Evolution__3rd_Edition

646 PART 5 / Macroevolution
Extinction is a relatively recent
discovery
Causes can sometimes be observed
...
allopatric speciation. However, his underlying reasoning is still part of the explanation
why adaptive radiations occur when the conditions (numbered 1–4 above) are present.
23.2 Causes and consequences of extinctions can be studied
in the fossil record
The discovery that species go extinct was made relatively recently in human history: it
dates from the late eighteenth and early nineteenth centuries. Fossils had been known
about long before that time, but when a fossil was found that differed from any known
species, it could still have been alive in some unexplored region of the globe. As the
global flora and fauna became better and better known through the eighteenth century,
it became increasingly likely that some fossil forms were no longer alive. By the end of
the century, several naturalists accepted that some marine invertebrate groups, such as
the ammonites, were extinct.
The best known taxa a the vertebrates, and mammals in particular a posed a special
problem, however. Fossil bones are preserved as isolated, disarticulated fragments, and
it is even more difficult to show that a single bone does not belong to any modern
species, than it is for a complete specimen (such as a shell). The decisive work is usually
credited to Cuvier. Cuvier reconstructed, with new standards of rigor, whole skeletons
from bone fragments. It is easier to see whether the whole skeleton, rather than just the
disarticulated bones, of a vertebrate belong to any living species. The most convincing
cases of extinction were for gigantic forms like mastodons a it was hardly plausible that
the explorers would have overlooked them. However, mistakes can still be made in
recognizing extinctions, and Box 23.1 describes “pseudoextinctions.”
Extinctions have two kinds of interest in evolutionary biology. One is the question of
causality. Why do species go extinct? Some modern extinctions have been witnessed
closely enough for the cause to be known with certainty. The enormous Steller’s sea
cow was discovered by a shipwrecked German naturalist called Georg Steller in 1742,
but he was the only naturalist ever to see it alive. The animals were completely tame a
Steller records how he could stroke them a and by 1769 they had been hunted to
extinction. The extinction of modern species by analogous human means is all too easy
to observe now, but for fossil species we do not have evidence as direct as sailors shooting
sea cows. The quality of the evidence depends on how recent the fossils are, and for
very recent fossils we can have quite convincing evidence about the cause of extinctions.
The most recent ice age, for example, which was at its peak about 18,000 years
ago, almost certainly caused many local extinctions. If a species disappears before the
advancing ice cap and does not return, there is little doubt what the cause of the extinction
was. The tulip tree and hemlock are only two of the species lost from the European
flora at that time, though both survived in North America.
As we move further back in time, the causes of extinctions of particular species
become more difficult to infer. We saw, in the well studied case of the Great American
Interchange how uncertain the evidence is about the causes of the many extinctions,
even though the Interchange took place only 2 million years ago and has left a good
fossil record (Section 17.8, p. 512). However, the causes of extinctions can still be
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