In contrast to the life span of a species, which is an evolved, positively selected life-history trait analogous to a biological "warranty period," aging is a byproduct of evolution analogous to the progressive deterioration of any system that increases the likelihood of failure. Inasmuch as the concept for designing a machine is fundamentally different from the concept for understanding its deterioration, it follows that a theoretical framework for understanding life span must necessarily be different from the theoretical framework for understanding aging. Therefore, the purpose of this section is to provide a brief overview of the main theories of aging. We begin with a description of the theory concerning why all sexually reproducing organisms grow old, followed by the more conventional theories ordered hierarchically—molecular, cellular, systemic, and evolutionary.
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