This chapter presents studies for multidisciplinary readers sharing interests at the crossroad of evolutionary theories of aging, molecular biology, and system biology. Its purpose is to discuss, from a mechanistic and systematic point of view, several aspects of our current understanding of aging as an evolutionary mechanism. By reviewing several illustrative studies, we emphasize how the incorporation in demographic models of aging of more detailed mechanistic descriptions of processes involved in the onset of aging can significantly improve the predictive power of these models and provide us with interesting hypotheses to explain the variety of aging patterns seen across the tree of life. Unsurprisingly, the results of these mechanistic and systemic approaches are in agreement with the main theories of aging. More interestingly, the studies which combine systemic dynamical models of mechanistic descriptions for the biological processes together with the effects of the forces of natural selection and environmental changes tend to demonstrate that these theories of aging, far from being exclusive, are actually complementary to each other. This could open the door for the elaboration of a unified and integrated theory of aging in the near future.