FEATURE ARTICLE
Aging: A Biological Perspective
A variety of techniques extend the lives of model organisms, and similar approaches might help human beings stay healthy longer
Robert Arking
Is Aging Adaptive?
"Nothing in biology makes sense except in the light of evolution," according to the well-known geneticist Theodosius Dobzhansky, and his statement applies to aging. By natural selection, some genetic variants of any population will be more successful—putting more copies of their genes into the next generation—than others, and the more numerous variants will be favored. Moreover, the high mortality rates resulting from predation, illness and accidents that are common among wild populations indicate that few, if any, individuals live long enough to show signs of aging and senescence. So any wild population consists primarily of young, breeding adults who make the genetic contributions to the next generation. Consequently, deleterious genetic variants that act late in life are not selected against because their carriers probably either die from environmental hazards before they reach old age or survive as post-reproductive adults. In either case, those genes are invisible to natural selection. In addition, long-lived genetic variants will not be selected for because they are expressed only in those few surviving post-reproductive individuals.
From an evolutionary perspective, the entire reproductive game revolves around passing copies of genes to the next generation. No trait, including extended longevity, provides evolutionary value unless it makes an individual more successful in this game. Living long enough to reproduce does merit evolutionary value, but living long enough to be post-reproductive supplies no increased fitness, at least in the case of non-social animals.
People already live long. Why then are we not capable of reproducing and living indefinitely or at least much longer than we do now? The answer involves energy. An organism must divide its energy between maintenance, repair and reproduction. Even a well-fed organism copes with energy limitations. As a result, organisms face a tough problem: What is the best allocation of finite metabolic energy to maximize reproduction and repair?
In 1977, Thomas Kirkwood of the University of Newcastle Upon Tyne showed theoretically that increasing the amount of energy expended on somatic repair results in increased survivorship but decreased fecundity, and vice versa. A choice must be made. Reproduction requires less energy than does repair. Therefore, allocating sufficient energy to maximize somatic repair will reduce fecundity, which decreases an organism's Darwinian fitness. In contrast, increasing fecundity will decrease the energy available for repair and probably result in a shortened longevity. In most cases, decreased fecundity over a longer life span yields fewer copies of an individual's genes in the next generation than does higher fecundity over a shorter lifetime. Accordingly, maximum fitness takes place at a repair level lower than that required for indefinite somatic repair, and organisms eventually die. This is the so-called disposable soma theory.
This theory reveals an intriguing point: An organism ages when energy allocations fail to make adequate repairs, not because of a genetically based aging program. As a result, humans are not required to age. So if people age only because there is no biological reason not to, then some intervention might stop—or at least slow—aging.
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