Punctuated Equilibrium and the Validation of Macroevolutionary Theory 971
extrapolation from the smallest scale of our experiments and personal observations.
Punctuated equilibrium has proven its mettle in:
- Elucidating and epitomizing what may be the primary process of a
distinctive level in the evolutionary hierarchy: the role of species as Darwinian
individuals, and the speciational reformulation of macroevolution—for leg one. - Defining (and, in part, thereby creating) the issue of stasis as a subject for
study, and in helping to explicate the structural rules that hold entities in active
stasis at various levels, but then permit rapid transition to qualitatively different
states—for leg two. - Stressing that level-bound punctuational breaks preclude the prediction or
full understanding of extensive temporal change from principles of anagenetic
transformation at the lowest level (a mode of evolution, moreover, that punctuated
equilibrium regards as rare in any case), thus emphasizing contingency and
denying extrapolationist premises and methodologies—for leg three.
In developing this set of implications, I do hold, in my obviously biased way,
that punctuated equilibrium has performed some worthy intellectual service. The
relative frequency of its truth-value, of course, must be regarded as another matter
entirely, and an issue that only time can fully resolve. But I would maintain that, in
the quarter century following its original formulation, punctuated equilibrium has
at least prevailed, against an initial skepticism of active and general force and
frequency, in three central empirical claims (quite independent of any theoretical
weight that evolutionary biology may ultimately wish to assign): (1)
documentation of the basic mechanism in cases now too numerous and too
minutely affirmed to deny status as an important phenomenon in
macroevolutionary pattern; (2) validation of stasis as a genuine, pervasive, and
active phenomenon in the geological history of most species; and (3) establishment
of predominant relative frequency in enough comprehensive and well-bounded
domains to assure the control of punctuated equilibrium over substantial aspects of
the phyletic geometry of macroevolution. A fourth, and ultimately more important,
issue for evolutionary theory remains unresolved: the implications of these
empirical findings for the role of genuine selection among species-individuals
(rather than merely descriptive species-sorting as an upwardly cascading
expression of conventional Darwinian selection acting at the organismic level) as
the causal foundation of macroevolutionary pattern.
Let me therefore end this chapter by restating the last paragraph of the review
article for Nature that Eldredge and I wrote (Gould and Eldredge, 1993, p. 227) to
celebrate the true majority, or coming of age—that is, the 21st birthday—of
punctuated equilibrium. We wrote this paragraph to assess the role of punctuated
equilibrium within a larger and far more general intellectual (and cultural)
movement that, obviously, punctuated equilibrium did not create or even instigate,
but that our theory didn't simply or slavishly follow either. We did, I think,
contribute some terms and concepts to the larger enterprise,