ABSTRACT The biological reasons for aging are now understood. Aging is the result of multiple stochastic events in molecules, cells, tissues and organs. These together produce the aged phenotype, senescence and ultimately death. Many of these changes can be directly linked to specific age-associated disease. However, there are also age-related changes that are not pathological. It can be said that aging has multiple causes, or is instead due to a general loss of molecular fidelity, that is, an increase in disorder. The complexity of organism means that they develop as ordered structures by obtaining energy from the environment. These ordered structures must be maintained by a wide variety of mechanisms which also depend on energy resources. Eventually these mechanisms fail, and senescence sets in. It is known that the efficiency of maintenance is correlated directly with the lifespan of different mammalian species. Also, these lifespans are inversely correlated with fecundity or reproductive potential. There is a trade off between investment of resources in maintenance of the body, or soma, and investment in reproduction.
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