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Biology 4218A

Chapter 15.1- 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 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) Why Do Organisms Age and Die? • Aging or Senescence- is a late-life decline in an individual’s 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 individual’s 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 theory o Many populations are not, in fact, up against intrinsic limits to longevity o They 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 o This 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 : 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 aging • What kind of mutation would cause death, but only at an advanced age? o One possibility is a mutation that reduces an organism’s ability to maintain itself in good repair • Germ
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