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Science Frontiers ONLINE No. 124: Jul-Aug 1999 |
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Preadaptive Evolution
"Preadaptive evolution" is a remarkable concept. Basically, it asserts that in some -- possibly many -- mutations, a useful response to an evolutionary challenge is "naturally" accompanied by useful responses to challenges that have not yet been posed to the life form in question. This prescience verges on the miraculous to the uninitiated, but mainstream biologists seem content to write the phenomenon off as merely good fortune -- like hitting two jackpots in a row on the same slot machine.
A good example of preadaptation occurs when bacteria are cultured in the presence of an antibiotic. Within a few weeks, they have evolved a resistance to that particular antibiotic. This well-known phenomenon is easily explained by evolution. However, often the newly evolved (or "adapted") bacteria are also resistant to several other antibiotics that work by different mechanisms. All of the multiple gene changes needed for the several different defense mechanisms are controlled by a single site on the same chromosome.
(Levy, Stuart B.; The Antibiotic Paradox, New York, 1992, p. 99. Cr. A. Mebane.)
Comments. How can bacteria prepare defenses against antibiotics they have not been exposed to? Luck, prescience, or some unrecognized mechanism?
In his Ever Since Darwin, S.J. Gould acknowledges that "preadaptation implies prescience although in actuality it means just the opposite! His explanation of "preadaptation is not easy to grasp.
"In short, the principle of preadaptation simply asserts that a structure can change its function radically without altering its form as much. We can bridge the limbo of intermediate stages by arguing for a retention of old functions while new ones are developing."