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The evolutionary senescence theory of aging
The most widely accepted overall theory of aging is currently the evolutionary senescence theory of aging. Unlike the earlier programmed theory of evolution and aging, which tried to find reasons why evolution might favor aging, evolutionary senescence theory focuses on the failure of natural selection to be able to affect late-life traits.
 Natural selection, because it operates via reproduction, can have little effect on later life. In the wild, predation and accidents guarantee that there are always more younger individuals reproducing than older ones. Genes and mutations that have harmful effects that appear only after reproduction is over do not affect reproductive success and therefore can be passed on to future generations. In 1952, Peter Medawar proposed that the inability of natural selection to influence late-life traits could mean that genes with detrimental late-life effects could continue to be passed from generation to generation. This theory is called the mutation accumulation theory.
A few years later, George Williams extrapolated on this idea by formulating the theory of “antagonistic pleiotropy.” Antagonistic pleiotropy means that some genes that increase the odds of successful reproduction early in life may have deleterious effects later in life. Because the gene’s harmful effects do not appear until after reproduction is over, they cannot be eliminated through natural selection. An example of antagonistic pleiotropy in humans is p53, a gene that directs damaged cells to stop reproducing or die. The gene helps prevent cancer in younger people, but may be partly responsible for aging by impairing the body’s ability to renew deteriorating tissues. Because of antagonistic pleiotropy, it is likely that tinkering with genes to improve late-life fitness could have a detrimental effect on health at younger ages.
Much experimental evidence exists to support the basic premises of the evolutionary senescence theory of aging. For example, the theory predicts that delaying the age of reproduction should delay aging, as it would increase the power of natural selection later in life. Experiments with fruit flies in which younger flies were prevented from mating, allowing only older flies to reproduce, confirmed this prediction. Aging in the fruit fly population was delayed. However, these long-lived flies were less fertile in early life than normal flies, giving support to the idea of antagonistic pleiotropy. In experiments with roundworms given a gene mutation that extended their life span, scientists found that these long-lived worms exhibited defects, such as reduced ability to enter a protective dauer stage, delayed development, and impaired reproduction.
In the 1970s, Thomas Kirkwood added to the evolutionary biology theory of aging with his “disposable soma” theory. He believed that organisms have to balance the demands of maintaining their body, or soma, cells and reproducing. Because an organism invests resources into reproduction, over time mutations and other cellular damage accumulate in the soma because the body cannot repair all of it. This idea explains some of the disparity in life span between different types of organisms. Species that are likely to die to predation, such as mice, invest more energy in reproduction than in maintaining health because an individual is unlikely to live long anyway. Humans, on the other hand, have few predators and can therefore allocate more resources to repairing physical damage since they will be able to reproduce over a longer period of time.
Research conducted by Steven Austad in the early 1990s provides interesting proof of this idea, namely, that hazardous environments favor early reproduction and short life spans, whereas safer environments favor the opposite. Studying opossums in Virginia, he found that animals living on a predator-free island aged much more slowly and reproduced later than opossums on the more dangerous mainland.
The disposable soma theory may also explain why some organisms, like salmon or certain kinds of spiders, reproduce only once and then die. If the animal is likely to die anyway before the next breeding season, then natural selection would favor allocating all an animal’s resources to reproduction, leaving nothing for somatic maintenance.
Although many scientists believe the evolutionary theory of aging needs further refinement, most agree that it is currently the best explanation for why we and other organisms age.
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