Failure of the neuroendocrine system would be expected to produce profound impairments in homeostatic systems, including loss of reproductive function and metabolic regulation, which occur with age. Failure of the immune system would be expected to produce an increased susceptibility to infection and a decreased ability to reject tumor cells. Furthermore, even if this theory explains some manifestations of aging in higher organisms, it does not explain aging in lower organisms, and little is known about the primary mechanism behind such weakness. Although data suggest that older organisms have altered proteins reflective of such genetic changes, this theory does little to explain most observed age-related changes. Teleologically, it suggests that the rate of aging within each species has developed for the good of each species.
Individual variation develops because of maladaption, exposure, and lifestyle.
In the wild, such maladapted individuals tend to die out and the well-adapted ones persist, altering longevity in the best interest of the species. Exactly what controls the rate of aging is unknown. It could be a gene that controls telomere shortening or some other process of cell division. Or it could be genetic control of another cellular process not involved in division, such as DNA repair, thus resulting in apoptosis. These include baldness, osteoporosis, and dry, wrinkled skin.