Evolution__3rd_Edition

..
Number of families
Number of families
(a)
1,000
800
600
400
200
Cm
0
V Ca O S D C P Tr J K T
(b)
1,200
1,000
800
600
400
200
0
V Ca O S D C P Tr J K T
Figure 23.15
The history of global biological diversity. (a) Sepkoski
originally found a logistic increase with a Paleozoic plateau,
followed by a steady increase since the Permian mass
extinction. He also recognized three distinct faunas:
Cambrian (Cm), Paleozoic (Pz), and Modern (Md).
(b) Benton compiled the data in a slightly different way,
. . . or an exponential model ...
Pz
Md
Number of genera
250
200
150
100
50
0
(c)
CHAPTER 23 / Extinction and Radiation 675
Median
95% CI
Ca O S D C P Tr J K T
550 450 350 250 150 50
Time (Myr)
and found a pattern that was indistinguishable from a steady
exponential increase. (c) A new compilation aims to correct
for stratigraphic incompleteness, and preliminary findings
suggest no increase in diversity between the Ordovician
and Tertiary. Redrawn, by permission of the publisher, from
Miller (1998) and Alroy et al. (2002). © 1998 American
Association for the Advancement of Science.
The difference between the Paleozoic and Modern parts of Sepkoski’s graph suggest
that the Permian mass extinction had a major creative influence for life on Earth today.
In the absence of the Permian mass extinction, maybe the Paleozoic fauna would have
continued to dominate life on Earth, and diversity might still be stuck at the Paleozoic
plateau. The “Modern” fauna seems to have been able to diversify more than the
Paleozoic fauna.
A second model a the exponential model afor the history of diversity was proposed
by Benton (1997). He used a different data compilation, including land organisms. He
suspected that Sepkoski’s result, including the Paleozoic plateau, was peculiar to
marine life. If all life is looked at together (Figure 23.15b), the full history of diversity
could just as well be explained by a steady (if noisy) exponential increase. Two implications
of Benton’s model are as follows. First, species have been persistently dividing and
subdividing ecological niches into finer and finer units a if there is a limit on total