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Distinctions between extinctions.

Organisms that have settled comfortably into their evolutionary niches, having survived millions of years of normal, or "background," extinction forces, could suddenly find that the evolutionary tables have turned on them during the relatively brief and rare episodes of mass extinction that punctuate the history of the earth. According to David Jablonski at the University of Chicago, adaptive traits that enhance survival and diversification of species during times of background extinction tend to have little in common with those traits that increase the chances of survival during mass extinctions.

Currently evolutionary theory is based almost exclusively on pattersn of background extinctions, Jablonski says; scientists have assumed that mass extinctions simply accelerate or emphasize trends of background extinctions so that the same kinds of organisms are wiped out by a mass extinction, only in much greater numbers. But Jablonski's finding that the two extinction regimes differ qualitatively as well should inspire a new view of the evolutionary forces that shape life.

Jablonski arrived at his conclusions by comparing the volutionary patterns of marine organisms that lived during the background extinctions in the last 16 million years of the Cretaceous period to those of marine life at the very end of the Cretaceous, 65 million years ago, when a mass extinction event killed off a large portion of species. He found that during background times, traits such as a broad geographic range and mobile larvae enhanced chances of species survival, and having many species in a clade (group of related species) increased the odds of clade survival. But these same traits were "ineffectual" during the mass extinction, which instead favored clades having wide geographic range, regardless of the number of member species. During mass extinctions, "evolution is channeled in directions that could not have been predicted on the basis of patterns that prevailed during background times," writes Jablonski in the Japan. 10 SCIENCE.
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Title Annotation:differences between background and mass extinctions
Publication:Science News
Date:Feb 1, 1986
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