Punctuated Equilibrium and the Validation of Macroevolutionary Theory 929
homology becomes all the more interesting if the particular efficient causes at
different scales—the actual pushers and movers of immediate change in each
case—remain evidently disparate, thus implying, by elimination, that the observed
commonality of pattern may arise from constraining channels of similar structural
properties across scales. If all roads lead to Rome, then the eternal city ranks as a
dominant and ineluctable attractor!
In the present case of punctuational patterns across markedly different scales
of time and component entities, claims for conceptual homology rest upon an
overarching hypothesis that punctuation records something quite general about the
nature of change itself, and that the differing causes of punctuational change at
each level—the waiting time between favorable mutations in bacterial anagenesis
(see next section), the scaling of ordinary speciation as a geological moment in
punctuated equilibrium, or the simultaneity of species deaths in mass extinction—
must run in a common structural channel that sets and constrains the episodic
nature of alteration.
If punctuated equilibrium gains this generality by conceptual homology, then
both components of its name should achieve such transfer across nature's
numerous scales of size and time. (The general mathematical models discussed in
the last section presume such meaningful transfer as a primary rationale for their
relevance.) The equilibrial component wins potential generality if active resistance
to change can be validated as an important structural property of systems discrete
and stable enough to be named and recognized as entities at any scale of nature
(whatever the causes of stability, whether internal to self-integration or imposed
from without upon an intrinsically less coherent structure—a fascinating question
that should become an object of research, not the subject of prior definition). This
property of active maintenance underlies our primary claim about stasis in
punctuated equilibrium, and our insistence that stasis must be conceptualized and
defined as a positive phenomenon, not as a disappointing or uninteresting absence
of anticipated change. (Throughout this chapter, I have provided evidence,
primarily in observed relative frequencies—far too high to originate either
passively or randomly in a world of natural selection and genetic drift—for
interpreting stasis as an actively generated property of systems, embodied in
species at the scale of punctuated equilibrium, but necessarily recognized in
structures of different status at scales both below and above species.)
The punctuational component, operationally measured by its short duration
relative to periods of stasis within definitive structures of the same scale, would
then achieve homological generality as the obverse to proposed reasons for stasis:
the reinterpretation of change—at least in its usual, if not canonical, expression—
as a rare and rapid event experienced by systems only when their previous
stabilities have been stretched beyond any capacity for equilibrial return, and when
they must therefore undertake a rapid excursion to a new position of stability under
changed conditions.
Obviously, these "brave" statements about conceptual homology across
disparate scales and immediate causalities must remain empty and meaningless
without operational criteria for distinguishing—if I may again use the