How did eusociality originate? 5/13/08

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 1. Cooperative care of young 2. Overlapping generations 3. Reproductive division of labor (“castes”): Fire ants

Eusocial species: those in order Hymenoptera Ants (all) Wasps (some) Bees (some)

Eusocial species: other Naked Mole Rat Damaraland Mole Rat Termites (all) Aphids (some) Thrips (some) Snapping shrimp (some)

Sterile individuals display extreme altruism Camponotus saundersi soldiers

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 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 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 eusociality Supporting evidence: 1.

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 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 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)

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

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 eusociality Supporting evidence: 2. Eusocial species tend to have a

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 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