Punctuated Equilibrium and the Validation of Macroevolutionary Theory 921
levels do active causal forces of evolutionary change and stability extend? Do such
causes generally weaken, or become restricted to peripheral impacts, at these
higher levels? If so, can we attribute such diminution to increasingly limited
opportunities for devising "glues" that might bind components into coherent
individuals at these higher levels? Can "glues" for higher units in ecological
hierarchies be strong enough, even in theory, to achieve the sufficient bounding
(and bonding) that higher levels of the genealogical hierarchy (like species) can
and do attain?
I do not know how this debate will develop, and how, or even whether, these
questions can be operationally defined and activated. We remain stymied, at the
moment, because so little thought, and so little empirical work, has been devoted to
operational criteria for distinguishing alternatives—particularly for defining the
different expectations of coordination as a passive consequence of joint endings vs.
an active result of ecological locking during intervals of stasis. Perhaps such
distinctions can be defined and recognized in the statistics of faunal associations
(varying strengths and numbers of paired correlations, similarities in joint ranges
and relative abundances of groups of species: what numbers of taxa, and what
intensities of coordination, imply active locking beyond the power of passive
response among independent items to accomplish?). Given the notorious
imperfections of the geological record, and the daunting problems of consensus in
taxonomic definition, I recognize the extreme difficulty of such questions. But the
issues raised are neither untestable nor non-operational, and the concepts involved
could not be more central to evolutionary theory. Whatever the future direction of
this debate, punctuated equilibrium has proven its mettle in prompting important
extensions beyond its original purview, and in proposing a fruitful strategy of
research, based on a new way of viewing the fossil record, that broke some
longstanding impasses in paleontological practice. At the very least, punctuated
equilibrium has raised some interesting and testable questions that could not be
framed under previous assumptions about evolutionary mechanisms and the
patterns of life's history.
As a final note and postscript, either extreme alternative for the explanation of
faunal stasis—passive consequence or active ecological locking— bears an
interesting implication for the significance of punctuated equilibrium. (Of course, I
would be shocked if either extreme eventually prevailed, or if a future consensus
simply melded aspects of both proposals into harmony. I suspect that the reasons
behind coordinated stasis are complex, multifarious, and informed by other modes
and styles of explanation as yet un-conceived.) If coordination arises as a passive
consequence, then our original version of punctuated equilibrium, proposed to
explain the pattern of individual species, also suffices to render this analogous
pattern at the higher level of faunas as well—thus increasing the range and strength
of our mechanism. But if coordination must be forged by higher-level mechanisms
of active ecological locking, then punctuated equilibrium provided the basis, both
logically and historically, for regarding species as evolutionary individuals, the