Evolution__3rd_Edition

674 PART 5 / Macroevolution
Changes in the total diversity of life
may fit ...
...a logistic model ...
low” for 40 million years until after the extinction of the dinosaurs. Only then did the
mammals radiate into formerly dinosaurian niches. Thus, the fossil evidence may still
be essentially correct. It shows when the main mammal orders proliferated, rather than
when they originated. This reconciliation between the molecular and fossil evidence is
similar to the case of the Cambrian explosion (Section 18.4, p. 535). The reconciliation
is plausible, and popular, but by no means confirmed a after all, we know nothing
about the Cretaceous representatives of the modern mammal orders, except that they
probably existed.
In summary, many examples exist through the history of life when one taxon
replaces another. One question we can ask about replacements is whether they were
competitive or independent. The main class of evidence comes from the time course of
the rise of one taxon, and the fall of the other (see Figure 23.11). Other evidence comes
from direct competition, such as bryozoan overgrowth. One possible example of independent
replacement, that of dinosaurs and mammals, has been reanalyzed recently
and, although the revised picture does not show that the replacement was competitive,
a formerly watertight case for independent replacement has sprung a leak. The role of
the Cretaceous mass extinction in the rise of the mammals is less certain than it
appeared to be 10 years ago, in the early to mid-1990s.
23.8 Species diversity may have increased logistically or
exponentially since the Cambrian, or it may have
increased little at all
The number of species alive on Earth today is uncertain, with estimates ranging from
10 to 100 million species. About 2 million species have been described. At some point in
the past, fewer species must have existed than now (this almost has to be true, if we trace
back far enough a even to the origin of life). But how have the number of species
changed over time? Early attempts to answer this question were made by Simpson,
Valentine, and others, but modern thinking about it begins with Sepkoski’s database a
the database for marine fossil animals we have met previously in this chapter.
Figure 23.15a illustrates Sepkoski’s classic result. The y-axis is for numbers of families,
but we can assume that numbers of species would show a similar pattern. Sepkoski
distinguished three faunas: the Cambrian, Paleozoic, and Modern. The three differ in
the kind of animals that were alive at the time. The data for the Cambrian is poor, and
the important features of the graph are: (i) the rapid increase in diversity after the
Cambrian, starting about 500 million years ago; (ii) the apparent “Paleozoic plateau”
of constant diversity; (iii) the reduction in diversity at the Permian mass extinction;
followed by (iv) the steadily increasing diversity since then (the end-Cretaceous mass
extinction is only a blip in the steady rise).
The increase of diversity, followed by a plateau, in the Paleozoic can be explained by
a logistic model. Logistic increase is the kind of increase seen by ecologists when new
resources are colonized. Numbers increase exponentially to begin with, due to the
absence of competition. Then, as competitors fill up the resource space, no new species
can be added except following the extinction of an existing species.
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