768 THE STRUCTURE OF EVOLUTIONARY THEORY
we need to formulate an appropriate definition of rapidity. (Punctuated equilibrium
makes no claim about the possibility of substantial change at rates that would be
called rapid by measuring rods of a human lifetime. Therefore, and especially,
punctuated equilibrium provides no insight into the old and contentious issue of
saltational or macromutational speciation.) As a first approach, the duration of a
bedding plane represents the practical limit of geological resolution. Any event of
speciation that occurs within the span of time recorded by most bedding planes will
rarely be resolvable because evidence for the entire transition will be compressed
onto a single stratigraphic layer, or "geological moment."
However, the limits of stratigraphic resolution vary widely, with bedding
planes representing years or seasons in rare and optimal cases of varved sediments,
but several thousand years in most circumstances. We therefore cannot formulate a
definition equating punctuation with "bedding plane simultaneity." (After all, such
a definition would, almost perversely, preclude the "dissection" of a punctuation in
admittedly rare, but precious, cases of sedimentation so complete and so rapid that
an event of speciation will not be compressed, as usual, onto a single bedding
plane, but will "spread out" over a sufficient stratigraphic interval to permit the
documentation of its rapid history.)
Punctuations must, instead, be defined relative to the subsequent duration of
the derived species in stasis—for punctuated equilibrium, as a theory of relative
timing, holds that species develop their distinctive features effectively "at birth,"
and then retain them in stasis for geologically long lifetimes. (These timings play
an important role in the recognition of species as Darwinian individuals—see
discussion on "vernacular" criteria of definable birth, death, and sufficient stability
for individuation—Chapter 8, pp. 602-608).
I know no rigorous way to transcend the arbitrary in trying to define the
permissible interval for punctuational origin. Since definitions must be theory-
bound, and since the possibility of recognizing species as Darwinian individuals in
macroevolution marks the major theoretical interest of punctuated equilibrium, an
analogy between speciation and gestation of an organism may not be ill conceived.
As the gestation time of a human being represents 1-2 percent of an ordinary
lifetime, perhaps we should permit the same general range for punctuational
speciation relative to later duration in stasis. At an average species lifetime of 4
million years, a 1-percent criterion allows 40,000 years for speciation. When we
recognize that such a span of time would be viewed as gradualistic—and extremely
slow paced at that—by any conventional microevolutionary scaling in human time;
and when we also acknowledge that the same span represents the resolvable
moment of a single bedding plane in a great majority of geological circumstances;
then we can understand why the punctuations of punctuated equilibrium do not
represent de Vriesian saltations, but rather denote the proper scaling of ordinary
speciation into geological time.
Punctuation does suffer the disadvantage of frequently compressed recording
on a single bedding plane (so that the temporal pattern of the full event