The Evolution of Life Span
Why do we live as long as we do?
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why do we age?
why do we live as long as we do?
Evolution Basics
Natural Selection -The process by which the individual with the greatest
fitness is selected from a population of genetically
variable individuals of one species.
Fitness = reproductive success
Individuals with the best reproductive success have more offspring.
And so on, and so on, until the adaptation (gene) that led to greater
reproductive success is present throughout the species.
Evolution (natural selection) will only act on genes (traits) that
lead to greater reproductive success.
Evolutionary Theory of Life
Span
- Huntington’s Disease, a dominant lethal mutation
How does Huntington’s stay in the population if it results in lethality?
*JBS Haldane
100
%
Natural
Selection
10
20
Age of onset for Huntington’s = ~35yr
30 40 50 60 70
age in years
80
Hypothesis: Aging results from a decline in the
force of natural selection.
Aging in Nature
Natural Selection
Aging Begins
100
%
Alive
Life Span in Nature
1
2
3
Life Span in the Lab
4
5
6
age in years
7
8
- Most organisms do not age in a natural environment.
Lifespan is proportional to extrinsic mortality!
-If mortality is high an organism will die from
predation or other hazards before it grows old.
-Therefore, if extrinsic mortality limits survival
there is no reason to evolve a life span
that is longer than an organism would
normally survive in nature.
Evolutionary Theories of Aging
Disposable Soma - Somatic cells are maintained only to ensure
continued reproductive success, following reproduction
the soma is disposable. (life span theory)
Antagonistic Pleiotropy - Genes that are beneficial at younger
ages are deleterious at older ages.
(Pleiotropism = The control by a single gene of
several distinct
and seemingly unrelated phenotypic effects)
Mutation Accumulation - Mutations that affect health at older
ages are not selected against (no strong evidence).
Evolutionary Theories of Aging
Disposable Soma
Natural Environment
Protected Environment
Antagonistic Pleiotropy
Mutation Accumulation
Nature 408, 233-38 (2000)
Life span is proportional to body mass
Traits That Correlate with Longevity
- Can evolution theory help explain these correlations?
- Body mass, brain mass
- Length of growth period, prolonged adolescence
(*typical for organisms with large brains)
- Metabolic rate
(inversely correlated)
- Fecundity, # of offspring
(inversely correlated)
Examples from Nature
Opossums and Life Span
- ultimate prey, ~ 80% die from predation
- typically reproduce once
- age very rapidly
-Hypothesis: The presence of predators limits life span, natural
selection favors somatic maintenance for only as long
as an average opossum can be expected to live.
-How could you test this hypothesis?
Steve Austad, U. of Idaho
Sapelo Island Opossums
- no predators (out in daytime)
- longer average life span
- reproduce twice (fewer offspring/litter)
-Are these changes due to a lack of predators, or a physiological
change that delays the aging process?
Physiological Change - Sapelo island opossums not only
live longer, they age slower than mainland animals.
-Sapelo Island opossums have less oxidative
damage than mainland opossums.
(collagen X-linking)
Evolution in the Laboratory
Michael Rose, U.C. Irvine
% Surviving
Drosophila Survival Curve
Average life span = ~40d
Age in Days
Selection at age of reproduction alters lifespan
Offspring of “old” flies are selected
% Surviving
- Reproductive period extended
- Stress resistant, -super flies
- Early adult fecundity reduced
*antagonistic pleiotropy
Normal
old flies selected
Age in Days
Evolution in the Laboratory
Offspring of “young” flies are selected
- Early adult fecundity increased
*antagonistic pleiotropy
% Surviving
old flies selected
Normal
young flies
selected
Age in Days
Summary of Drosophila Selection
1) Selection at age of reproduction can alter the lifespan of
Drosophila (lifespan has been doubled by this technique).
2) Increase in lifespan has a cost, reduced fecundity (reproduction).
- antagonistic pleiotropy -
3) Long-lived flies are stress resistant (heat shock, oxidants).
Exceptions to the Rule
Some organisms evolve unique adaptations that allow the
subsequent evolution of exceptional life span.
Rats and Bats: Rats live for ~3 years, Bats live for ~30 years
Bats evolved a mechanism (flight) that reduced extrinsic mortality
and allowed for the subsequent evolution of a long life span.
What other adaptations might lead to prolonged life span?
Exceptional Life Span in Eusocial Insects
Queen Bees and Queen Ants have exceptional life spans!
•Small size
•Many offspring
Why do they live so long?
- Protected from the environment, therefore extrinsic mortality is low!
Extraordinary lifespans in ants: A test of evolutionary theories of ageing
85
80
10
5
0
0
10
20
30
Keller L, Genoud M: Extraordinary lifespans in ants: A test of evolutionary theories of ageing. Nature (London) 1997, 389:958-960
Exceptional Life Span is due to Social Organization
Keller L, Genoud M: Extraordinary lifespans in ants: A test of evolutionary theories of ageing. Nature (London) 1997, 389:958-960
What does this tell us about aging?
-Body size is correlated with lifespan, but is not
necessarily a determinant of lifespan.
-Reproduction / metabolism does not control life span.
•Some queen ants produce their body weight in offspring each day
-Life Span results from selective pressures.
What about Humans?
-Unlike most animals, humans and some related primates age in
a natural environment.
-Menopause is also unique to humans. How can nature select for
a process that limits reproduction?
-How does parental care influence the evolution of human life span?
Age specific mortality in humans
Evolutionary models that integrate parental investment can more
accurately predict age-specific mortality rates.
Rethinking the evolutionary theory of aging: Transfers, not births, shape senescence in social species
(PNAS Vol.100 No.16) Ronald D. Lee UC Berkeley
The p53 Tumor Suppressor
Stress
Genome Stress
DNA damage
Oxidative Stress
p53
Apoptosis
Senescence
Growth Inhibition
- Loss of p53 function results in an increased incidence of cancer
- p53 is mutated in ~80% of all human tumors
p53 may promote aging…
p53
Cancer
Aging
Why did we evolve a system that
limits our lifespan?
-to protect against cancer!
(Antagonistic Pleiotropy again!)
Life Span versus Aging
Aging - can not be selected for, results from an absence
of natural selection.
Life Span - results from selection and extrinsic mortality
Environmental Selection - predators, natural hazards
Social Selection - parental investment, sexual behavior
Main Ideas
1. Life span results from selective pressure.
2. Life span is inversely proportional to extrinsic mortality.
3. Aging results from a lack of natural selection with age.