"Nothing in biology makes sense except in the light of evolution." This statement by the well-known geneticist Theodosius Dobzhansky has been verified by the study of aging. The operation of natural selection means that some genetic variants of any population will be more successful
(i.e., leave more copies of their genes in the next generation) than will other variants, and the first variant will be favored.
Most known populations are structured by age; that is, the population is composed of individuals of different age classes, each of which represent a different proportion of the population. The high mortality rates resulting from predation, illness, and accidents that are common among wild populations indicate that only a few, if any, individuals live long enough to show signs of aging and senescence. Thus, in any wild population there are many more young breeding adults than old adults, and in each generation the genetic contributions to the next generation come predominantly from young adults. One consequence of this age structure is that deleterious genetic variants that act late in life are not selected against because their carriers probably will have died from environmental hazards before they reach old age or will have survived, but as postreproductive adults. In either case they are invisible to the operations of natural selection. Another consequence is that long-lived genetic variants will not be selected because they are expressed only in those few surviving postreproductive individuals.
From an evolutionary point of view, the "name of the game is to play again"; that is, the whole point of being a reproductive adult is to pass copies of one's genes to the next generation. This is a game that no one can win but anyone can lose simply by not transmitting sufficient copies of his or her genes to the next generation. There is no evolutionary value (i.e., Darwinian fitness) in any trait, including extended longevity, if that trait does not materially assist one in playing the game. There is evolutionary value in living long enough to reproduce, but there usually is no increased fitness associated with living so long that an individual is postreproductive (see Rose for review and references).
However, because people live so long already, why are they not capable of reproducing and living indefinitely or at least much longer than they do now? The answer to this question involves energy. Organisms must channel and apportion their energies into reproductive activities as well as into the maintenance and repair of the soma. Although the energy cost of making an egg or sperm probably stays more or less constant over time and is therefore the same for both young and old, this is not the only energy cost incurred in reproduction. The energy costs of courtship, pregnancy, and child rearing are high and represent a significant investment of energy by an organism. In addition, some energy must be devoted to the repair and maintenance of the soma if an organism is to survive reproduction. It is reasonable to assume that even a well-fed organism has only a limited amount of energy available to it. Thus, the problem facing the organism is how best to allocate its finite metabolic energy to maximize both reproduction and repair.
A theoretical analysis by Kirkwood (1987) showed 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 and thus decrease an organism's Darwinian fitness. In contrast, increasing fecundity will decrease the energy available for repair and thus probably result in 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. Thus, fitness is maximized at a repair level lower than that required for indefinite somatic repair. Hence, people die. It is easy to see how this theory came to be known as the disposable soma theory. This process is nothing more than the cost-benefit analysis most people make when faced with the decision whether to continue to invest their hard-earned money in repairs to the old car or invest it in purchasing a new car. At some point the cost of repairs exceeds the cost of purchase, and so the old car is junked and a new one is obtained.
Because modern humans have a very low and culturally controlled rate of reproduction, it is reasonable to question whether the disposable soma theory still applies to human beings. It does, for people evolved under its aegis and the control mechanisms of the body that set fitness and repair levels are not reversed by one or two centuries of nonheritable demographic change. This concept provides a plausible mechanism by which evolution can act and has made people what they are today. Shakespeare foresaw this relationship in Sonnet 12:
When I do count the clock that tells the time, and see the brave day sunk in hideous night; ... Then of thy beauty do I question make, That thou among the wastes of time must go, ... And nothing 'gainst Time's scythe can make defence Save breed, to brave him when he takes thee hence.
Therefore, people age not because of a philosophically satisfying cosmic reason that requires senescence and death but simply because the body's energy allocations are such that failure to repair ensures that there is no reason not to age. This biological conclusion may seem dark:. Who, after all, wants to believe that his or her death serves no larger purpose? The major religions of the world are based on the opposite premise (but see Holliday). Some people, however, find it liberating. Jacob compared embryonic development to adult aging and saw a paradox. What biogerontologists see in the early twenty-first century is the fact that there is no evidence for the existence of a genetically based aging program. People do not have an organismal death program built into their genes. Human beings are not required to age. It follows that if people age only because there is no biological reason for them not to age, this clearly implies that people need not age (or at least not age so quickly) if they can supply their bodies with a relevant biological reason not to age. It is the business of biogerontologists, then, to provide those reasons (de Grey, 2002).
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