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Chap 20.doc

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Maryann Vaughan

Basic Issues in Life History AnalysisFig 132 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 tissuesoAfter she became sexually mature the female stopped growing thereafter using her energy for metabolism repair and reproductionChanges in life history are caused by changes in the allocation of energyoFor example a different female opossum might stop allocating energy to growth at an earlier age thereby reaching sexual maturity more quicklyThis strategy involves a tradeoff The femalealso matures at a smaller size which means that she will produce smaller litters babiesoStill 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 tradeoff Allocating less energy to reproduction means having smaller litters babiesWhy Do Organisms Age and DieAging or Senescence is a latelife 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 fitnessoTherefore aging should be opposed by natural selectionTwo theories on why aging persistsoRateofLiving theoryInvokes an evolutionary constraint Posits that populations lack the genetic variation to respond any further to selection against agingoEvolutionary theoryInvokes a tradeoff between the allocation of energy to reproduction versus repair The RateofLiving Theory of AgingHolds that aging is caused by the accumulation of irreparable damage to cells and tissuesDamage to cells and tissues is caused by errors during replication transcription and translation and by the accumulation of poisonous metabolic byproducts Under this theoryall 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 predictionsoBecause cell and tissue damage is caused in part by the byproducts of metabolism the aging rate should be correlated with the metabolic rateoBecause 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 oThis theory holds that aging is a function of metabolic rate but data on variation in metabolic rate and aging among mammals deny this theoryoMany populations are not in fact up against intrinsic limits to longevityoThey 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
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