Punctuated Equilibrium and the Validation of Macroevolutionary Theory 927
new. The fossil record of the latter process would then resemble 'punctuated
equilibrium': a pattern of stasis interrupted by sudden change, which some
paleontologists now believe to be the norm in real evolution... This same pattern
of stasis punctuated by sudden change also showed up in a number of other
ecosystem models presented at the workshop, even when those models seemed
superficially quite different. Does this mean some more general mechanism is at
work, some theory that could account for the behavior of these models—and
perhaps real life—no matter how they are structured?"
A false and counterproductive argument has enveloped this work during the
past few years. Bak, in particular, has noted that punctuations at the highest level,
corresponding to simultaneous extinction of a high percentage of components in a
system, can be generated from internal dynamics alone, and require no
environmental trigger of corresponding (or even of any) magnitude. He and others
then draw the overextended inference that because such large-scale punctuations
can arise endogenously, the actual mass extinctions of the fossil record therefore
need no exogenous trigger of environmental catastrophe, or any other external
prod. This claim, emanating from a theoretical physicist with little knowledge of
the empirical archives of geology and paleontology, and emerging just as
persuasive evidence seems to have sealed the case for bolide impact as a trigger of
at least one actual mass extinction (the end Cretaceous event 65 million years ago),
could hardly fail to raise the hackles of observationally minded scientists who, for
reasons both understandable and lamentable, already bear considerable animus
towards any pure theoretician's claim that success in modelling logically entails
reification in nature.
The obvious solution—if human emotions matched human logic in clarity, or
the empirical world in complexity—would welcome the mathematical validation of
potential endogenous triggers (often of small initial extent) for punctuational
change as a partner with well-documented exogenous triggers (of great extent in
one well documented case, but perhaps also of potentially small magnitude as
well). Instead of waging a false battle for preference or exclusivity of one
alternative between two plausible arguments, we should recognize instead the
complementary and general theme behind both proposals—their common role as
sources for punctuational change (which then achieves higher status as a truly
general pattern in nature), and in their mutual reinforcement for revising and
expanding the Darwinian paradigm on all three supporting legs of its essential
tripod. For the punctuational style of change—disfavored by Darwin, who
recognized the necessary status of gradualism within the logic of his world view—
now emerges as a primary consequence of repairs and reinforcements upon all legs
of the tripod: the expansion beyond small uniformitarian inputs for the external
triggers and causes of leg three (thus granting environment an even greater role
than Darwin himself, who so brilliantly introduced the concept to defeat previous
internalist theories of change, had envisioned); and the recognition that constraints
of systems (leg two)—not only overt natural selection—acting at all levels of