Evolution__3rd_Edition

658 PART 5 / Macroevolution
Darwin suspected mass extinctions
were sedimentary artifacts
vulcanism were invoked to explain real extinction patterns. However, at least since
Lyell, in the mid-nineteenth century, some paleontologists have been skeptical about
the observed changes in extinction rates. The apparently high extinction rates at the
end of the major geological eras were known about in Lyell’s time. But the high extinction
rates could be artifacts, due to gaps in the fossil record, rather than real events.
Darwin, for instance, wrote in the section on extinctions in On the Origin of Species
(1859), “the old notion of all the inhabitants having been swept away by catastrophes
at successive periods is very generally given up, even by those geologists, as Elie de
Beaumont, Murchison, Barrande, &c., whose general views would naturally lead them
to this conclusion. On the contrary, we have every reason to believe, from the study
of the tertiary formations, that species and groups of species gradually disappear, one
after another.”
When Darwin wrote, absolute dates for rocks were not available. Absolute dates
came in as the radioisotope method was developed in the twentieth century. Without
these dates, a sudden transition such as from the Cretaceous to the Tertiary faunas
could simply have reflected a prolonged gap in the fossil record. The end of the
Cretaceous might have been 50 million years before the beginning of the Tertiary.
Radioisotope dates ruled out that possibility. In fact the end of the Cretaceous runs
straight into the beginning of the Tertiary, around 65 million years ago (Figure 18.1,
p. 526). The mass extinctions, therefore, look real (Figure 23.2). Most paleontologists
have come to accept that the history of life contains a number of catastrophic mass
extinctions. This is an important respect in which the modern view of the history of life
differs from Darwin’s.
However, some of the changes in extinction rate observed in the fossil record could
still be caused by changes in the sedimentary record. Box 23.2 looks at a comprehensive
study by Peters & Foote (2002). The implications of their study remain undecided. A
conservative conclusion would be that mass extinctions are real events, as is generally
believed. But their work also allows a radical conclusion a that all the observed changes
in extinction rates, including elevated extinction rates at “mass” extinctions, are
sedimentary artifacts. Such a radical conclusion would take more work to establish,
however. For now, paleobiologists will probably continue to study changes in extinction
rates, but perhaps with more of an eye on artifacts in the data.
23.6 Species selection
23.6.1 Characters that evolve within taxa may influence extinction and
speciation rates, as illustrated by snails with planktonic and
direct development
What factors determine the patterns of speciation and radiation? The question has
been studied in various ways and in this and the next section we shall concentrate on
two ideas: one in which the attributes of the organisms may influence a taxon’s probabilities
of survival and speciation, and the other in which external ecological factors
may show such an influence.
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