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Why does ageing occur?
Intro:
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Bio def: persistent decline in the age-specific fitness components of an organism due to internal
physiological deterioration, not simply getting older.
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Seen in both captive and natural populations.
Not biologically inevitable as there are extensive mechanisms at the cellular level which can
repair somatic damage, DNA damage repair mechanisms eg photoreactivation of crosslinked
bases. These systems use up <2% of energy budget
Rate of ageing- senescence- is highly varied between species, even in benign captive conditions.
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Paradox of evolution. How can a trait which involves a loss in fitness arise from natural selection.
Natural selection:
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Without extrinsic mortality, reproduction is equally likely during any time in life, but as extrinsic
mortality is highly likely in populations- cumulative chance of extrinsic death increases rapidly
with time. This mens organisms with a high chance of extrinsic death will be selected to breed
earlier in life as this will contribute more to lifetime reproduction success.
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If ageing has been naturally selected we should see that ageing correlates temporally with
baseline mortality. This is seen in bird and mammal studies.
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In social insects- queen is protected from extrinsic hazards and lives up to 100x longer than
works. Workers who leave the nest for food age faster than those whose tasks are within the
nest. – NAH THOUGH they’re genetically identical this doesn’t explain it.
Separation of germline and soma:
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cost of immortal germ line is a mortal somatic body.
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Soma is maintained to maximise LRS. Reproduction causes somatic damage, decrease in repair
leads to ageing. This optimises life history.
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Immortal jellyfish- doesn't separate soma and germline.
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When reproduction is prevented, ageing may slow as shown by the Korean enuchs, however this
study is highly questionable. But sterilising Drosophila increases the female lifespan 2 fold.
Maladaptive factors:
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Drosophila studies in lab- some selected to breed early in life (lifespan decreases) others
selected to breed later in life (lifespan increases) but the force of selection turns out to decline
with age. Older individuals are less sensitive to selection pressures.
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This permits the expression of deleterious genes later in life as these are not as strongly selected
against. EG huntingdon's disease- loss of cognitive and motor control from mutation in Htt gene
on chromosome 4.
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Variation in genetic fitness found to increase with age in red deer.
Biological mechanisms that cause ageing:
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Free radical hypothesis: Metabolic processes create reactive oxygen species which are unstable
and can react and damage vital macromolecules such as DNA, particularly mtDNA (which could
be explanation as to why diet restriction leads to longer lifespan)- cellular mechanisms for
limiting this damage eventually are overwhelmed.
Telomere shortening: Telomere caps at ends of chromosomes, shorten at every replication.
Telomerase mostly prevents this but this enzyme is degraded over time and eventually
telomeres degrade completely. Chromosomes join together by accident and cell dies.
Investment in organs during development: Low birth weight babies are more likely to suffer
cardiovascular disease in later life during to limited investment in this organ during development.
Conclusion:
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Phenotypic gambit often used for complex traits, clearly appropriate for use with ageing which is
a highly complex mechanism. But can we attribute the mechanism on the genetic level at all?
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Antagonistic pleiotropy hypothesis- 1 gene controls for multiple characteristics (may be a
transcription factor) where at least one is advantageous and expressed early in life and another
unfavorable and expressed later in life. Huntingdons disease gene also promotes increase in p53
tumour repressor gene, decreases cancer chance. As selection not as sensitive later in life, the
benefits of these genes outweigh the costs- the costs being ageing and eventual intrinsic death,
this shows how in fact, ageing is an evolutionarily advantageous trait just like any other, there is
an overall benefit to the organism.