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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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slide2

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?
slide3

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?
slide4

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?
slide5

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
slide6

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
slide7

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
slide9

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
slide10

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
slide11

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
slide12

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
slide13

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
slide14

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
slide15

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...
slide16

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...
slide17

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...
slide18

multiple mating

    • multiple mating  overall relatedness drops; but relatedness to male offspring is unchanged
    • more male-biased sex investment ratio
slide19

multiple mating

    • multiple mating  overall relatedness drops; but relatedness to male offspring is unchanged
    • more male-biased sex investment ratio
slide20

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)
slide21

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)
slide22

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)
slide23

polygyny

    • polygyny  relatedness drops
    • related queens  relatedness asymmetry changes
    • unrelated queens  relatedness asymmetry unchanged
    • polygyny  greater cost of gynes?
slide24

polygyny

    • polygyny  relatedness drops
    • related queens  relatedness asymmetry changes
    • unrelated queens  relatedness asymmetry unchanged
    • polygyny  greater cost of gynes?
slide25

polygyny

    • polygyny  relatedness drops
    • related queens  relatedness asymmetry changes
    • unrelated queens  relatedness asymmetry unchanged
    • polygyny  greater cost of gynes?
slide26

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)
slide27

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)
slide28

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)
slide29

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)
slide30

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)
slide31

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)
slide32

nepotism?

    • polyandry and polygyny  selection for nepotism in workers
    • nepotism is rare or absent!
slide33

nepotism?

    • polyandry and polygyny  selection for nepotism in workers
    • nepotism is rare or absent!
slide34

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
slide35

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
slide36

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
slide37

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)
slide38

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)
slide39

so what’s the alternative?

    • reviews of kin selection in Hymenoptera omit competing hypotheses!
    • what about group selection?
slide40

so what’s the alternative?

    • reviews of kin selection in Hymenoptera omit competing hypotheses!
    • what about group selection?
slide41

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
slide42

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
slide43

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
slide44

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
slide45

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
slide46

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
slide47

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
slide48

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
slide49

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
slide50

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
slide51

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
slide52

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
slide53

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
slide54

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
slide55

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
slide56

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
slide57

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
slide58

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
slide59

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
slide60

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
slide61

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
slide62

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
slide63

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
slide64

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
slide65

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
slide66

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
slide67

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?
slide68

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?
slide69

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?
slide70

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
slide71

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
slide72

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
slide73

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
slide74

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
slide75

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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