Evolution__3rd_Edition

..
Taxonomic replacements can be
competitive or independent
Figure 23.11
The exact pattern of
replacement of one group by
another suggests whether or
not competition was at work.
(a) If the initially dominant
group declines before the
second group expands, it
suggests that the replacement
was not caused by competitive
displacement. The dominant
group may decline either
gradually or catastrophically.
(b) If the dominant group
declines as the other group
gains at its expense,
competition and relative
adaptation are more likely
to have influenced the
replacement.
CHAPTER 23 / Extinction and Radiation 669
23.7 One higher taxon may replace another, because of
chance, environmental change, or competitive
replacement
23.7.1 Taxonomic patterns through time can provide evidence about
the cause of replacements
After the dinosaurs went extinct at the end of the Cretaceous, the mammals radiated
rapidly and filled the ecological niches for large land vertebrates formerly occupied by
dinosaurs (see Figure 23.1). Earlier in the Cretaceous, the angiosperms had radiated,
apparently at the expense of the gymnosperms, which simultaneously declined (Figure
18.8, p. 539). These are both examples of evolutionary replacements, in which one
taxonomic group comes to occupy the ecological space formerly occupied by another
taxonomic group.
Why should one higher taxon replace another higher taxon of ecologically similar
species? Two theories can be tested. One (competitive displacement) says that the later
group outcompeted the first, and drove it extinct. The other (independent replacement)
says that the first group declined and went extinct for some reason unrelated to
the presence of the second group, and the second group only radiated after the first had
been cleared away. The pattern of change in diversity of the two groups provides the
best evidence to test between the two theories (Figure 23.11). If the first group declines
before the second expands, it suggests competition was not influential. If the first group
declines in proportion to the increase in the second, it suggests competition; this
pattern (Figure 23.11b) is sometimes called a double-wedge pattern.
In the case of independent replacement, we can distinguish two possibilities (Figure
23.11a). One is that the environment changed, and the earlier group went extinct due
to poor adaptation to the new environmental conditions. The second is that a catastrophic
mass extinction occurred, for instance following an asteroid impact, and one
dominant taxonomic group went extinct while another taxonomic group had a few
survivors. The reason why one group went completely extinct while the other survived
might mainly be luck.
The test between competitive and independent replacement in Figure 23.11 is not
foolproof. The double-wedge pattern characteristic of competitive replacement could
Time
(a) Passive replacement
Gradual extinction Mass extinction
Diversity
Time
Diversity
Time
(b) Competitive replacement
Diversity