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Evolution__3rd_Edition

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..<br />

Early research established the main<br />

faunal transitions<br />

The asteroid theory inspired a new<br />

set of ideas ...<br />

. . . some of which are holding up<br />

better than others<br />

CHAPTER 23 / Extinction and Radiation 677<br />

23.9 Conclusion: biologists and paleontologists have<br />

held a range of views about the importance of mass<br />

extinctions in the history of life<br />

Biological diversity a the range of life forms on Earth a is clearly influenced by radiations<br />

and extinctions. The most controversial question in this subject has concerned<br />

the importance of mass extinctions. We can distinguish four historic phases in our<br />

understanding of mass extinctions. The four phases correspond to changing views<br />

about the reality, distinctiveness, and evolutionary significance of mass extinctions.<br />

The faunal transitions that we now recognize as mass extinctions were first discovered<br />

in the early nineteenth century as the main stages, and substages, of the fossil<br />

record were also established. This research forms Phase 1. The faunal transitions<br />

between the main stages of the fossil record were then often explained by rounds of<br />

catastrophic extinctions. Phase 2 begins with Lyell in the 1830s. Lyell doubted whether<br />

the observed faunal transitions were real catastrophes. He explained the faunal changes<br />

by changes in the environmental and sedimentary conditons. Darwin continued this<br />

line of thought. However, absolute geological dates later showed that the faunal transitions<br />

did not correspond to hiatuses in the fossil record. They seemed to be real mass<br />

extinctions.<br />

Phase 3 can conveniently be dated to about 1980. Strong evidence then suggested<br />

that the Cretaceous–Tertiary mass extinction was caused by an asteroid impact. This<br />

was one of several components in a “1980s” view that mass extictions were real, distinct<br />

events, and had a major influence in the history of life. We saw in this chapter how mass<br />

extinctions have been thought to clear space and permit radiations of new taxa, such as<br />

the modern mammal and bird orders in the Tertiary. Also, the Permian mass extinction<br />

seemed to precipitate a faunal transition, leading to a modern set of life forms that<br />

diversified more than the earlier Paleozoic fauna. Rare asteroid impacts are not the sort<br />

of event that life will evolve adaptations to survive. Which taxa do survive, and which<br />

go extinct, in the exceptional circumstances of mass extinctions, may be largely a matter<br />

of luck, and have little to do with the microevolutionary process of adaptation and<br />

natural selection. Thus, there could be distinct “macroevolutionary regimes” during,<br />

and between, mass extinctions. In this set of ideas, mass extinctions are the key to<br />

understanding much of evolutionary history.<br />

Some of these Phase 3 ideas will probably endure. However, recent research has<br />

moved on to a Phase 4. Paleobiologists and molecular phylogeneticists have at least<br />

picked holes in, and sometimes seriously challenged, earlier work. Extinction rates<br />

appear to be “fractal.” A similar set of causes are at work when extinction rates are low,<br />

high, or anywhere between. Random factors determine whether that set of causes<br />

results in high or low extinction rates. The search for the “cause” (or “causes”) of mass<br />

extinctions may be mistaken; the same set of causes are at work all the time. It is even<br />

possible that mass extinctions are artifacts, though few experts advance that as more<br />

than a hypothetical possibility.<br />

The molecular date for the origin of modern mammal orders could revise our<br />

understanding of the nature of mass extinctions, and of the influence of mass extinctions<br />

in evolutionary replacements. Almost all the modern orders of mammals (and

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