inclusive fitness and ants

  • 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...


Group selection inclusive fitness and ants

  • 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...


Group selection inclusive fitness and ants

  • 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...


Group selection inclusive fitness and ants

  • multiple mating

    • multiple mating  overall relatedness drops; but relatedness to male offspring is unchanged

    • more male-biased sex investment ratio


Group selection inclusive fitness and ants

  • multiple mating

    • multiple mating  overall relatedness drops; but relatedness to male offspring is unchanged

    • more male-biased sex investment ratio


Group selection inclusive fitness and ants

  • 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)


Group selection inclusive fitness and ants

  • 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)


Group selection inclusive fitness and ants

  • 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)


Group selection inclusive fitness and ants

  • polygyny

    • polygyny  relatedness drops

    • related queens  relatedness asymmetry changes

    • unrelated queens  relatedness asymmetry unchanged

    • polygyny  greater cost of gynes?


Group selection inclusive fitness and ants

  • polygyny

    • polygyny  relatedness drops

    • related queens  relatedness asymmetry changes

    • unrelated queens  relatedness asymmetry unchanged

    • polygyny  greater cost of gynes?


Group selection inclusive fitness and ants

  • polygyny

    • polygyny  relatedness drops

    • related queens  relatedness asymmetry changes

    • unrelated queens  relatedness asymmetry unchanged

    • polygyny  greater cost of gynes?


Group selection inclusive fitness and ants

  • 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)


Group selection inclusive fitness and ants

  • 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)


Group selection inclusive fitness and ants

  • 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)


Group selection inclusive fitness and ants

  • 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)


Group selection inclusive fitness and ants

  • 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)


Group selection inclusive fitness and ants

  • 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)


Group selection inclusive fitness and ants

  • nepotism?

    • polyandry and polygyny  selection for nepotism in workers

    • nepotism is rare or absent!


Group selection inclusive fitness and ants

  • nepotism?

    • polyandry and polygyny  selection for nepotism in workers

    • nepotism is rare or absent!


Group selection inclusive fitness and ants

  • 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


Group selection inclusive fitness and ants

  • 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


Group selection inclusive fitness and ants

  • 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


Group selection inclusive fitness and ants

  • 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)


Group selection inclusive fitness and ants

  • 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)


Group selection inclusive fitness and ants

  • so what’s the alternative?

    • reviews of kin selection in Hymenoptera omit competing hypotheses!

    • what about group selection?


Group selection inclusive fitness and ants

  • so what’s the alternative?

    • reviews of kin selection in Hymenoptera omit competing hypotheses!

    • what about group selection?


Group selection inclusive fitness and ants

  • 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


Group selection inclusive fitness and ants

  • 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


Group selection inclusive fitness and ants

  • 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


Group selection inclusive fitness and ants

  • 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


Group selection inclusive fitness and ants

  • 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


Group selection inclusive fitness and ants

  • 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


Group selection inclusive fitness and ants

  • 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


Group selection inclusive fitness and ants

  • 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


Group selection inclusive fitness and ants

  • 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


Group selection inclusive fitness and ants

  • 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


Group selection inclusive fitness and ants

  • 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


Group selection inclusive fitness and ants

  • 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


Group selection inclusive fitness and ants

  • group selection vs. kin selection

    • within-group relatedness increases between-group partitioning of variance

    • the two are compatible explanations, not strict alternatives

    • but predictions & explanatory value differ


Group selection inclusive fitness and ants

  • group selection vs. kin selection

    • within-group relatedness increases between-group partitioning of variance

    • the two are compatible explanations, not strict alternatives

    • but predictions & explanatory value differ


Group selection inclusive fitness and ants

  • group selection vs. kin selection

    • within-group relatedness increases between-group partitioning of variance

    • the two are compatible explanations, not strict alternatives

    • but predictions & explanatory value differ


Group selection inclusive fitness and ants

  • is relatedness the right metric?

    • Solenopsis invicta, Linepithema humile, other major invasives characterized by unicoloniality:

      • low or absent between-nest aggression, often across large areas

      • relatedness in nests approaches 0


Group selection inclusive fitness and ants

  • is relatedness the right metric?

    • Linepithema humile:

      • although relatedness is ~0, genetic similarity is high

      • low relatedness may be a measurement artifact

    • Solenopsis invicta:

      • between-nest & between-queen cooperation determined by Gp-9

      • single-locus identity, and not relatedness, controls social form


Group selection inclusive fitness and ants

  • is relatedness the right metric?

    • Linepithema humile:

      • although relatedness is ~0, genetic similarity is high

      • low relatedness may be a measurement artifact

    • Solenopsis invicta:

      • between-nest & between-queen cooperation determined by Gp-9

      • single-locus identity, and not relatedness, controls social form


Group selection inclusive fitness and ants

  • do colony characteristics determine success?

    • Linepithema & Solenopsis:

      • increased colony size  large competitive advantage

    • Formica:

      • increased colony size  exploitation of concentrated, long-term resources

    • Pogonomyrmex: decreased relatedness  increased colony growth


Group selection inclusive fitness and ants

  • do colony characteristics determine success?

    • Linepithema & Solenopsis:

      • increased colony size  large competitive advantage

    • Formica:

      • increased colony size  exploitation of concentrated, long-term resources

    • Pogonomyrmex: decreased relatedness  increased colony growth


Group selection inclusive fitness and ants

  • do colony characteristics determine success?

    • Linepithema & Solenopsis:

      • increased colony size  large competitive advantage

    • Formica:

      • increased colony size  exploitation of concentrated, long-term resources

    • Pogonomyrmex: decreased relatedness  increased colony growth


Group selection inclusive fitness and ants

  • explaining inclusive fitness gaps?

    • absence of nepotism, tolerance of polygyny & low relatedness  more efficient colony function, better colony-level performance

    • split sex ratios may be determined by colony-level factors:

      • resource limitation

      • local mate competition

      • habitat saturation


Group selection inclusive fitness and ants

  • explaining inclusive fitness gaps?

    • absence of nepotism, tolerance of polygyny & low relatedness  more efficient colony function, better colony-level performance

    • split sex ratios may be determined by colony-level factors:

      • resource limitation

      • local mate competition

      • habitat saturation


Group selection inclusive fitness and ants

  • but: the forgotten variables

    • cost & benefit terms of Hamilton’s rule generally ignored

    • can we accomodate everything by hiding it in those variables?

      • yes, you can!

      • but there is no explanatory or predictive value

      • and it obscures explanatory integration


Group selection inclusive fitness and ants

  • but: the forgotten variables

    • cost & benefit terms of Hamilton’s rule generally ignored

    • can we accomodate everything by hiding it in those variables?

      • yes, you can!

      • but there is no explanatory or predictive value

      • and it obscures explanatory integration


Group selection inclusive fitness and ants

  • but: the forgotten variables

    • cost & benefit terms of Hamilton’s rule generally ignored

    • can we accomodate everything by hiding it in those variables?

      • yes, you can!

      • but there is no explanatory or predictive value

      • and it obscures explanatory integration


Group selection inclusive fitness and ants

  • integration

    • Pogonomyrmex genetic variation & productivity:

      • is this analogous to heterozygote advantages?

    • split sex ratios:

      • aren’t we looking at evolution of gonochorism, one level up?

    • do we want to reinvent the wheel for group-level explanations?


Group selection inclusive fitness and ants

  • integration

    • Pogonomyrmex genetic variation & productivity:

      • is this analogous to heterozygote advantages?

    • split sex ratios:

      • aren’t we looking at evolution of gonochorism, one level up?

    • do we want to reinvent the wheel for group-level explanations?


Group selection inclusive fitness and ants

  • integration

    • Pogonomyrmex genetic variation & productivity:

      • is this analogous to heterozygote advantages?

    • split sex ratios:

      • aren’t we looking at evolution of gonochorism, one level up?

    • do we want to reinvent the wheel for group-level explanations?


Group selection inclusive fitness and ants

  • conclusions

    • origins of eusociality not tractable in ants

    • kin selection predictions for sex ratio hold for monogyny + monandry, fail in other cases

    • kin selection explanations neglect male fitness; predict nepotism; rely on constraints


Group selection inclusive fitness and ants

  • conclusions

    • origins of eusociality not tractable in ants

    • kin selection predictions for sex ratio hold for monogyny + monandry, fail in other cases

    • kin selection explanations neglect male fitness; predict nepotism; rely on constraints


Group selection inclusive fitness and ants

  • conclusions

    • origins of eusociality not tractable in ants

    • kin selection predictions for sex ratio hold for monogyny + monandry, fail in other cases

    • kin selection explanations neglect male fitness; predict nepotism; rely on constraints


Group selection inclusive fitness and ants

  • conclusions

    • group selection (or multilevel selection) includes kin selection explanations

    • accounts for variance independent of relatedness

    • aids explanation of colony functionality, fosters integration of theory across levels


Group selection inclusive fitness and ants

  • conclusions

    • group selection (or multilevel selection) includes kin selection explanations

    • accounts for variance independent of relatedness

    • aids explanation of colony functionality, fosters integration of theory across levels


Group selection inclusive fitness and ants

  • conclusions

    • group selection (or multilevel selection) includes kin selection explanations

    • accounts for variance independent of relatedness

    • aids explanation of colony functionality, fosters integration of theory across levels


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