Punctuated Equilibrium and the Validation of Macroevolutionary Theory 795
But an appropriate and non-arbitrary criterion exists—and has been fully
enunciated, featured as crucial, and subjected to frequent test, from the early days
of punctuated equilibrium. We can distinguish the punctuations of rapid anagenesis
from those of branching speciation by invoking the eminently testable criterion of
ancestral survival following the origin of a descendant species. If the ancestor
survives, then the new species has arisen by branching. If the ancestor does not
survive, then we must count the case either as indecisive, or as good evidence for
rapid anagenesis—but in any instance, certainly not as evidence for punctuated
equilibrium.
Moreover, by using this criterion, we obey the methodological requirement
that existing biases must work against a theory under test. When ancestors do not
survive following the first appearance of descendants, the pattern may still be
recording an event of branching speciation—hence affirmation for punctuated
equilibrium. But we cannot count such cases in our favor, for the plausible
alternative of rapid anagenesis cannot be disproven. By restricting affirmations to
cases where ancestors demonstrably survive, we accept only a subset of events
actually caused by speciation. Thus, we underestimate the frequency of punctuated
equilibrium—as we must do in the face of an unresolvable bias affecting a
hypothesis under test.
In our first papers, we did not recognize or articulate the importance of
tabulating cases of ancestral survival following punctuational origin of a de-
scendant as a criterion for distinguishing punctuated equilibrium from other forms
of punctuational change. (Both of our original examples in Eldredge and Gould,
1972, did feature—and prominently discuss—ancestral survival as an important
aspect of the total pattern. We had a proper "gut feeling" about best cases, but we
did not formalize the criterion.) But, beginning in 1982, and continuing thereafter,
we have stressed the centrality of this criterion in claims for speciation as the
mechanism of punctuated equilibrium. Contrasting the difference in
paleontological expression between Wright's shifting balance and punctuated
equilibrium by speciation, for example, I wrote (Gould, 1982c, p. 100): "Since
punctuational events can occur in the phyletic mode under shifting balance, but by
branching speciation under punctuated equilibrium, the persistence of ancestors
following the abrupt appearance of a descendant is the surest sign of punctuated
equilibrium."
This criterion has been actively applied, in an increasingly routine manner (as
researchers recognize its importance), in the expanding literature on empirical
study of evolutionary tempos and modes in well-documented fossil sequences.
Cases of probable anagenetic transformation have been documented (no ancestral
survival when good stratigraphic resolution should have recorded such persistence,
had it occurred), especially in planktic marine Foraminifera, where long oceanic
cores often provide unusually complete evidence (Banner and Lowry, 1985;
Malmgren and Kennett, 1981, who coined the appropriate term "punctuated
anagenesis" for this phenomenon).
However, abundant cases of ancestral survival, and consequent punctuational
origin of descendant taxa by branching speciation, have also been affirmed as