The evolution of eusociality Flashcards
What is Eusociality?
The original definition of eusocility
* Cooperative care young (not just mother)
* Reproductive division of labor, with more or less sterile individuals working on behalf of fecund nestmates
* Overlap generations (mother, adult offspring, young offspring)
Problems with this definition:
- Reproductive division of labour is vague
- what importance does overlapping generations have?
- Definition of based on particular group of bees rather than the evolution of eusocilaity
- Falsely defines certain cooperative breeders as being eusocial. (Mole rat)
Revised definition:
* Presence of castes - groups of individuals that become irreversibly behaviourally distinct prior to reproductive maturity
->This allows the evolution of the extreme morphologies
* Helping by the less reproductive caste (helpers & breeders)
* Loss of behavioral totipotency of one (faculative) or both (obligate) castes
Diversity and specialization of castes
Due to the division of labour between castes extreme morphologies are able to evolve.
(Can even get different caste forms within helpers_
Examples:
- Army ants: they live in moving colonies and send out rampages to clear the path. So strong they are used as stitches
- Honey pot ants: they store honey for when resources are low
- Soldiers of turtle ants: There heads are shaped to block the entrance to the colony
The stimuli are normally envrionmental:
- the amount and quality of food received as a larva or nymph,
- the amount of yolk received while still an embryo in the egg
- the temperature of the nest,
- photoperiod,
- the presence or absence of pheromones from the queen or other colony
Few exmaples of being genetically determined:
- Singless bee species, queens appear to be females that are fully heterozygous for two independently segregating loci and also get specific diet
Why did eusociality evolve?- Two key points
1) Strict life time monogamy keeping relatedness high so that offspring of queens were just as related to siblings as there own offspring (R1 and R2 cancel)
2) An initial benefit to cooperation: life time insurance or fortree defence
Natural selection or kin selection leading to eusociliaty? hamilton’s rule?
To explain the evolution of eusociality we must explain with evolution of sterile workers which are showing extreme altruism.
natural selection?
- This cannot be explained by natural selection as natural selection favours genes that are better at making copies of themselves (promote reproductive success)
- Sterile workers are not making direct copies of there genesand do not directly contribute to the next generation.
Kin selection?
- Kin selection can explain eusociality as genes are getting to the next generation as the sterile worker is indirectly enabling the transfer of another copy by benefiting the fitness of a relative.
- This can be explained using hamiltons rule.
RB-C>0
- Altruism will be favoured if the relatedness and the benefits to the indivdual are high and the cost to the actor is low.
- For evolution of eusocilaty we must include the relative fitness benefits of either helping your own offspring or the queens offspring. This is done by including relatedness of the worker to its siblings and its offspring in the equation.
R1B-R2C>0
R1: relatedness of parent to sibling
R2: relatedness of parent to offspring
Monogamy vs pologamy leading to eusociality? Hamilton’s rule
Polygamous: Workers are more related to offspring (0.5) than to sibling (half sibs= 0.25)
- R2 is greater than R1 so altruism will not be favoured
Monogamous: Workers are equally related to offspring (0.5) and to siblings (0.5)
- R1 and R2 are equal so as long as the benefits are greater than the costs altruism will be favoured.
- Division of labour allows low costs and high birth rate of queen allows big benefits (e.g. high performing termite queen can produce 164 mill eggs in her lifetime)
- Data supports this and most eusocial colonies are monogamous (any pologamous have evolved secondarily)
- -> This is week evidence as are not able to compare evolution in mongamous and polygamous collonies
- -> possible to do this with birds
Birds
- Studies reveal that cooperation is lower in promiscuous species of birds and that evolution of cooperation is always associated with monogamy or low rates of promiscuity in birds
Asexuality
- Asexuality works in the same way as monogamy as offspring are equall related to sibling and their offspirng (1)
- Depends on the balance between costs and benefits
Why did eusociliaty intially evolve?
Hamiltons rule explains why eusociality is able to persist, but not how it initially evolved. What leads to the evolution of a high b/c?
Life insurance:
- When there is extended parental care the parent may die before the offspring is independent
- All the resources used producing the offspring will be wasted
- If parents help eachother and cooperate then they can ensure their offspring will survive
Example: Tropical paper wasp
- egg developement is 62 days and survival is 0.12
- Cooperating has a relative fitness advatage of 3.6
- This provide the benefits needing in hamilton’s rule.
Example: tropical hover wasp
- Part reared offsrping can be reared by group members-> relatedness threshold as low as 0.1
- Study removed different numbers of helpers from nests to see what happened to number of offspring.
- Larger, more valuable extra brood were reared through to maturity, but not smaller, less valuable brood. (same number of large reared as if no helper removed, but less small)
- Rearing extra brood did not increase adult mortality or brood development time
- 100 day developement with 10-30% survival
Fortress defense:
- Fortress defrense is important and has very high benefits
- Workers/ soldiers are required to defend the nest. (defend food sites or breeder’s from predators)
Example: Gail forming aphid
- Study manipulated contents of gails to change the type of aphid (non- soldiers)
- When there were no soldiers no predators were killed and all aphids died.
Overview
The definition of Eusociality has evolved away from a definition describing one group of organisms and now encompases the evolution of eusociality.
Hamilton’s rule can be used to explain the evolution of Eusociliaty.
R1B-R2C>0
Monogamous: R1=R2 so as long as C>B eusociality/ altruism can evolve.
For evolution to occur you need high b/c
- Fortress defense
- Life insurance
