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Evolution__3rd_Edition

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646 PART 5 / Macroevolution<br />

Extinction is a relatively recent<br />

discovery<br />

Causes can sometimes be observed<br />

...<br />

allopatric speciation. However, his underlying reasoning is still part of the explanation<br />

why adaptive radiations occur when the conditions (numbered 1–4 above) are present.<br />

23.2 Causes and consequences of extinctions can be studied<br />

in the fossil record<br />

The discovery that species go extinct was made relatively recently in human history: it<br />

dates from the late eighteenth and early nineteenth centuries. Fossils had been known<br />

about long before that time, but when a fossil was found that differed from any known<br />

species, it could still have been alive in some unexplored region of the globe. As the<br />

global flora and fauna became better and better known through the eighteenth century,<br />

it became increasingly likely that some fossil forms were no longer alive. By the end of<br />

the century, several naturalists accepted that some marine invertebrate groups, such as<br />

the ammonites, were extinct.<br />

The best known taxa a the vertebrates, and mammals in particular a posed a special<br />

problem, however. Fossil bones are preserved as isolated, disarticulated fragments, and<br />

it is even more difficult to show that a single bone does not belong to any modern<br />

species, than it is for a complete specimen (such as a shell). The decisive work is usually<br />

credited to Cuvier. Cuvier reconstructed, with new standards of rigor, whole skeletons<br />

from bone fragments. It is easier to see whether the whole skeleton, rather than just the<br />

disarticulated bones, of a vertebrate belong to any living species. The most convincing<br />

cases of extinction were for gigantic forms like mastodons a it was hardly plausible that<br />

the explorers would have overlooked them. However, mistakes can still be made in<br />

recognizing extinctions, and Box 23.1 describes “pseudoextinctions.”<br />

Extinctions have two kinds of interest in evolutionary biology. One is the question of<br />

causality. Why do species go extinct? Some modern extinctions have been witnessed<br />

closely enough for the cause to be known with certainty. The enormous Steller’s sea<br />

cow was discovered by a shipwrecked German naturalist called Georg Steller in 1742,<br />

but he was the only naturalist ever to see it alive. The animals were completely tame a<br />

Steller records how he could stroke them a and by 1769 they had been hunted to<br />

extinction. The extinction of modern species by analogous human means is all too easy<br />

to observe now, but for fossil species we do not have evidence as direct as sailors shooting<br />

sea cows. The quality of the evidence depends on how recent the fossils are, and for<br />

very recent fossils we can have quite convincing evidence about the cause of extinctions.<br />

The most recent ice age, for example, which was at its peak about 18,000 years<br />

ago, almost certainly caused many local extinctions. If a species disappears before the<br />

advancing ice cap and does not return, there is little doubt what the cause of the extinction<br />

was. The tulip tree and hemlock are only two of the species lost from the European<br />

flora at that time, though both survived in North America.<br />

As we move further back in time, the causes of extinctions of particular species<br />

become more difficult to infer. We saw, in the well studied case of the Great American<br />

Interchange how uncertain the evidence is about the causes of the many extinctions,<br />

even though the Interchange took place only 2 million years ago and has left a good<br />

fossil record (Section 17.8, p. 512). However, the causes of extinctions can still be<br />

..

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