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theory of aging

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BIOL 1602
Rebecca Jubis

 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 r
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