766 THE STRUCTURE OF EVOLUTIONARY THEORY
and proponents of punctuated equilibrium would become dull specialists if they did
not take an interest in the different mechanisms responsible for similarities in the
general features of stability and change across nature's varied domains, for science
has always sought unity in this form of abstraction. But punctuated equilibrium—a
particular punctuational theory of change and stability for one central phenomenon
of evolution—does not directly address the potentially coordinated history of
faunas, or the limits of viable mutational change between a parental organism and
its offspring in the next generation.
The theory of punctuated equilibrium attempts to explain the
macroevolutionary role of species and speciation as expressed in geological time.
Its statements about rapidity and stability describe the history of individual species;
and its claims about rates and styles of change treat the mapping of these
individual histories into the unfamiliar domain of "deep" or geological time—
where the span of a human life passes beneath all possible notice, and the entire
history of human civilization stands to the duration of primate phylogeny as an eye
blink to a human lifetime. The claims of punctuated equilibrium presuppose the
proper scaling of microevolutionary processes into this geological immensity—the
central point that Darwin missed when he falsely assumed that "slowness" of
modification in domesticated animals or crop plants, as measured in ordinary
human time (where all of our history, and so many human generations, have
witnessed substantial change within populations, but no origin of new species),
would translate into geological time as the continuity and slowness of phyletic
gradualism.
Once we recognize that definitions for the two key concepts of stasis and
punctuation describe the history of individual species scaled into geological time,
we can establish sensible and operational criteria. As a central proposition,
punctuated equilibrium holds that the great majority of species, as evidenced by
their anatomical and geographical histories in the fossil record, originate in
geological moments (punctuations) and then persist in stasis throughout their long
durations (Sepkoski, 1997, gives a low estimate of 4 million years for the average
duration of fossil species; mean values vary widely across groups and times, with
terrestrial vertebrates at lesser durations and most marine invertebrates in the
higher ranges; in any case, geological longevity achieves its primary measure in
millions of years, not thousands). As the primary macroevolutionary implication of
this pattern, species meet all definitional criteria for operating as Darwinian
individuals (see pp. 602-613) in the domain of macroevolution.
This central proposition embodies three concepts requiring definite
operational meanings: stasis, punctuation, and dominant relative frequency. (I am
not forgetting the thorny problems associated with the definition of species from
fossil data, where anatomy prevails as a major criterion and reproductive isolation
can almost never be assessed directly—and also with the putative correspondence
of morphological "packages" that paleontologists designate as species with the
concept as understood and practiced by students of modern populations of sexually
reproducing organisms. I shall treat these issues on pages 784-796.)