Revision summary
Physiological theories include oxidative damage, telomeres, and endocrine decline. Hayflick limits and repair failure describe cells, not purpose. Medawar: late harmful mutations accumulate. Williams: genes good early can be bad late. Kirkwood: the soma is disposable once reproduction is served.
Model answer
Introduction
Aging is the rise of mortality and the loss of function after maturity. Physiological theories name what breaks. Evolutionary theories name why selection allowed the break.
Body
Physiological theories
- Wear and tear and rate-of-living ideas treat the body as a machine that rusts. They are intuitive and too simple.
- Free radical and oxidative stress theories, after Harman, blame mitochondrial leaks. Antioxidant evidence in humans is mixed.
- Telomere shortening and cellular senescence limit division. Hayflick showed cells do not divide forever.
- Endocrine and immunological aging: less growth hormone and sex steroid, more inflammation, weaker vaccine response.
- Protein cross-links, DNA repair failure, and stem-cell exhaustion are further cellular accounts. They describe mechanism, not why the clock exists.
Evolutionary theories
- Medawar’s mutation accumulation: late-acting harmful alleles hide from selection because most ancestors had already reproduced.
- Williams’ antagonistic pleiotropy: a gene that helps youth can harm age. Selection prefers the youth effect.
- Kirkwood’s disposable soma: energy goes to reproduction over perfect repair, a life-history bargain.
- Grandmother hypotheses argue that post-menopausal life can still be selected if it raises kin fertility, as in some forager demography.
- Humans live long beside a large brain and late adolescent spurt. Aging is not a failure of erect living alone; it is scheduled by life history.
Together
- Evolution explains the schedule. Physiology names the rust. Neither is a complete gerontology without nutrition and class.
Flow diagram
flowchart TD AG[Aging] --> PHY[Damage telomere endocrine] AG --> EV[Medawar Williams Kirkwood] EV --> LH[Life history reproduction vs repair]
Conclusion
Physiological theories track damage, telomeres, and hormones. Evolutionary theories—Medawar, Williams, Kirkwood—explain why selection did not delete old age. Human longevity still sits in a social environment.
Quick related
Students also ask
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Explain the structural analysis of kinship as proposed by Levi-Strauss
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Is aging a disease?
It is a process that raises disease risk. Evolutionary theory treats it as a by-product of selection on early life.
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Do Inuit or Tibetans age by different laws?
The evolutionary logic is shared. Climate, diet, and care change the physiological path.
PYQ trend
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