Geoscience Reference
In-Depth Information
The core concept of natural selection is that species continuously
adapt to their environment. This means that organisms tend to be
well suited to the normal stresses they encounter, even those that
occur on a time scale of thousands or hundreds of thousands of years.
(Note that Raup said that the stress has to be unfamiliar not only
to
individuals
but to entire
species.)
Normal environmental changes,
such as a gradual lowering or raising of sea level, or a gradual alter-
ation in climate, cannot by themselves cause a mass extinction—they
allow species time to adapt or to migrate to more favorable climes.
Some species, it is true, will be unable to do either and will become
extinct, but those few do not a mass extinction make. This is more
than theory. Another Chicago paleontologist, David Jablonski, exam-
ined the fossil record from major mass extinctions in the geologic
record to see if they correlated in any way with known changes in sea
level, global climate, and mountain building; he found that they did
not.
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Of course, catastrophic events such as floods, earthquakes, and
volcanic eruptions can and do kill many individuals and, on rare
occasions, even species (if they occur in a limited geographical area),
but these are not global or even continental in their reach. For spe-
cies, the opposite of the old saying, "What you don't know can't hurt
you" is true: It is what a species does know that can't hurt it.
But suppose that Raup is wrong and that global cooling can cause
a mass extinction. Then the evidence should be readily at hand, for
the earth has just suffered a succession of ice ages, the last one end-
ing only some 15,000 years ago. Temperatures fell so far that huge ice
sheets advanced thousands of miles, covering, for example, most of
the northern half of the United States. Many of the large mammal
species (of perennial fascination to
Homo sapiens)
did become ex-
tinct during the last ice age, but the overall extinction rate was far
below that of the five major mass extinctions and barely makes it
onto a chart of extinction intensity (Figure 16). Admittedly, scientists
FIGURE I 6
Variation in intensity
of extinction for the last 600 million
years broken up into 1-million-year
intervals. [After David Raup.] The
Big Five are out on the right tail;
Pi refers to the Pleistocene extinction
rate, which is far below that of a mass
extinction, even though climate and
sea level changed drastically.
