Evolution__3rd_Edition

676 PART 5 / Macroevolution
. . . or some other model
The data to test the models is
currently ambiguous ...
. . . and subject to active statistical
analysis
diversity, as the logistic model assumes, then that limit has not yet been reached. 2
Secondly, mass extinctions have been less important for the history of diversity. Diversity
now would be much the same if no mass extinctions had occurred. Other models
beside those by Sepkoski and Benton have also been proposed (see Miller 1998).
Which model is correct? The question has been difficult to answer, because of limitations
in the data. Even Sepkoski’s compilation has gaps and biases. Jackson & Johnson
(2001), for instance, investigated Sepkoski’s data for tropical Bryozoa. They found a
huge underestimate of the diversity of this regional taxon for the Pliocene. The reason
was simply that Europe and North America were no longer tropical in the Pliocene.
Most paleontologists are European or North American, and most of them work locally.
Thus, the high diversity of tropical bryozoan fauna has been much better studied for
earlier times, when Europe and North America were tropical, than for the Pliocene,
when they were temperate. If Sepkoski’s data are adjusted for this bias, Jackson and
Johnson suggest it fits better with the exponential model. However, the main point is
that we need a database that has been more systematically corrected for bias. The two
main kinds of bias come from the amount of rock preserved from different time intervals,
and the amount of that preserved rock that has been studied by paleontologists.
Geographic and taxonomic biases (as paleontologists study some regions, and some
taxa, more than others) interact with these two main kinds of bias.
A new database is being compiled by a team of paleobiologists, and is being housed at
the University of California at Santa Barbara. The new database aims to correct for
biases, by appropriate statistical corrections to the raw data. The preliminary results are
remarkable (Figure 23.15c). The estimated global species diversity seems to show little
change from the Paleozoic to the Oligocene. If the result holds up as the database is
developed, it will suggest that diversity has a tighter limit than had previously been
suspected. Speciation and extinction may have been in balance for much of the past
500 million years, with mass extinctions having had little effect on global species
diversity. That such a dramatically different picture can be produced now highlights
the importance of statistical corrections to the data. The main difference between
Figure 23.15a and 23.15c is due to the statistical corrections for bias.
In summary, we have seen how there is an active research program that aims to
describe the history of global species diversity. The main problems lie in data compilation,
and correction for biases and gaps. Paleobiologists can be found who support at
least three models of history: a logistic increase to a Paleozoic plateau, followed by a
steady increase; steady exponential increase; and a prolonged plateau. The influence
of mass extinctions on diversity is uncertain. It is also uncertain whether modern life
subdivides ecological niches more finely than earlier life did. These uncertainties exist
not because the problems have been forgotten about, but because of a multipronged
modern attack by research paleobiologists.
2 Ultimately, species diversity will be limited, among sexual creatures, by the time it takes to find a mate.
As species split and split, the numbers of individuals per species will go down (because the total biomass on
Earth is limited by the energy input from the sun). As the members of a species become rarer, and sparser, the
energetic cost of finding a mate will ultimately become prohibitive. Hybridization, despite its genetic costs
(Section 14.4, p. 389) will eventually be favored.
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