794 THE STRUCTURE OF EVOLUTIONARY THEORY
designated as separate paleospecies by any standard criterion, and also
genealogically close enough to support a hypothesis of direct ancestry and descent.
What more do we need? Does this situation not affirm punctuated equilibrium ipso
facto?
But critics charge (and I must agree) that such evidence cannot be persuasive
by itself, because punctuated equilibrium explicitly links punctuational patterns to
events of branching speciation. Therefore, recorded punctuations produced for
other reasons do not affirm punctuated equilibrium—and may even challenge the
theory if their frequency be high and, especially, if they cannot be distinguished in
principle (or frequently enough in practice) from events of cladogenetic branching.
Punctuational patterns often originate (at all scales in evolutionary hierarchies
of levels and times) for reasons other than geologically instantaneous speciation—
and I welcome such evidence as an affirmation of pervasive importance (see p. 922
et seq.) for a general style of nongradualistic change, with punctuated equilibrium
as its usual mode of expression at the speciational scale under consideration in this
chapter. But testable, and generally applicable, criteria have been formulated for
distinguishing punctuated equilibrium from other reasons for punctuational
patterns—and available evidence amply confirms the importance and high relative
frequency of punctuated equilibrium.
Of the two major reasons for punctuational patterns not due to speciation,
Darwin's own classic argument of imperfection—geological gradualism that
appears punctuational because most steps of a continuum have not been preserved
in the fossil record—retains pride of place by venerable ancestry. I have already
presented my reasons for regarding this argument as inconsequential (see pp. 765-
774). I do not, of course, deny that many (or most) breaks in geological sequences
only reflect missing evidence. But proponents of punctuated equilibrium do not
base their claims on such inadequate examples that cannot be decided in either
direction. The test cases of our best literature—whether their outcomes be
punctuational or gradualistic—have been generated from stratigraphic situations
where temporal resolution and density of sampling can make appropriate
distinctions by recorded evidence, not conjectures about missing data.
The second reason has been highlighted by some critics, but unfairly I think,
because punctuated equilibrium has always recognized the argument and has,
moreover, enunciated and explicitly tested proper criteria for making the necessary
distinctions. To state the supposed problem: what can we conclude when we
document a truly punctuational sequence that cannot be attributed to imperfections
of the fossil record? How do we know that such a pattern records an event of
branching speciation, as the theory of punctuated equilibrium requires? When
ancestral Species A abruptly yields to descendant Species B in a vertical sequence
of strata, we may only be witnessing an anagenetic transformation through a
population bottleneck, or perhaps an event of migration, where Species B, having
evolved gradualistically from Species A in another region, invades the geographic
range, and abruptly wipes out its ancestor.