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Chapter 13

BIOC51H3 Chapter Notes - Chapter 13: Senescence, Pleiotropy, Dna Mismatch Repair

Biological Sciences
Course Code
Maydianne Andrade

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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
oThis 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
oAfter 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
oFor 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)
oStill 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 individual’s fertility and probability of
Documentation of a bird, a mammal, and an insect, all show declines in both fertility and
If everything else remains equal, aging reduces an individual’s fitness
oTherefore aging should be opposed by natural selection
Two theories on why aging persists:
oRate-of-Living theory
Invokes an evolutionary constraint
Posits that populations lack the genetic variation to respond any further to
selection against aging
oEvolutionary theory
Invokes a trade-off between the allocation of energy to reproduction versus
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
oThey have reached the limit of biologically possible repair
oIn 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:
oBecause cell and tissue damage is caused in part by the by-products of metabolism,
the aging rate should be correlated with the metabolic rate
oBecause 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

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oThis theory holds that aging is a function of metabolic rate but data on variation in
metabolic rate and aging among mammals deny this theory
oMany populations are not, in fact, up against intrinsic limits to longevity
oThey harbour 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
oThis failure to fully repair leads to gradual decay and ultimate collapse
Given that organisms are capable of 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 :
oDeleterious mutations
oTrade-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
oIndividuals carrying such a mutation would have an expected lifetime reproductive
success of zero
oBoth mutations causing death after reproduction has begun are selected against less
oThe later in life that such mutations exert their deleterious effects, the more weakly
they are selected against
oMutations that are selected against only weakly can persist in mutation-selection
oThe accumulation in populations of deleterious mutations whose effects occur only
late in life is one evolutionary explanation for aging
What kind of mutation would cause death, but only at an advanced age?
oOne possibility is a mutation that reduces an organism’s ability to maintain itself in
good repair
Germ-line mutations in DNA mismatch repair genes cause a form of cancer in humans called
hereditary nonpolyposis colon cancer
Most people carrying mutations in the genes for DNA mismatch repair enzymes do not suffer
the deleterious consequences of the mutations until well after the age at which reproduction
In an evolutionary sense, hereditary nonpolyposis colon cancer is a manifestation of
senescence that is caused by deleterious mutations
othese deleterious mutations persist in populations because they reduce survival only
late in life
deleterious mutations that contribute to aging can accumulate rapidly
Trade-Offs and Aging: The Antagonistic Pleitropy Hypothesis
mutation involves a trade-off between reproduction early in life and survival late in life; its
pleiotropic effects are antagonistic
omutation causes reproductive maturation at age 2 instead of age 3, and mutation
causes death at age 10
mutation is pleiotropic
most of the individuals born with the mutation will live long enough to reap the benefit of
earlier reproduction, but few will survive long enough to pay the cost of early aging
othis mutation that causes both early maturation and early senescence is therefore
favoured by selection
selection for alleles with pleiotropic effects that are advantageous early in life and
deleterious late in life is a second evolutionary explanation for aging
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