Chapter 13- Aging and Other Life History Characters
The branch of evolutionary biology that attempts to make sense of the diversity in
reproductive strategies is called life history analysis
An organism truly perfected for reproduction would mature at birth, continuously produce
high-quality offspring in large numbers, and live forever
o This type of organism is called a Darwinian demon----no such organism exists
Trade-offs constrain the evolution of adaptations
13.1- Basic Issues in Life History Analysis
Fig 13.2 shows how a female opossum got her energy at different stages of her life, and the
functions to which she allocated that finite energy supply
Female possum before she became sexually mature, the female used her energy for growth,
metabolic functions like thermoregulation, and the repair of damaged tissues
o After she became sexually mature, the female stopped growing, thereafter using her
energy for metabolism, repair, and reproduction
Changes in life history are caused by changes in the allocation of energy
o For example, a different female opossum might stop allocating energy to growth at an
earlier age, thereby reaching sexual maturity more quickly
This strategy involves a trade-off: The female also matures at a smaller size,
which means that she will produce smaller litters (babies)
o Still another female might, after reaching sexual maturity, allocate less energy to
reproduction and more to repair, thereby keeping her tissues in better condition
Again there is a trade-off: Allocating less energy to reproduction means having
smaller litters (babies)
13.2- Why Do Organisms Age and Die?
Aging or Senescence- is a late-life decline in an individuals fertility and probability of survivial
Documentation of a bird, a mammal, and an insect, all show declines in both fertility and
survival
If everything else remains equal, aging reduces an individuals fitness
o Therefore aging should be opposed by natural selection
Two theories on why aging persists:
o Rate-of-Living theory
Invokes an evolutionary constraint
Posits that populations lack the genetic variation to respond any further to
selection against aging
o Evolutionary theory
Invokes a trade-off between the allocation of energy to reproduction versus repair
The Rate-of-Living Theory of Aging
Holds that aging is caused by the accumulation of irreparable damage to cells and tissues
Damage to cells and tissues is caused by errors during replication, transcription, and
translation, and by the accumulation of poisonous metabolic by-products
Under this theory-all organism have been selected to resist and repair cell and tissue damage
to the maximum extent physiologically possible
o They have reached the limit of biologically possible repair
o In other words, populations lack the genetic variation that would enable them to evolve
more effective repair mechanisms than they already have
This theory makes two predictions:
o Because cell and tissue damage is caused in part by the by-products of metabolism, the
aging rate should be correlated with the metabolic rate
o Because organisms have been selected to resist and repair damage to the maximum
extent possible, species should not be able to evolve longer life spans, whether
subjected to natural or artificial selection
o This theory holds that aging is a function of metabolic rate but data on variation in
metabolic rate and aging among mammals deny this theoryo Many populations are not, in fact, up against intrinsic limits to longevity
o They harbor genetic variation that would allow the evolution of longer life spans
And yet, longer life spans have not evolved
The Evolutionary Theory of Aging
Under the evolutionary theory, aging is caused not so much by cell and tissue damage itself as
by the failure of organisms to completely repair such damage
o This failure to fully repair leads to gradual decay and ultimate collapse
Given that organisms are capable fo constructing themselves from scratch, they should also be
capable of maintaining their organs and tissues once formed
Organisms do have remarkable abilities to replace or repair damaged parts; yet in many
organisms repair is incomplete
Under the evolutionary theory of senescence, the failure to completely repair damage is
ultimately caused by either :
o Deleterious mutations
o Trade-offs between repair and reproduction
Deleterious Mutations and Aging: The Mutation Accumulation Hypothesis
Many mutations causing death are highly deleterious
o A mutation causing death at age 2, would be selected against strongly
o Individuals carrying such a mutation would have an expected lifetime reproductive
success of zero
o Both mutations causing death after reproduction has begun are selected against less
strongly
o The later in life that such mutations exert their deleterious effects, the more weakly
they are selected against
o Mutations that are selected against only weakly can persist in mutation-selection
balance
o The accumulation in populations of deleterious mutations whose effects occur only late
in life is one evolutionary explanation for ag
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