Evolution__3rd_Edition

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23Extinction and Radiation
T his chapter looks at the two factors that govern diversity
in the fossil record: extinction and radiation. Radiations
increase, and extinctions decrease, the diversity of life.
Interactions between the two processes explain much of the
history of biological diversity. We begin the chapter by
looking at the circumstances in which adaptive radiations
occur. We then look at extinction rates since the Cambrian,
and see that extinction rates have been elevated during
certain mass extinctions. The causes of mass extinctions
have been much studied, and we look at the evidence for
an asteroid impact at the end of the Cretaceous. We look at
statistical evidence that the causes of mass extinctions are not
distinct from other extinctions b that extinction rates at all
times fit the same power law. We also look at the possibility
that most changes in observed extinction rates are artifacts,
due to changes in the quality of the sedimentary record.
We then turn to three topics that combine radiation and
extinction. The first is species selection; the second is
evolutionary replacement; and the third is the history of
biological diversity on the global scale over geological time.