Punctuated Equilibrium and the Validation of Macroevolutionary Theory 773
for the existence of a phenomenon. Such issues cannot be resolved by anecdote, or
the documentation, however elegant, of individual cases. If anyone ever doubted
that punctuated equilibrium exists as a phenomenon, then this issue, at least, has
been put to rest by two decades of study following the presentation of our theory,
and by clear and copious documentation of many cases (see pp. 822-874).
Nonetheless, as pleased as Eldredge and I have been both by the extent of this
research and the frequency of its success, the "ideal case study" method cannot
validate our theory.
Punctuated equilibrium does not merely assert the existence of a
phenomenon, but ventures a stronger claim for a dominant role as a
macroevolutionary pattern in geological time. But how can this vernacular notion
of "dominant" be translated into a quantitative prediction for testing? At this point
in the argument, we encounter the difficult (and pervasive) methodological issue of
assessing relative frequency in sciences of natural history. If species were like
identical beans in the beanbag of classical thought experiments in probability, then
we could devise a sampling scheme based on enumerative induction. Enough
randomly selected cases could establish a pattern at a desired level of statistical
resolution. But species are irreducibly unique, and the set of all species does not
exhibit a distribution consistent with requirement of standard statistical procedures.
It matters crucially whether we study a clam or a mammal, a Cambrian or a
Tertiary taxon, a species in the stable tropics, or at volatile high latitudes.
Moreover—and especially—the "ideal case study" method has often failed, and led
to parochialisms and false generalities, precisely because we tend to select unusual
cases and ignore, often quite unconsciously, a dominant pattern. Indeed,
proclamations for the supposed "truth" of gradualism—asserted against every
working paleontologist's knowledge of its rarity—emerged largely from such a
restriction of attention to exceedingly rare cases under the false belief that they
alone provided a record of evolution at all! The falsification of most "textbook
classics" upon restudy only accentuates the fallacy of the "case study" method, and
its root in prior expectation rather than objective reading of the fossil record.
Punctuated equilibrium must therefore be tested by relative frequencies
among all taxa (or in a truly randomized subset) in a particular fauna, a particular
clade, a particular place and time, etc. If we can say, as Ager did (see p. 753) that
all but one Mesozoic brachiopod species displays stasis, or as Imbrie did (see p.
760) that all but one Devonian species from the Michigan Basin shows no change,
then we have specified a dominant pattern, at least within a particular, well-defined
and evolutionarily meaningful package. I cannot give a firm percentage for what
constitutes a "dominant" relative frequency—for, again, we encounter a theory-
bound claim, where "dominant" specifies a weight, beyond which the
morphological history of a clade must be explicated primarily by the differential
success of species treated as stable entities, or Darwinian individuals in
macroevolution—and not by anagenetic change within species. More research
must be done, largely in the testing of mathematical models under realistic
circumstances, to learn the relative frequencies