How did eusociality originate?

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How did eusociality originate?
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6/18/08 Social behavior II Eusociality

• Lecture objectives
• Be able to state the characteristics that define
  eusocial species
• Understand how high relatedness might have
  promoted the evolution of eusociality
• Understand how ecological factors might have
  promoted the evolution of eusociality

R
B/C
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Eusocial species are characterized by 3 traits
1. Cooperative care of young 2. Overlapping
generations 3. Reproductive division of labor
(castes)
Fire ants
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Eusocial species those in order Hymenoptera
Ants (all)
Wasps (some)
Bees (some)
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Eusocial species other
Damaraland Mole Rat
Naked Mole Rat
Termites (all)
Aphids (some)
Thrips (some)
Snapping shrimp (some)
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Sterile individuals display extreme altruism
Camponotus saundersi soldiers
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Why might sterile individuals display such
altruism?
Rrelatedness (between donor and
recipient) Ccost to donor
Bbenefit to receiver
Hamiltons Rule R x B gt C
How did eusociality originate? Two hypotheses
Genetic hypothesis (haplodiploid hypothesis)
promoted the evolution of eusociality Ecologi
cal hypothesis promoted the evolution of
eusociality
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High relatedness promoted the evolution of
eusociality
What causes high relatedness between individuals?
1.
Some thrips
2.
Some aphids
3.
Fertilize egg?
Bees, ants, wasps, thrips
yes
no
Female
Male
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High relatedness promoted the evolution of
eusociality
How might haplodiploidy promote helping behavior?
R (female daughter)
A female could pass along more of her genes
R (female sister)
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High relatedness promoted the evolution of
eusociality
Supporting evidence 1.
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High relatedness promoted the evolution of
eusociality
Supporting evidence 2. Haplodiploid workers
should be more likely to favor sisters when their
queen is
Workers are all full sisters (R 0.75)
Workers have full sisters (R 0.75) and half
sisters (R 0.25) (overall, R lt 0.75)
Test this in a species where a queen may be
monogamous OR polyandrous
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High relatedness promoted the evolution of
eusociality
Supporting evidence 2. Haplodiploid workers
should be more likely to favor sisters when their
queen is monogamous versus polyandrous
Prediction Formica ant workers with a
____________ queen will bias their rearing toward
sisters compared to workers with a _________
queen
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High relatedness promoted the evolution of
eusociality
Supporting evidence 3.
Eusociality independently arose at least 12 times
in the Hymenoptera (ants, bees, wasps), but
only 1-2 times in Isoptera (termites)
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High relatedness promoted the evolution of
eusociality
Supporting evidence 4. Monogamy should be
________ in eusocial Hymenoptera
Monogamy was the ancestral state for 8 of the
independent origins of eusociality
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High relatedness promoted the evolution of
eusociality
Evidence that there might be other factors
involved 1. High relatedness is
______________ for eusociality to exist (high
relatedness may be a ____________________)
Termites are diploid
Mole rats are not typically inbred
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High relatedness promoted the evolution of
eusociality
Evidence that there might be other factors
involved 2. High relatedness is ___________
for evolution of eusociality There are
non-social haplodiploid species
Haplodiploid but not eusocial Solitary bees
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Why might sterile individuals display such
altruism?
Rrelatedness (between donor and
recipient) Ccost to donor
Bbenefit to receiver
Hamiltons Rule R x B gt C
How did eusociality originate? Two hypotheses
Genetic hypothesis (haplodiploid hypothesis)
high relatedness (high R) promoted the
evolution of eusociality Ecological
hypothesis high benefit-to-cost ratio (high
B/C) promoted the evolution of eusociality
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Ecological factors promoted the evolution of
eusociality
What causes a high benefit-to-cost ratio for
helping behavior?
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Ecological factors promoted the evolution of
eusociality
Supporting evidence1.
Example Drywood termites flexible caste
policy retain ability to develop into
reproductives
Prediction Drywood termites should be more
likely to disperse when costs to staying at home
are high
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Ecological factors promoted the evolution of
eusociality
Supporting evidence2. Eusocial species tend to
have a
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Ecological factors promoted the evolution of
eusociality
Supporting evidence2. Eusocial species tend to
have a fortress that requires defense
Example Gall-forming aphids have soldiers that
help defend gall
Prediction Galls with soldiers will be more
likely to be attacked by insect predators than
those without soldiers
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Ecological factors promoted the evolution of
eusociality
Evidence that there might be other factors
involved
7 species of African mole rats build communal
tunnels Many species of rodents build complex
burrows
Wild norway rats