Longevity: Selection

The key to the use of organisms with increased longevity is that they must have slowed or abrogated the normal processes of aging. Organisms with reduced longevity may die because of novel pathologies, unrelated to normal mechanisms of aging. But this problem does not apply to longer-lived organisms. Consistent differences between organisms with increased longevity and closely related, normal-lived, controls must be related in some way to the control of aging.

An important qualification is the term consistent. If one longer-lived population is compared with one control population, then there may be genetic or physiological differences between them that are due solely to genetic drift. This problem can be solved, however, by replication of selected and control populations, since such random differentiation will separate selected populations from each other, as well as differentiating controls from each other. Such differentiation can then be partitioned out using analysis of variance techniques, leaving only the differences between selected and control that are specifically associated with the effects of selection. This method is now routinely used in studies of increased longevity in fruit flies.

An additional qualification is that some changes that are produced by selection for increased longevity may not themselves increase longevity. They could instead be side-effects of the changes that increase longevity. This problem may be resolvable through the use of further selection. For example, one character associated with increased longevity in some fruit fly populations is resistance to dying from total starvation. Additional selection for increased starvation resistance also increases longevity, indicating that starvation resistance is part of a mechanism controlling longevity.

One benefit of the comparison of longerlived with normal organisms is that any kind of character can be studied. In fruit flies, researchers have already studied life-history characters, behavioral performance, organismal physiology, biochemical composition, single-locus genetics, and gene expression. A general pattern is that, while many characters and genes have little association with increased longevity, a moderate number of characters are associated with increased longevity. This shows that selection can indeed be used to reveal the controls on longevity.