Evolution__3rd_Edition

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Several factors may work
interactively
Frequency
CHAPTER 23 / Extinction and Radiation 655
mass extinction (Figure 23.5) and this factor is unlikely to be a general cause of all mass
extinctions. Volcanic eruptions on a large scale may also cause mass extinctions. Three
mass extinctions, including the two largest, are associated with large areas of rock that
were deposited after volcanic eruptions.
The various factors are not mutually exclusive. A large asteroid impact could trigger
volcanic activity or a change in climate a which could in turn affect sea level. The platetectonic
pattern also influences sea level and climate, and tectonic activity influences
vulcanism.
In summary, research on the causes of mass extinctions is considering various factors,
including asteroid impacts, vulcanism, sea level changes, climate, and plate tectonics.
Currently, it looks unlikely that any one factor acts as a general cause of all mass extinctions.
The complex pattern of evidence in Figure 23.5 suggests that several factors
may operate, in various combinations, to cause the observed pattern of extinctions.
This impression will be strengthened in the next two sections.
23.4 Distributions of extinction rates may fit a power law
Are mass extinctions a distinct kind of event? We can distinguish two conceptual
possibilities. One is that through the history of life the probability of extinction has
been approximately constant, though the probability varies by chance. Sometimes, by
chance, many taxa will go extinct in a time interval, at other times, by chance, few taxa
will go extinct. The total distribution of extinction rates per unit time interval, for all
history, will be continuous, ranging from low to high. The distribution, for instance,
could look something like Figure 23.6a: a Poisson distribution. This distribution arises
when the chance that any species goes extinct has, at all times, some small probability
(a) Continuum of macroevolutionary regimes (b) Two macroevolutionary regimes
Probability of extinction
Figure 23.6
(a) If there is a continuum of macroevolutionary regimes,
the frequency distribution of extinction probabilities in the
fossil record will have a continuous Poisson distribution.
(b) Whereas if there are two macroevolutionary regimes, the
frequency distribution of extinction probabilities in the fossil
record should be bimodal. (c) The actual distribution for
Frequency
Probability of extinction
Number of stages
40
30
20
10
(c) Real pattern of extinction probabilities
Cretaceous–Tertiary
0 0 40 60 120 160
Extinctions per stage
2,316 marine animal families in the 79 generally recognized
divisions of geological time since the Cambrian is continuous.
The extinction intensity at the Cretaceous–Tertiary boundary
is indicated for comparison. Redrawn, by permission of the
publisher, from Raup (1986). © 1986 American Association
for the Advancement of Science.