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What Are We Doing to Evolution?
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17197 |
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NATURAL SCIENCE
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| Issue
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12 / 1990 |
2,632 Words |
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Warren D. Allmon
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One of the questions that scientists who study evolution are most often asked is "What will humans evolve into?" Such inquiries must always be answered in the same way: We have absolutely no idea what the descendants of modern humans will look like a million years or more from now. Yet this answer expresses one of the most important aspects of the evolutionary process that we think we do understand. Evolution, governed as we believe it is by Darwin's principle of natural selection, is inherently unpredictable. It is controlled by the reaction of populations of organisms to their environmental conditions. Since both reactions and conditions are so complex and can occur in so many ways, it is impossible to foresee how any particular species might evolve.
This does not mean, however, that we can say nothing about future evolution. While we cannot predict with certainty, we can look backward into the geological past. We can detect regularities in the changes life has undergone since its origin more than three and a half billion years ago, and particularly over the last 600 million years for which we have a good fossil record; we cause these regularities to make statements about what may or may not happen to life, given a particular set of circumstances.
Of the processes that contribute to evolution, the fossil record tells us most about extinction. Because the record is imperfect, it is often ambiguous about whether changes occurred within lineages, and if so how fast, or under what circumstances new species arise. Disappearance of species, in contrast, is usually unmistakable. This is particularly true when large numbers of species disappear over a relatively short interval, a phenomenon paleontologists call "mass extinction." The fossil record indicates not only that extinction has been the fate of many species but also that mass extinctions have occurred many times.
Indeed, the geological time scale itself, the system of long names that daunt the memories of the beginning student and provide the alphabet for all historical geology, is based on the appearance and disappearance of fossil species. The boundaries between the three eras, Paleozoic, Mesozoic, and Cenozoic, are defined by major extinction episodes. This is because fossils from the three eras, taken as a whole, look very different.
Marine invertebrates make up the vast majority of the fossil record and are the basic for recognizing mass extinctions. There is really no mistaking an "average" assemblage of Paleozoic marine invertebrates, dominated as it is by trilobites, brachiopods, stalked crinoids, and straight nautiloid cephalopods, or a typical Mesozoic assemblage filled with distinctive clams and ammonites. These differences are due in large part to the episodes of mass extinction that ended these eras. To a degree and by means still imperfectly understood, these events wiped the slate clean and allowed new assemblages of organisms to replace old. They reorganized the living world so substantially that not only are most of the organisms different in each era but they are often put together in communities in very different ways. Reefs in the Paleozoic, for example, have little resemblance to reefs in the Mesozoic or Cenozoic; their composition and structure are almost completely different.
The geological record of evolution, and particularly of extinction, tells us that species are interconnected and that individual species are linked to their
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