Evolution__3rd_Edition

648 PART 5 / Macroevolution
. . . but are usually statistically
inferred
A general survey of extinction rates
...
. . . shows a decreasing average
rate...
. . . and up to five mass extinctions
studied with fossil evidence. Instead of looking at particular species, we look at patterns
among large numbers of species or higher taxa. The Red Queen hypothesis (Section 22.8,
p. 638), provides an example of this general approach. The Red Queen hypothesis is a
theory about the cause of extinctions. It suggests that species go extinct when they are
outcompeted by other species that have made evolutionary advances. The validity of
the hypothesis is uncertain, but what matters here is that it was deduced from a general
pattern in fossil extinction a the log linearity of taxonomic survivorship curves. The
Red Queen posits a biological cause for extinctions. In this chapter we shall be more
concerned with non-biological causes, such as asteroid impacts and changes in the
physical environment.
The second evolutionary interest of extinctions lies in their consequences. When a
species goes extinct, it vacates ecological space that can be exploited by another species.
The sudden extinction of an entire larger taxonomic group may vacate a larger space
and permit a new adaptive radiation by a competing group (Figure 23.1). Radiations
and extinctions can be related events, and we look at the relation between the two later
in the chapter.
23.3 Mass extinctions
23.3.1 The fossil record of extinction rates shows recurrent rounds of
mass extinctions
Sepkoski, in a series of papers from 1981 on, compiled, from the paleontological
literature, the time distributions in the fossil record of all the families and genera of
marine organisms. Sepkoski’s compilation was not the first of its kind, but it is the most
comprehensive and the most widely used. Figure 23.2 shows how extinction rates
change over time in the fossil record.
We can notice two features of Figure 23.2. One is that the average extinction rate
appears to decrease from the Cambrian to the present. The explanation for the decrease
is not agreed on. The decrease may be an artifact of some kind a caused by changes
over geological time in the quality of the sedimentary record or the degree to which taxa
have been “split” by taxonomists. Or the decrease may be real. For instance, life may
have initially colonized relatively “central” niches that became subject to intense competition.
These niches may have a relatively rapid turnover of occupying species. Then,
over time, life also colonized more marginal niches, where competition is less intense.
The occupants of a marginal niche may stay there fairly permanently. These ideas are
clearly vague and uncertain at present.
The second remarkable feature in Figure 23.2 is the series of peak times when
extinction rates appear to be exceptionally high. These peaks are called mass extinctions.
The exact definition of a mass extinction is arbitrary, and different paleontologists
recognize different numbers of mass extinctions in the history of life. The evidence for
the Cambrian is sufficiently poor that we cannot say for sure whether extinction rates
were exceptionally high at any time then. From the Ordovician onwards, the five largest
extinction events were in the late Ordovician, the late Devonian, the end of the
..