L2, Theories of Aging Flashcards
1
Q
List the 4 key theories of aging: (2x Ev, 2x Mech.)
A
- Mutation accumulation theory of aging (Evolutionary)
- Antagonistic pleiotropy (Evolutionary)
- Disposable soma theory of aging (mechanistic)
- Aging due to biological constraints (mechanistic)
2
Q
Haldane’s Huntington’s disease study:
A
Early evolutionary theories of aging:
- Mathematical population study
- Investigated HD allele; highly deleterious, onset in 30s/40s, 1/10,000 in USA
- Harmful phenotype is not being ‘seen’ by natural selection
- Reproductive lifespan not coinciding with onset of disease (genotype already passed on) -> weak or no natural selection against disease alleles
- Particularly important to consider demographics in prehistory -> young individuals making the major contribution to genetics
3
Q
Why do late acting alleles have less of an impact on population selection?
A
- Basic level: Due to extrinsic mortality, much fewer old than young individuals in nature
- Further: Reproduction patterns; young reproducing more than old
4
Q
Selection shadow:
A
- Allowing late onset diseases to persist
- Due to extrinsic mortality, force of negative selection is weak at older ages
- Less ‘optimised’
- See diagram
5
Q
Describe the mutation accumulation theory of aging: (MATA)
A
Proposed by Medawar in 1950s based on idea of selection shadow -> ‘Unsolved problem in biology’…
- Ageing is (partly) genetic and is driven by an accumulation of late-acting deleterious genetic variants over evolutionary time
- Due to weak natural selection, late-acting harmful mutations are being acquired by genetic drift
6
Q
Genetic drift:
A
- Evolution by random chance
- ‘Change in allele frequency in a population over generations due to random sampling’
- Heavily influenced by population size effects i.e. faster in smaller populations
7
Q
+ Public vs private mechanisms of aging
A
- Public: Shared across different, distantly related evolutionary lineages
- Private: Peculiar to particular evolutionary lineages
- Evidence for existence of public mechanisms: Universal effect of DR and reduced reproduction on lifespans
8
Q
+ Inbreeding depression
A
- Reduced biological fitness, potentially due to inbreeding
- Loss of genetic diversity in inbred populations due to small size
9
Q
+ Two models of antagonistic pleiotropy:
A
- A: Late action model (early and late actions of a pleiotropic gene have distinct effects on fitness e.g. daf2 in C.elegans)
- B: Latent damage model (Both late and early effects occur due to early actions, with the latter effect effect occurring long after the period of action)
10
Q
Antagonistic pleiotropy theory of aging:
A
- Mutations beneficial early in life but harmful later will be positively selected for
11
Q
Predicted effect of extrinsic mortality on aging according to evolutionary theories…
A
- Where an organisms ecology involves higher extrinsic mortality, more/worse late-acting deleterious genetic variants should have escaped selection -> aging faster, shorter lifespan
- When correlating body mass and lifespan, prey species appear to perform worst (rat, buffalo, gazelle etc) -> Greater extrinsic mortality
12
Q
Austad’s possum study (lifespan comparison within species):
A
- Landmark study comparing mainland possum’s with island population
- Mainland population faced greater extrinsic mortality (predation, cars etc) -> age faster, reproduce once, more collagen damage)
- Island population had no predators; longer life, reproducing twice -> less collagen damage
- Lower extrinsic mortality -> selection for longer life to maximise reproductive output
13
Q
Does selection using delayed reproduction remove late-harmful alleles and extend lifespan? - Include evidence
A
- Berkeley study on flies; over 50 generations, older individuals were isolated to reproduce each time
- Reproductive period extended, stress resistant individuals with better lifespans
- However, early adult fecundity was reduced (APTA!)
- Verifies delayed reproduction idea
14
Q
Disposable soma theory of aging: (DSTA)
A
- Proposed by Kirkwood in 1970s
- Limited energy budget implies a trade-off between soma and germline
- He suggested that the soma (i.e. all somatic cells) are built to last only as long as they would in nature; survival beyond this is ‘maintenance overshoot’
- Aging is due to unrepaired damage
- Reproduction and repair compete for energy allocation
- Can explain disparities in species lifespans (see: r vs k strategies)
15
Q
+ r/K selection theory
A
- Selection of a combination of traits in an organism to trade off between quantity and quality of offspring
- r-strategists: Rabbits (many offspring, low individual parent investment) -> precocial offspring
- k-strategists: Whales, eagles (few offspring, investing greatly in terms of time and care demands) -> altricial offspring
- However, most organism are not primarily r- or k-strategists, but have traits of both (e.g. trees) -> think of as a continuous spectrum
