Thesis # 15 – Aging evolves because of the previously adduced evolutionary genetic limitations to the forces of natural selection, which are affected by physiology, but aging is nonetheless not a merely physiological process.
Do the last five theses imply that physiology does not matter? No, not at all. Physiological features of organisms matter a great deal for the evolution of their aging.
Both insects and mammals are thought to age universally. That is to say, no one has ever found an insect or mammalian species in which some part of adulthood is not marked by endogenous deterioration, regardless of how diligently they are cared for by their owners, zookeepers, or attentive laboratory scientists. When studied with enough care, all these species show some type of aging pattern that features both functional impairment and increased risk of death, along with declining reproduction in those species that reproduce more than once.
But is their physiology irrelevant? Insects and mammals differ significantly in their capacity to produce new cells as adults. Mammals have a great deal of cell proliferation as adults. Insects have fairly little, with most of this cell proliferation confined to their reproductive organs and their kidney-like organs, the Malpighian tubules. Mammalian aging often features an increased risk of cancer, a disorder of highly proliferative somatic cells. Insect aging rarely features cancer; insect cell proliferation is so stringently controlled in adults that very few aging insects have been found in which cancer is detectable. It happens, but without deliberately introducing mutations in a laboratory strain, insect cancer is very rare. Thus the cell biology of mammals and insects has material effects on their patterns of aging, including the presence or absence of cancer.
This example illustrates the point that the physiological features of a group of animal species affects how their aging evolves qualitatively. Previously, we have shown how other features of a group of species affects how their aging evolves quantitatively. Thus growing a shell, or not, strongly affects the evolution of aging. Some of the longest-lived animals are burrowing bivalves, which benefit from the protection of both their burrows and their shells.
One way to think of this relationship between aging and physiology would be as follows. Physiology at every level, from molecular biology to functional ecology, conditions the mortality risks and the patterns of reproduction of a species. These mortality risks and reproductive patterns determine the terms in the equations which define the Forces of Natural Selection. Those forces then condition the evolution of patterns of aging, subject to the availability of genetic variation, rates of mutation, and other evolutionary genetic factors. So physiology certainly conditions how evolution shapes aging. But it does not require, or by itself generate, aging.
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