Punctuated Equilibrium and the Validation of Macroevolutionary Theory 841
equilibrium—a theory about cladogenesis—would be demoted or negated, while
the important ancillary concept of explaining trends, within a hierarchical model
(see Chapter 8), as differential success of species within clades, would also become
marginalized.
The fourth, or migrational, alternative may resolve a local issue in a given
section, but can only indicate, for questions about tempo and mode of speciation, a
need for additional information of wider geographic scope. For we must still learn
whether the new species, arriving as a punctuational migrant, arose by anagenesis
or cladogenesis, and at either a gradual or punctuational tempo, in its natal area.
But if the migrant invades the territory of a surviving ancestor—a common pattern
in recorded literature—then, at least, we have documented the cladogenetic origin
that punctuated equilibrium requires.
Against the charge that our theory cannot be adequately tested, participants in
the empirical debate about punctuated equilibrium have long recognized, and
generally utilized, an excellent criterion possessing the two cardinal virtues of a
probing agent for scientific hypotheses: ready (and unambiguous) application in
most cases, and an inherent bias against punctuated equilibrium by
underrepresentation of actual cases. I presented this tool—ancestral survival
following punctuational origin—earlier in the chapter (see pages 793-796), while
leaving the primary documentation for this section.
The criterion of ancestral survival invokes paleontological data of the most
conventional and easily acquired kind—specimens in local sections, forming
samples of sufficient size for basic taxonomic identifications—and not distant
inferences from models or from fossil data to unobservable correlates in behavior
or physiology. Moreover, the criterion is properly biased against punctuated
equilibrium in recognizing only the subset of legitimate cases with documented
ancestral survival (therefore leaving in limbo all genuine cases where ancestors
may have survived in other regions, or may not yet have been found). Any
tabulation based on the criterion of ancestral survival must therefore underestimate
the true relative frequency of punctuated equilibrium.
One potential biasing factor, however, might lead to an overestimate for
punctuated equilibrium under this criterion, and must therefore be scrutinized and
avoided. Under the fourth explanation presented above for literal observations of
punctuation between a descendant and a surviving ancestor in a local section,
migration of the descendant from a different region (where it might have originated
gradually), rather than punctuational evolution in situ, could produce an artificial
boost in frequency if falsely counted as a proven case of punctuated equilibrium. (I
suspect that most cases in this mode do represented punctuated equilibrium, based
on general arguments that most speciation events in unobserved regions of the
geographic range will themselves be punctuational, but we obviously cannot count
these examples favorably, because our entire case would then become circular by
assuming the premise supposedly under test.)
The proper solution to such unresolvable cases lies in proper scrutiny, and