Longevity: Selection

While there are other organisms that have been subjected to this type of selection, some insects and mice in particular, most of the studies that use natural selection to increase longevity have employed Drosophila melanogaster, the common laboratory fruit fly. Though a handful of experiments of similar design were performed in the period before 1980, none were designed specifically to increase longevity using natural selection. Since 1980, a variety of laboratories have employed the basic method of delaying fruit fly culture reproduction to increase longevity using natural selection.

The most consistent result found in these experiments is an increase in adult longevity, often by more than 50 percent. Another common result has been improvement in stress resistance in flies that live longer, although not all laboratories have found identical results with respect to increased stress resistance. A few laboratories have studied other aspects of the functional biology of longer-lived fruit flies. Such fruit flies appear to conserve water and store calories. There is no general reduction in metabolic rate: longer-lived fruit flies do not appear to "live less, longer." This is a notable contrast with nematode mutants having increased longevity, but decreased metabolic rate. Longer-lived fruit flies also appear to have improved flight stamina, along with increased reproductive performance in mid- and late-life. The general impression that the data give is that longer-lived fruit flies are more robust adults.

There are some controversies, however. The most prominent of these concerns the relationship between early fertility and longevity. In some of the populations selected for increased longevity, early female fecundity has been reduced. But in other populations that have shown increased longevity, early female fecundity has not been reduced. To some extent this disparity has been put into perspective by the experimental demonstration that the correlation between selection for increased longevity and any secondary effect on early reproduction depends on the specific environment in which early fecundity is measured. The same population may show a trade-off between early reproduction and longevity, or an absence of this trade-off, depending on such details as the amount of dietary yeast, the period of egg-laying, and so on. The most reasonable conclusion is that the trade-off between early reproduction and longevity is environment-dependent, rather than universal.

Overall, Drosophila have been successfully selected for increased longevity by manipulating natural selection. These organisms are largely free of inbreeding depression. Another notable feature of these experiments is that they have been extensively replicated, in several laboratories, with multiple selected populations and multiple control populations. This makes them excellent material for further research.