Transcript Social behavior II

How did eusociality originate?
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6/18/08: Social behavior II: Eusociality
Lecture objectives:
1. Be able to state the characteristics that
define eusocial species
R
2. Understand how high relatedness
might have promoted the evolution of
eusociality
B/C
3. Understand how ecological factors
might have promoted the evolution of
eusociality
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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)
Bees (some)
Wasps (some)
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Eusocial species: other
Termites (all)
Naked Mole Rat
Damaraland Mole Rat
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?
Hamilton’s Rule:
RxB>C
R=relatedness
(between donor and recipient)
C=cost to donor
B=benefit to receiver
How did eusociality originate? Two hypotheses:
Genetic hypothesis (haplodiploid hypothesis):
promoted the evolution of eusociality
Ecological 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
Female
no
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 < 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
http://www.sciencemag.org.prox
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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?
Hamilton’s Rule:
RxB>C
R=relatedness
(between donor and recipient)
C=cost to donor
B=benefit to receiver
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 evidence:
1.
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 evidence:
2. Eusocial species tend to have
a
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Ecological factors promoted the evolution of eusociality
Supporting evidence:
2. 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
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