Punctuated Equilibrium and the Validation of Macroevolutionary Theory 793
erection of a species name for every recognizable morphological variant (even for
odd individuals rather than populations), has greatly inflated the roster of
legitimate names in many cases, particularly for fossil groups last monographed
several generations ago. (Our literature even recognizes the half-facetious term
"monographic burst" for peaks of diversity thus artificially created. But this
problem of past over splitting cannot be construed as either uniquely or even
especially paleontological, for neontological systematics then followed the same
practices as well.) The grossly uneven, and often greatly over split, construction of
species-level taxonomy in paleontology has acted as a strong impediment for the
entire research program of the prominent school of "taxon-counting" (Raup,
1975,1985). For this reason, the genus has traditionally been regarded as the lowest
unit of rough comparability in paleontological data (see Newell, 1949). Sepkoski
(1982) therefore compiled his two great compendia—the basis for so much
research in the history of life's fluctuating diversity—at the family, and then at the
genus, level (but explicitly not at the species level in recognition of frequent over
splitting and extreme imbalance in practice of research among specialists on
various groups).
Although this problem has proved far more serious for taxon-counters than
for proponents of punctuated equilibrium, a potential bias towards
overrepresentation also poses a threat for our theory, as Levinton (1988, p. 364)
rightly recognizes: "The problem is not very new. Meyer (1878) claimed that the
ability to recognize gradual evolutionary change in Micraster [a famous sequence
of Cretaceous echinoids] was obscured by the rampant naming of separate species
by previous taxonomists."
This issue would cause me serious concern—for the claim of overestimation
does, after all, fall into the worrisome category of biases favoring a preferred
hypothesis under test—if two arguments and realities did not obviate the danger.
First, if supporters of punctuated equilibrium did try to affirm their hypothesis by
using names recorded in the literature as primary data for judging the strength and
effect of speciation upon evolutionary trends, then we would face a serious
difficulty. But I cannot think of any study that utilized this invalid approach—for
paleontologists recognize and generally avoid the dangers of this well-known
directional bias. Punctuated equilibrium, to my knowledge, has never been
defended by taxon counting at the species level. All confirmatory studies employ
measured morphometric patterns, not the geological ranges of names recorded in
literature.
Second, as stated above, all students of this subject seem to agree that if a
systematic bias exists in relative numbers of paleospecies and biospecies, fossil
data should be skewed in the opposite direction of recognizing fewer paleospecies
than biospecies—an acceptable bias operating against the confirmation of
punctuated equilibrium.
Reasons why an observed punctuational pattern might not
represent speciation
Suppose that we have empirical evidence for a punctuational event separating two
distinct morphological packages regarded as both different enough to be