Punctuated Equilibrium and the Validation of Macroevolutionary Theory 891
may experience a reduced average geological longevity relative to surface dwelling
(for rodents) and planktonic (for mollusks) clade members. But these impediments
may be overbalanced by enhancement of speciation rates, thus driving the trend.
Second, both the low N for species in clades (relative to organisms in most
populations), and the remarkably (and, for most people, counterintuitively) high
frequency of fortuitous but significant correlations between pairs of traits in
systems built by genealogical branching (Raup and Gould, 1974) virtually
guarantee that trending by drift will be much more common in sorting of species-
individuals than in conventional sorting of organism-individuals. After all,
branching evolution imparts a set of autapomorphic traits (through a unique
common ancestor) to any subclade of species—and we can scarcely believe that
each of these traits establishes the basis of selective existence and success for each
species in the entire monophyletic group. Therefore, any process that favors the
relative proliferation of any subclade for any reason will automatically engender a
positive trend for any included autapomorphic character, whatever the causal basis
of the general trend.
When we combine this spur to drift by hitchhiking with the observation that
many successful clades go through severe bottlenecks (often as single surviving
species) during their geological existence, we obtain even more compelling reasons
for considering drift as a major source of macroevolutionary trends, however much
we may reject analogous processes as substantial generators of trends in
phenotypic characters controlled by organismal selection. For example, ammonites
endured two severe bottlenecks at two major mass extinction events—suffering
reduction, perhaps to two surviving lineages in the Permo-Triassic debacle, and to
a single lineage in the closing Triassic event. In this light, why should we regard
explanations based on general biomechanical advantages for organisms as
preferable to the obvious blessing of good fortune upon any trait belonging to the
phenotype of single lineages that manage to squeak through such profound
bottlenecks? Few other evolutionary processes can promote traits from partial
representation to exclusivity within a population (of species-individuals within a
clade in this case) so quickly and so decisively.
The paleontological literature has just begun to reconceptualize trends in
speciational terms. Initial results offer much encouragement, both for revising
traditional explanations of particular temporal sequences, and for posing new
questions requiring tests by different kinds of data. New insights often emerge just
by framing the subject in terms of numbers and longevities of taxa rather than
gradual fluxes of form. In their study of trends in Mississippian crinoids, for
example, Kammer, Baumiller, and Ausich (1997) reach a conclusion that surprised
them only because such a reasonable idea had not previously been formulated in
operational terms (p. 221): "Results of this study indicate that among Mississippian
crinoids niche generalists had greater species longevity than niche specialists.
Although logical, few data have previously been developed to test this
relationship."
The complexity of the subject then becomes apparent when the authors