Punctuated Equilibrium and the Validation of Macroevolutionary Theory 803
by relative frequencies. No exquisite case of punctuated equilibrium—and many
have been documented—can "prove" our theory; while no beautiful example of
gradualism—and such have been discovered as well—can refute us. The key
question has never been "whether," but rather "how often," "with what range of
variation in what circumstances of time, taxon, and environment," and especially,
"to what degree of control over patterns in phylogeny?" A single good case can
only validate the reality of the phenomenon—and the simple claim for existence
has not, surely, been an issue for more than 20 years. Similarly, an opposite case of
gradualism can only prove that punctuated equilibrium lacks universal validity, and
neither we nor anyone else ever made such a foolish and vainglorious claim in the
first place. The empirical debate about punctuated equilibrium has always, and
properly, focussed upon issues of relative frequency.
I shall present the empirical arguments for asserting dominant relative
frequency, rather than mere occurrence, for punctuated equilibrium on pages 854-
- If we ask, by contrast, whether strong evidence for predominant gradualism
has been asserted for any major taxon, time or environment, one case stands out as
a potentially general refutation of punctuated equilibrium in one important domain
at least: the claim for anagenetic gradualism as a primary phylogenetic pattern in
the evolution of Cenozoic planktonic Foraminifera.
This case gains potential power and generality from the unusually favorable
stratigraphic context, and the consequent nature of sampling, in such studies. The
data come from deep oceanic cores, with stratigraphic records presumably
unmatched in general completeness, for these environments receive a continuous
supply of sediment (including foraminiferal tests) from the water column above.
Moreover, these microscopic organisms can usually be extracted in large and
closely spaced samples (sieved from disaggregated sediments), even from the
restricted volume of a single oceanic core. Thus, forams in oceanic cores should
provide our most consistently satisfactory information—in terms of large samples
with good stratigraphic resolution—for the study of phylogenetic pattern. If
gradualistic anagenesis prevails in such situations of maximal information—even if
punctuated equilibrium predominates in the conventional fossil record of marine
invertebrates from shallow water sediments—shouldn't we then conclude that
Darwin's old argument must be valid after all; that punctuational patterns represent
an artifact of missing data; and that more complete information will affirm genuine
gradualism as the characteristic signal of phylogeny?
I acknowledge the highest relative frequency of recorded gradualism for
foraminiferal data of this type, and I also admire the procedural rigor and
informational richness in several of these studies. But I do not regard this case as a
general argument against punctuated equilibrium—and neither, I think, do most of
my paleontological colleagues, whatever their overall opinion about our theory, for
the following reasons based upon well-known features of the fossil record in
general, and the biology of forams in particular.