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The reliability theory of aging
Another theory of why organisms age, called reliability theory, attempts to apply mathematical theories of reliability often used in engineering to predict systems failure in machines. The basic premise is that complex systems, whether manmade or biological, are made up of numerous subsystems, many of which are redundant. Because of this redundancy, the failure of one or more subsystems does not result in the death of the organism or the complete failure of the machine, but as subsystems fail, the efficiency of the organism or machine deteriorates. This period of deterioration is equivalent to aging in biological organisms, which do not die until a critical number of subsystems have failed.
 This theory explains why animals do not necessarily die as soon as they stop reproducing. Natural selection engineers animals to live long enough to successfully reproduce. Once they have finished their reproductive years, it takes variable lengths of time for enough of an animal’s subsystems to fail for redundancy to be eliminated and leave the animal vulnerable to death upon further system failure.
This theory also explains why some one-celled, simple biological organisms do not display signs of aging. They have no subsystems. When any kind of failure occurs, they die.
This theory does not, however, explain why certain species live longer than others. Some scientists, such as Leonid Gavrilov and Natalia Gavrilova, who have written extensively on the biological applications of reliability theory, believe that reliability theory is compatible with many aspects of the evolutionary senescence theory of aging and that combining aspects of the two theories may yield improved understanding.
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