Evolution__3rd_Edition

..
Early research established the main
faunal transitions
The asteroid theory inspired a new
set of ideas ...
. . . some of which are holding up
better than others
CHAPTER 23 / Extinction and Radiation 677
23.9 Conclusion: biologists and paleontologists have
held a range of views about the importance of mass
extinctions in the history of life
Biological diversity a the range of life forms on Earth a is clearly influenced by radiations
and extinctions. The most controversial question in this subject has concerned
the importance of mass extinctions. We can distinguish four historic phases in our
understanding of mass extinctions. The four phases correspond to changing views
about the reality, distinctiveness, and evolutionary significance of mass extinctions.
The faunal transitions that we now recognize as mass extinctions were first discovered
in the early nineteenth century as the main stages, and substages, of the fossil
record were also established. This research forms Phase 1. The faunal transitions
between the main stages of the fossil record were then often explained by rounds of
catastrophic extinctions. Phase 2 begins with Lyell in the 1830s. Lyell doubted whether
the observed faunal transitions were real catastrophes. He explained the faunal changes
by changes in the environmental and sedimentary conditons. Darwin continued this
line of thought. However, absolute geological dates later showed that the faunal transitions
did not correspond to hiatuses in the fossil record. They seemed to be real mass
extinctions.
Phase 3 can conveniently be dated to about 1980. Strong evidence then suggested
that the Cretaceous–Tertiary mass extinction was caused by an asteroid impact. This
was one of several components in a “1980s” view that mass extictions were real, distinct
events, and had a major influence in the history of life. We saw in this chapter how mass
extinctions have been thought to clear space and permit radiations of new taxa, such as
the modern mammal and bird orders in the Tertiary. Also, the Permian mass extinction
seemed to precipitate a faunal transition, leading to a modern set of life forms that
diversified more than the earlier Paleozoic fauna. Rare asteroid impacts are not the sort
of event that life will evolve adaptations to survive. Which taxa do survive, and which
go extinct, in the exceptional circumstances of mass extinctions, may be largely a matter
of luck, and have little to do with the microevolutionary process of adaptation and
natural selection. Thus, there could be distinct “macroevolutionary regimes” during,
and between, mass extinctions. In this set of ideas, mass extinctions are the key to
understanding much of evolutionary history.
Some of these Phase 3 ideas will probably endure. However, recent research has
moved on to a Phase 4. Paleobiologists and molecular phylogeneticists have at least
picked holes in, and sometimes seriously challenged, earlier work. Extinction rates
appear to be “fractal.” A similar set of causes are at work when extinction rates are low,
high, or anywhere between. Random factors determine whether that set of causes
results in high or low extinction rates. The search for the “cause” (or “causes”) of mass
extinctions may be mistaken; the same set of causes are at work all the time. It is even
possible that mass extinctions are artifacts, though few experts advance that as more
than a hypothetical possibility.
The molecular date for the origin of modern mammal orders could revise our
understanding of the nature of mass extinctions, and of the influence of mass extinctions
in evolutionary replacements. Almost all the modern orders of mammals (and