840 THE STRUCTURE OE EVOLUTIONARY THEORY
literal punctuations in the fossil record; and the specification of fine-scale
"anatomy" (data of timing, mode, morphology, geography, etc.) for punctuational
events.
The inference of cladogenesis by the criterion of ancestral survival I doubt
that any professional paleontologist would dispute the statement that a great
majority of paleospecies makes a geologically abrupt first appearance in the fossil
record. But this statement about an observed, literal pattern carries almost no
interpretive weight because the phenomenon so described can be explained by such
a wide variety of putative causes, including the following distinctly different
proposals:
THE TRADITIONAL GRADUALIST VIEW. The species arose by geologically
gradual transformation of an ancestral population, but our woefully imperfect fossil
record did not preserve the intermediary stages.
THE CLADOGENETIC PROPOSAL OF PUNCTUATED EQUILIBRIUM. The literal
record represents the expected geological scaling of biological processes
responsible for the origin of species. New species arise by isolation and branching
of a segment of the ancestral population. The branch evolves to a new species by
continuous transformation, but at a rate that, however "slow" by the inappropriate
standard of a human lifetime, runs to completion during the "geological moment"
of a bedding plane in most cases.
PUNCTUATED ANAGENESIS. The new species arises by continuous
transformation, in toto and without branching, of an ancestral species, but at a rate
too rapid for geological resolution of intermediary stages.
SUDDEN APPEARANCE BY MIGRATION INTO A LOCAL SECTION. The new species
arose in another region at a rate and mode that cannot be determined from local
evidence. A punctuational first appearance in any particular geographic region
records a process of migration from another area of earlier origin.
These four proposals have strikingly different implications for the validation
of punctuated equilibrium. The first opposes punctuated equilibrium
unambiguously and would disprove the theory, or at least consign it to irrelevancy
as a cause of pattern in the history of life, if this mode of classical gradualism
could be affirmed at dominant relative frequency for the origin of new species.
Punctuated equilibrium predicts that the second explanation must hold as the
primary generator of the dominant empirical signal of punctuational origin for
paleospecies. If most species did not arise by rapid cladogenesis at appropriate
geological scales, then punctuated equilibrium would be disproven as a major
cause of evolutionary pattern (and would be relegated to a status of marginality and
insignificance in the history of life).
The third explanation may fall within the "spirit" of punctuated equilibrium,
by identifying a genuine geological punctuation, rather than a false appearance
based on missing data in gradualistic sequences, as the source for an empirical
observation of abrupt origin. But if punctuated anagenesis could be validated at
high relative frequency for the origin of paleospecies, then punctuated