Thursday, May 06, 2010

Laboratory Life

April 13, 2010, 7:00 pm

Here’s a problem: evolution never stops.

Imagine you’re a wild fruit fly, of the species Drosophila melanogaster. You’re happily feasting on some yeast that’s growing on rotting fruit when, whoomf, you get sucked into a bottle and taken to a laboratory. From now on, this is your home.

Life in a bottle — or cage — is different from life in the wild. In nature, for example, fruit flies reproduce throughout their adult lives. Often, in the laboratory, they do not: flies grown in bottles may only be allowed to reproduce for the first five or six days after emerging from the pupa. (Wild flies can live for more than 80 days.) In nature, flies choose their mates. Often, in the laboratory, they do not: they are often assigned to one, and that one may be a close relative. On top of that, the food is different; infectious diseases are rare; predators are absent.

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In short, the pressures of daily life have been transformed — and traits that were an advantage Out There may no longer be so Inside. Similarly, traits that would have killed you in the wild may help you get along inside a bottle.

If, for example, older flies are never allowed to reproduce, the ability to lay eggs later in life becomes irrelevant, so there’s nothing to prevent the appearance of mutations that interfere with that ability. Indeed, if those mutations increase early fertility, they may even be favored: the most fecund young flies are likely to leave the most descendants.

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A second area where laboratory evolution can be a serious problem is in the study of subjects like the evolution of aging, and the diseases associated with it. For example, the study of laboratory populations may give a misleading impression of how easy it is to extend lifespans: since laboratory organisms tend to have unnaturally short lifespans, discovering ways to make them live longer may not be especially informative. We may simply be reversing the unnatural shortening that we created in the first place, a view supported by the fact that selection to increase lifespan in laboratory populations often simply restores it to levels seen in the wild.

Such realizations have led an increasing number of scientists to argue that long-established laboratory populations are “suspect starting material” for understanding aging, and that comparisons with wild populations “support the pessimistic interpretation that laboratory-adapted stocks of rodents may be particularly inappropriate for the analysis of the genetic and physiological factors that regulate aging in mammals.”


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