Punctuated Equilibrium and the Validation of Macroevolutionary Theory 959
adaptationist paradigm. She writes (1991, p. 12): "Gradualist paradigms imply that
systems can 'accept' virtually any change, any time, as long as it is small enough;
big changes result from the insensible accumulation of small ones. In contrast,
punctuated equilibrium suggests that, for most of systems' histories, there are limits
beyond which change is actively prevented, rather than always potential but merely
suppressed because no adaptive advantage would accrue."
Gersick's lists of commonalities among her six levels, both for periods of
stasis and for episodes of punctuation, satisfy the strategy of conjoints, while her
ranked list of scales, and especially her linkages of particular categories to the two
most overarching theories of punctuational change—Kuhn's for human thought
and Prigogine's for the natural world—meet the criterion of generalization. For
example, her chart of comparison among the six levels for "equilibrium periods"
cites both commonalities and conjoints, with an interestingly different emphasis
(from our concerns with biological systems) upon the potential for strong
limitation placed upon incremental pathways within a plateau—an important theme
for the Lamarckian character of human cultural change. She lists as
"commonalities" (p. 17): "During equilibrium periods, systems maintain and carry
out the choices of their deep structure. Systems make adjustments that preserve the
deep structure against internal and external perturbations, and move incrementally
along paths built into the deep structure. Pursuit of stable deep structure choices
may result in behavior that is turbulent on the surface."
Similarly, her chart for punctuational episodes stresses the unpredictability
and potential nonprogressionism of outcomes, a surprising theme for human
systems based on supposed and explicit goals, but a notion that we did not
emphasize in formulating punctuated equilibrium because biological evolution
proceeds in a highly contingent manner for so many other reasons (see Gould,
1989c), some recognized and emphasized by Darwin himself. Thus, this important
theme, while equally central within the structure of our theory, did not have similar
salience for us—and we thank Gersick for her insight and generalization. Gersick
writes in her heading (p. 20): "Revolutions are relatively brief periods when a
system's deep structure comes apart, leaving it in disarray until the period ends,
with the 'choices' around which a new deep structure forms. Revolutionary
outcomes, based on interactions of systems' historical resources with current
events, are not predictable; they may or may not leave a system better off.
Revolutions vary in magnitude."
As another example of fruitful borrowing across disciplines, Mokyr (1990, p.
350) begins his study of technological change by noting that Alfred Marshall's
advocacy of gradualism in economics played a similar role in the human sciences
to Darwin's impact upon the natural sciences: "Charles Darwin and Alfred
Marshall were both extremely influential men... Darwin and Marshall both
believed that nature does not make leaps. Both were influenced by a long and
venerable tradition that harked back to Leibniz rooted in the Aristotelian notion of
the continuity of space and time." (On the same