Why do we age

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SCIENCE CAFE
Why do we age?
Tom Sherratt Department of Biology
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Outline
What is ageing? Is it heritable? Why does
natural selection not select for immortality?
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What is ageing?
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What is ageing?
Individual level Physiological breakdown of the
body with increasing chronological age
Demographic level Increased mortality
and/or decreased fecundity rate with
chronological age
Ageing is not simply growing old, but
deteriorating
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The physiological characteristics of ageing
Flying Phil Rabinowitz age 100
100 m dash world records (men)
Usain Bolt age 22
Time (s)
Age
30.86 seconds
9.58 seconds
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The demographic characteristics of ageing
The bad news probability of mortality doubles
every 8 years of life
The good news once you reach 95-100 then
mortality reaches a plateau
Male
Female
National Center for Health Statistics. Vital
Statistics of the United States 2000
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Ageing is a near-universal phenomenon
Mortality rate
Adult age
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There is wide variation in longevities of species
Maximum lifespan 10 years
Maximum lifespan 8 years
Maximum lifespan 180 years
Maximum lifespan 80 years
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Is the rate of ageing heritable?
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There is indeed a heritable basis to ageing
INDY (Im Not Dead Yet) Locus in Fruitflies
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You can select for longevity in the lab
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If ageing is bad for an individuals reproductive
success and longevity is heritable, why do
individuals age at all?
AND HAVE MOST CHILDREN
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Why do we age?
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• Inevitable wear and tear
• For the good of the species
• For the good of the family
• Natural selection does not care about oldies
• Trade-off between early and late success
• Reproduction vs. repair

8. Rate of living
7. Telomeres / Free radicals / Mitochondria
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Inevitable wear and tear?
Reactive oxygen species
Unlike fridges, organisms are capable of evolving
protection and self repair.
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Passing the buck on telomeres and Hayflick limits
Cellular characteristics may not cause lifespan,
and even if they do, we still need to explain
variation in cellular characteristics!
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Good of the species organisms age to make room
for future generations
as consumers of nourishment in a constantly
increasing degree, are an injury to those
successors. Natural selection therefore weeds
them out, and in many cases favours such races as
die almost immediately after they have left
successors..
Wallace 1865
Weismann 1866

• Counter-selection for longer-lived cheats
• No evidence for widespread genes resulting in
  programmed death

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Good of the family organisms die to help their
relatives
Self-destruct mechanisms in octupii
Senescence still occurs in species without kin
structure
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Natural selection does not care about old folk
Benign neglect If a gene has a harmful effect
late enough in individual life, its consequences
may be completely unimportant to reproductive
success
Mutation accumulation theory Genes with
late-acting deleterious effects build up in
populations because there is little
counter-selection to remove them.
Huntingtons disease, some types of cancer
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Quiz Two strategies Live fast, die young Live
two years, two offspring per year Steady as she
goes Live five years, one offspring per year
What strategy would spread quickest?
Selection for early success, even if it comes at
the expense of later success
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Tradeoffs live now, pay later
Testosterone Calcium deposition
Antagonistic pleiotropy theory
Live now, pay later ageing is the hangover after
the reproductive party
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The Rolls-Royce Hypothesis
Problem surely ageing is not all due to genes
with late-acting harmful effects. Damage is
important too!
Repair and protection mechanisms only evolve to
be as good as they need to be to protect
organisms for the natural duration of their lives
you dont need a Rolls-Royce engine if you are
going to scrap the car within 5 years.
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Why do we age? a) Its not as simple as wear and
tear b) Cellular based explanations pass the
buck c) Organisms do not age for the good of
their species d) Some organisms may age for the
good of their kin, but it cant be a general
explanation e) There is little doubt that the
force of natural selection declines with age f)
Trade-offs may also be involved live now, pay
later g) We need to think about damage and repair

The future prospects for human longevity.
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SCIENCE CAFE
QUESTIONS ?
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Damselflies RULE!
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Some species live a long time why dont we all
do this?
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Mortality plateaus
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Disposable Soma Theory
Resources invested in reproduction cannot be used
for damage repair. Ageing arises as a
consequence of selection for investment rules, in
which reproduction is favoured over somatic
repair.
An investment strategy preferring reproduction
over repair
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Disposable soma
Repair
Reproduction
Energy
Problems - repair mechanisms are physical
things, not genetic thermostats - low calorie
diets can extend life - there is not always a
trade-off between longevity and reproduction
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Field evidence shows that rates of senescence are
malleable
Derelict sites
Natural pastures
Grown from seed in same conditions
Weeks
Months
Months later
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Ageing has a long evolutionary history