tion at a higher faunal level. Punctuation must be scaled relative to the later
duration of species in stasis, and we suggest 1-2 percent (analogous to human
gestation vs. the length of human life) as an upper bound. Punctuated equilibrium
can be distinguished from other causes of rapid change (including anagenetic
passage through bottlenecks and the traditional claim of imperfect preservation for
a truly gradualistic event) by the criterion of ancestral survival following the
branching of a descendant. Punctuations can be revealed by positive evidence
(rather than inferred from compression on a single bedding plane) in admittedly
rare situations, but not so infrequent in absolute number, of unusual fineness of
stratigraphic resolution or ability to date the individual specimens of a single
bedding plane. Stasis is not defined as absolute phenotypic immobility, but as
fluctuation of means through time at a magnitude not statistically broader than the
range of geographic variation among modern populations of similar species, and
not directional in any preferred way, especially not towards the phenotype of
descendants. Punctuated equilibrium will be validated, as all such theories in
natural history must be (including natural selection itself), by predominant relative
frequency, not by exclusivity. Gradualism certainly can and does occur, but at very
low relative frequencies when all species of a fauna are tabulated, and when we
overcome our conventional bias for studying only the small percentage of species
qualitatively recognized beforehand as having changed through time.
- Punctuated equilibrium emerges as the expected scaling of ordinary
allopatric speciation into geological time, and does not suggest or imply radically
different evolutionary mechanisms at the level of the origin of species. (Other
proposed mechanisms of speciation, including most sympatric modes, envision
rates of speciation even faster than conventional allopatry, and are therefore even
more consistent with punctuated equilibrium.) The theoretically radical features of
punctuated equilibrium flow from its proposals for macroevolution, with species
treated as higher-level Darwinian individuals analogous to organisms in
microevolution. - The difficulty of defining species in the fossil record does not threaten the
validity of punctuated equilibrium for several reasons. First, in the few studies with
adequate data for genetic and experimental resolution, paleospecies (even for such
difficult and morphologically labile species as colonial cheilostome bryozoans)
have been documented as excellent surrogates, comparable as units to conventional
biospecies. Second, the potential underestimation of biospecies by paleospecies
only imposes a bias that makes punctuated equilibrium harder to recognize. The
fossil record's strongly positive signal for punctuated equilibrium, in the light of
this bias, only increases the probability of the pattern's importance and high
relative frequency. Third, the potential overestimation of biospecies by
paleospecies is probably false in any case, and also of little practical concern
because no paleontologist would assert punctuated equilibrium from the evidence
of oversplit taxa in faunal lists, but only from direct biometric study of stasis and
punctuation in actual data. - We originally, and probably wrongly, tried to validate punctuated
equilibrium by asserting that, in principle, most evolutionary change should be
Defining and Revising the Structure of Evolutionary Theory 77