Lecture 12 - Eusociality Flashcards
why did Darwin question eusociality?
how can sterility evolve and be selected for in a population considering natural selection involves the capacity to reproduce
describe the diversity of eusociality
1) Hymenoptera - bees, wasps, ants
2) isoptera - termites
3) homoptera - aphids
what are 3 key features of eusociality
- Cooperative brood care
- Sterile castes
- Overlapping generations
why are eusocial systems ecologically important?
- 14,000 spp of social insect (cf. 10,000 birds, 4,000 mammals)
- driver ant colony: 22 million ants
- Brazilian rainforest: 70-80% of insect, 33% animal biomass
- sophisticated communication: waggle dance
- specialisation
INCREDIBlY SUCCESSFUL
brief explanation of the lifecycle of a typical hymnoptera insect
1) queen founds nest and produces sterile workers
2) takes 9 years before the colony is big enough to stat reproducing
3) after 9 years the queen switches from just producing female workers to also producing winged females and males that reproduce
4) males and females fly up and mate - male then dies and females lose winds and found a new nest
what are the 2 models for how eusociality evolved?
- both start off assuming ancestors were solitary parasitoids
1) staying at home hypothesis
2) sharing a nest
describe the ‘staying at home’ hypothesis
solitary parasitoid -> nest guarding by female -> young stay and help defend/build -> young permanently at home and never breed
what is the genetic predisposition for the ‘staying at home’ hypothesis
For daughters, raising full siblings (coeff of relatedness, r = 0.5), is as good as raising own offspring (r = 0.5).
For queen, producing offspring (r = 0.5) is better than producing grand-offspring (r = 0.25).
Queen should prefer daughters to stay as workers
describe the ‘sharing a nest’ hypothesis
sisters build nest close together -> cooperative defence, separate reproduction -> one female dominates reproduction -> young females become workers
what is the genetic predisposition for the ‘sharing a nest’ hypothesis
- For sisters that cooperate, there is a benefit from raising dominant’s offspring (i.e. nieces and nephews).
- This may outweigh the benefit of breeding alone if that is a high risk activity
in what species is the ‘staying at home’ (subsocial) hypothesis seen
halictine bees
in what species is the ‘sharing a nest’ (parasocial) hypothesis seen
polistes and stenogastrine wasps
what did hamilton say about haplodiploidy?
haplodiploidy predisposes Hymenoptera to evolve sterile castes
what is the difference between males and females that are haplodiploids?
- Males from unfertilized eggs – haploid
- Females from fertilized eggs – diploid
- Males form gametes without meiosis
- Females form gametes with meiosis
where do sons and daughters in haplodiploid species receive their genes from?
- daughters receive identical genes from their father and the other half from diploid mother
- sons dont have a father - receive genes only from their mother
what is the consequence of haplodiploidy for relatedness for sister-sister?
Via mother:
r = 0.5 x 0.5 = 0.25
\+
Via father:
r = 0.5 x 1 = 0.5
r = 0.75
what is the consequence of haplodiploidy for relatedness for sister-brother?
Via mother:
r = 0.5 x 0.5 = 0.25
\+
Via father:
r = 0.5 x 0 = 0
r = 0.25
- the father passes no genes to his son
what is the consequence of haplodiploidy for relatedness for brother-sister ?
Via mother:
r = 1 x 0.5 = 0.5
\+
Via father:
r = 0 x 1 = 0
r = 0.5
- a brother is more related to his sister than she is too him - asymmetry in relatedness
what affect does the asymmetry in relatedness have on female workers
for a female worker it is better to produce sisters than daughters
- explains why females (not males) rear sisters
compare haplodiploidy to diploid termites
males and females equally related to siblings and both sexes become sterile workers
compare haplodiploidy to clonal aphids
sterility is not an evolutionary puzzle because there’s no conflict over reproduction - every individual is related to each other by 1, they just want to maximise overall reproductive output of the clonal group
describe the conflict between queens and workers over sex ratio of broods
- queens are equally related to sons and daughters
- workers are more closely related to their sisters
- queens want to produce an even sex ratio
- workers have a preference for females
- queens can choose to fertilise eggs or not to produce different sexes but workers can choose which sex to invest more into
what did trivers and hare test in their haplodiploidy experiement?
- tested sex ratio in 21 ant species
- results suggest the workers ‘win’
what are the 2 main criticisms with the trivers and hare experiment?
1) Local mate competition - brothers may compete to mate so it may pay females to bias sex ratio too
2) Queen mating frequency affects relatedness - if the female mates more than once the relatedness gets reduced
what is now known about multiple mating?
- multiple mating is actually frequent making the patterns of relatedness less extreme
- this messes up with the predictions of what workers would prefer- actually depends more on the queens mating frequency
