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How did eusociality originate?. 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

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6 18 08 social behavior ii eusociality
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

eusocial species are characterized by 3 traits
Eusocial species are characterized by 3 traits

1. Cooperative care of young

2. Overlapping generations

3. Reproductive division of labor (“castes”):

Fire ants

eusocial species those in order hymenoptera
Eusocial species: those in order Hymenoptera

Ants (all)

Wasps (some)

Bees (some)

eusocial species other
Eusocial species: other

Naked Mole Rat

Damaraland Mole Rat

Termites (all)

Aphids (some)

Thrips (some)

Snapping shrimp (some)

why might sterile individuals display such altruism
Why might sterile individuals display such altruism?

R=relatedness (between donor and recipient)

C=cost to donor B=benefit to receiver

Hamilton’s Rule:R x B > C

How did eusociality originate? Two hypotheses:

Genetic hypothesis (haplodiploid hypothesis): promoted the evolution of eusociality

Ecological hypothesis: promoted the evolution of eusociality

high relatedness promoted the evolution of eusociality
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

high relatedness promoted the evolution of eusociality9
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) =

high relatedness promoted the evolution of eusociality11
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

high relatedness promoted the evolution of eusociality12
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

high relatedness promoted the evolution of eusociality13
High relatedness promoted the evolution of eusociality

Supporting evidence: 3.

Eusociality independently arose at least 12 timesin the Hymenoptera (ants, bees, wasps),

but only 1-2 times in Isoptera (termites)

high relatedness promoted the evolution of eusociality14
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.proxy1.cl.msu.edu:2047/content/vol320/issue5880/images/large/320_1213_F1.jpeg

high relatedness promoted the evolution of eusociality15
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

high relatedness promoted the evolution of eusociality16
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

why might sterile individuals display such altruism17
Why might sterile individuals display such altruism?

R=relatedness (between donor and recipient)

C=cost to donor B=benefit to receiver

Hamilton’s Rule:R x B > 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

ecological factors promoted the evolution of eusociality
Ecological factors promoted the evolution of eusociality

What causes a high benefit-to-cost ratio for helping behavior?

ecological factors promoted the evolution of eusociality19
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

ecological factors promoted the evolution of eusociality20
Ecological factors promoted the evolution of eusociality

Supporting evidence:2. Eusocial species tend to have a

ecological factors promoted the evolution of eusociality21
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

ecological factors promoted the evolution of eusociality22
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