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Chapter 13


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McMaster University

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 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 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 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-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 begins • In an evolutionary sense, hereditary nonpolyposis colon cancer is a manifestation of senescence that is caused by deleterious mutations o these 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 o mutation 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 o this 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 • Because natural selection is weaker late in life, alleles that enhance early-life reproduction may be favoured even if they also hasten death • What kind of mutation could increase reproduction early in life at the same time it reduced reproduction or survival late in life? o Perhaps a mutation that causes less energy to be allocated to repair early in life and more energy to be allocated to reproduction • A cost-free mutation that extends life span and increases tolerance to a variety of stresses would challenge the evolutionary theory of senescence • Researchers have documented a trade-off between reproductive effort early in life and reproductive success late in life • When an experiment was carried out on a flycatcher population on an island: o Researchers followed the life histories of individual birds from hatching to death o Researchers found that some female fly catchers begin breeding at age 1, whereas others wait until age 2 o Females that breed at age 1 have smaller clutch sizes throughout life, indicating that there is a cost later in life to breeding early o To investigate further, researchers manipulated early reproductive effort by giving some first-year breeders extra eggs  Females given extra eggs had progressively smaller clutch sizes in subsequent years, whereas control females did not begin to show reproductive senescence until age 4  Researchers concluded that there is a trade-off in collared flycatchers between early-life and late-life reproduction  Researchers noted that despite this trade-off, first-year breeders had higher lifetime reproductive success than second-year breeders A Natural Experiment on the Evolution of Aging  Ecological mortality- mortality caused by extrinsic factors such as predators, diseases, and accidents  Physiological mortality- mortality caused by processes intrinsic to the organism; wearing out of body parts  Evolutionary theory of senescence predicts that populations with lower rates of ecological mortality will evolve delayed senescence (they will age later in life) o Both of the evolutionary mechanisms that lead to senescence have reduced effectiveness in populations with lower ecological mortality rates  In the case of late-acting deleterious mutations, lower ecological mortality means that a higher fraction of zygotes will live long enough to experience the deleterious effects (if there is low chance of being killed by the ecological factors, then the organism will live long enough to experience the late-acting bad mutations and its effects)  Late-acting deleterious mutations are thus more strongly selected against and will be held at lower frequency in mutation-selection balance  In the case of mutatio
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