Evolution__3rd_Edition

678 PART 5 / Macroevolution
Summary
1 An adaptive radiation occurs when one or a small
number of ancestral species evolves over time into a
larger number of descendant species, occupying a
range of ecological niches. Adaptive radiations occur
following the colonization of a new area where competitors
are absent; the extinction of competitors; and
adaptive break-throughs.
2 The observed extinction rate varies through geological
time. At certain moments, extinction rates have
increased to a peak; these moments are called mass
extinctions. From two to five major mass extinctions
have been observed in the past. The two most important
are at the end of the Permian and the end of
the Cretaceous. Three others were at the end of the
Ordovician, Devonian, and Triassic.
3 Alvarez et al.’s discovery of anomalously large
concentrations of the rare earth element iridium at
the Cretaceous–Tertiary boundary rocks at Gubbio, in
Italy, suggested that the mass extinction may have
been caused by the collision of an asteroid, about 7.5
miles (12 km) in diameter, with the Earth. Much evidence
now supports their idea.
4 The impact theory of mass extinctions predicts that
extinctions in different taxa should be sudden, synchronous,
and global; the evidence for the Cretaceous–
birds) may have existed in the Cretaceous, and survived the extinction at the end of the
Cretaceous. The fossil evidence overestimates the massiveness of the mass extinction,
because no Cretaceous fossils of these taxa are known. The Cretaceous mass extinction
may still have enabled the rise of the mammals, but the picture is less clear than before.
Finally, the influence of mass extinctions on global diversity is being challenged. In
the 1980s, the Permian mass extinction was thought to be a key event, as diversity
steadily increased after it, following a plateau before. The most recent, and statistically
best adjusted, evidence suggests that diversity could have been constant. Maybe
the diversity of life today would be much the same if the mass extinctions had never
happened.
Much remains uncertain. The future lies with improvements in data collection,
taxonomy, and statistical adjustment for biases in the data through time. As further
results unfold, we shall see whether the Phase 3 or Phase 4 ideas hold up better a and
what Phase 5 will bring in.
Tertiary extinction mainly fits the prediction, though
other interpretations of the pattern are possible.
5 Asteroidal collisions, volcanic eruption, climatic
cooling, sea level changes, and changes in habitat area
caused by plate-tectonic movements are the five
potentially general theories of mass extinction. The
evidence does not suggest mass extinctions are generally
caused by asteroidal collisions; the effects of
climatic sea level changes need to be tested systematically;
and the effect of plate tectonics is difficult to test
at present.
6 The distribution of extinction rates may fit a power
law. This would suggest that the same basic causal pattern
is always at work, and random factors determine
whether extinction rates are high, low, or in between.
“Mass” extinctions may not have distinct causes as
opposed to extinctions at other times (sometimes
called “background” extinctions).
7 Almost all changes in extinction rates may be
accounted for by changes in the amount of sedimentary
rock per geological time interval. This would suggest
either that many changes in extinction rates are
artifactual, or that a common factor drives changes
both in the real extinction rate and the amount of sedimentary
rock.
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