Manual The chances of death : and other studies in evolution

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This work was reproduced from the original artifact, and remains as true to the original work as possible. Therefore, you will see the original copyright references, library stamps as most of these works have been housed in our most important libraries around the world , and other notations in the work. This work is in the public domain in the United States of America, and possibly other nations. Within the United States, you may freely copy and distribute this work, as no entity individual or corporate has a copyright on the body of the work.

As a reproduction of a historical artifact, this work may contain missing or blurred pages, poor pictures, errant marks, etc. Scholars believe, and we concur, that this work is important enough to be preserved, reproduced, and made generally available to the public. We appreciate your support of the preservation process, and thank you for being an important part of keeping this knowledge alive and relevant. Seller Inventory LHB More information about this seller Contact this seller 1.

Seller Inventory APC More information about this seller Contact this seller 2. More information about this seller Contact this seller 3. Item added to your basket View basket. Proceed to Basket. View basket. Why do we age and die? This poses an evolutionary paradox: natural selection designs organisms for optimal survival and reproductive success Darwinian fitness , so why does evolution not prevent aging in the first place? Shown here is a case of severe atherosclerosis of the aorta. Image a courtesy of Wikipedia.

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Image b courtesy of Rakesh Ahuja, M. Image c courtesy of Dr. Edwin P. Ewing, Jr. For centuries, beginning with Aristotle, scientists and philosophers have struggled to resolve this enigma. The Roman poet and philosopher Lucretius, for example, argued in his De Rerum Natura On the Nature of Things that aging and death are beneficial because they make room for the next generation Bailey , a view that persisted among biologists well into the 20th century. The famous 19th century German biologist, August Weissmann, for instance, suggested — similar to Lucretius — that selection might favor the evolution of a death mechanism that ensures species survival by making space for more youthful, reproductively prolific individuals Weissmann But this explanation turns out to be wrong.

Since the cost of death to individuals likely exceeds the benefit to the group or species, and because long-lived individuals leave more offspring than short-lived individuals given equivalent reproductive output , selection would not favor such a death mechanism. A more parsimonious evolutionary explanation for the existence of aging therefore requires an explanation that is based on individual fitness and selection, not on group selection.

This was understood in the 's and 's by three evolutionary biologists, J.

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Haldane, Peter B. Medawar and George C. Williams, who realized that aging does not evolve for the "good of the species". Instead, they argued, aging evolves because natural selection becomes inefficient at maintaining function and fitness at old age. Their ideas were later mathematically formalized by William D.

Hamilton and Brian Charlesworth in the 's and 's, and today they are empirically well supported.

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Below we review these major evolutionary insights and the empirical evidence for why we grow old and die. As mentioned above, the key conceptual insight that allowed Medawar, Williams, and others, to develop the evolutionary theory of aging is based on the notion that the force of natural selection, a measure of how effectively selection acts on survival rate or fecundity as a function of age, declines with progressive age see Hamilton , Charlesworth , Rose et al.

This was first noted, though not formally analyzed, by Fisher in his famous book The Genetical Theory of Natural Selection , and both Haldane and Medawar , came to the same conclusion. Thus, the disease would not have been "seen" by, or subject to, selection. Based on Fisher's and Haldane's ideas, Medawar , worked out the first complete verbal and graphical model of how aging evolves also see next section.

The gist of Medawar's argument is as follows. First, for most organisms, the natural world is dangerous since it abounds with competitors, predators, pathogens, accidents, and other hazards. It follows from this that in natural populations most individuals die or get killed before they can grow old and suffer the symptoms of aging: thus, individuals have a very small overall probability of being alive and reproductive at an advanced age e.

Second, the strength of natural selection declines with increasing age Figure 2 , such that selection ignores the performance of individuals late in life. As a consequence, selection is unable to favor beneficial effects, or to counteract deleterious effects, when these effects are expressed at advanced ages. For example, if a beneficial or deleterious mutation occurs only after reproduction has ceased, then it will not affect fitness reproductive success and can therefore not be efficiently selected for or against.

However, even if a mutation occurs earlier, say during the reproductive period, its effects may not be visible to selection since, if extrinsic, environmentally imposed mortality is high, individuals that could express the mutation are likely to be dead already. Haldane and Peter B.

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  • Medawar that was later mathematically formalized by William D. In the shaded area the "selection shadow" selection cannot "see" deleterious mutations whose effects are confined to late ages: a harmful mutation that has a negative effect that is restricted to late life will likely already have been passed on to the offspring of the individuals bearing it, and selection will thus be inefficient at eliminating such a mutation from the population.

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    The concept of the declining force of selection is the fundamental basis for the evolutionary theories of aging also see Figure 3. Medawar , and Williams realized that these deductions, later mathematically expressed by Hamilton , also see Rose et al. Following the logic outlined above, Medawar , reasoned that, if the effects of a deleterious mutation were restricted to late ages, when reproduction has largely stopped and future survival is unlikely, carriers of the negative mutation would have already passed it on to the next generation before the negative late-life effects would become apparent.

    In such a situation, natural selection would be weak and inefficient at eliminating such a mutation, and over evolutionary time such effectively neutral mutations would accumulate in the population by genetic drift, which in turn would lead to the evolution of aging.

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    The effects of such a mutation accumulation process would only become manifest at the organismal level after the environment changes such that individuals experience less extrinsic mortality e. Top A : Mutation Accumulation. Medawar realized that, if the force of selection declines with age, mutations or alleles that are neutral i. Such late-life deleterious genetic variants can lead to the evolution of aging, an idea called the mutation accumulation MA theory of aging.

    Bottom B : Antagonistic Pleiotropy. Williams developed Medawar's idea further by realizing that strong selection at early ages might favor mutations or alleles with beneficial effects on survival and reproduction, even if these same mutations or alleles exhibit pleiotropic deleterious effects at advanced ages. At late ages, selection is weak and therefore inefficient at opposing such harmful genetic effects shaded area , especially when the same variants have positive effects that are favored by intense selection early.

    Williams' idea is known as the antagonistic pleiotropy AP theory of aging. Together, the MA and AP theories form the cornerstones of the evolutionary theory of aging. All rights reserved. Medawars' MA hypothesis was later put on firm mathematical ground by Charlesworth , In an influential paper published in Evolution, George C.

    Catalog Record: The chances of death, and other studies in evolution | HathiTrust Digital Library

    Williams took Medawar's ideas a step further. If it is true that selection cannot counteract deleterious effects at old age, he argued, then mutations or alleles might exist that have opposite, pleiotropic effects at different ages: genetic variants that on the one hand exhibit beneficial effects on fitness early in life, when selection is strong, but that on the other hand have deleterious effects late in life, when selection is already weak. Williams pointed out that, if the beneficial effects of such mutations early in life outweigh their deleterious effects at advanced age, such genetic variants would be favored and enriched in a population, thus leading to the evolution of aging.

    Thus, under Williams' hypothesis, the evolution of aging can be seen as a maladaptive byproduct of selection for survival and reproduction during youth. The idea here is that the evolution of a higher investment is unlikely to pay off since the return from such an investment may never be realized due to extrinsic mortality.

    Moreover, investment into reproduction — or early fitness components in general — might withdraw limited resources that could otherwise be used for somatic maintenance and repair. Such resource allocation trade-offs can thus been seen as a physiological extension of Williams' AP model. Whether such trade-offs are physiologically caused by competitive energy or resource allocation — as would be expected under the DS hypothesis — remains somewhat controversial, but the trade-offs themselves are well established see Flatt These elegant experiments represent the first solid empirical tests of the evolutionary theory of aging Rose The classical evolutionary theory of aging has therefore two fundamental cornerstones: MA and AP.

    However, it is worth noting that both models are conceptually very similar: under MA, aging evolves through the accumulation of effectively neutral mutations with deleterious late-life effects, whereas, under AP, aging occurs due to mutations with beneficial early- and deleterious late-life effects.

    In reality, probably both types of mutations occur in populations, yet their relative frequencies remain unknown. Furthermore, the age distribution of mutational effects may be much more complicated than these two scenarios suggest e. Different species vary dramatically in how long they life.

    Even older than giant tortoises are certain trees, such as the yew Taxus baccata , with some specimens between 4, and 5, years old c. A few other organisms, such as freshwater polyps of the genus Hydra , are thought to age at a negligible rate or to be even potentially immortal, although this is still somewhat controversial d.

    here Image a courtesy of Matthew Field. Image b courtesy of Fritz Geller-Grimm. Image c courtesy of Wikipedia.