Punctuated Equilibrium and the Validation of Macroevolutionary Theory 949
most cases, to invoke the central concept of punctuational change: origin in a tiny
fraction of later existence in stasis.)
For example, Kerr (1994) begins his report on Peter Sheehan's work (in a
commentary entitled "Between extinctions, evolutionary stasis") by writing (Kerr,
1994, p. 29): "More and more, paleontologists are learning that the full measure of
a mass extinction can't be found in its immediate toll. Just as important is the
wholesale reorganization of living communities that takes place afterward. And
those brief recovery periods, lasting just a few million years, are all the more
important because during the tens of or hundreds of millions of years that follow,
until the next mass extinction, not much may happen."
Sheehan divides the last 640 million years into six major faunal packages that
he calls EEU's, or Ecologic Evolutionary Units. Each lasts for 35 to 147 million
years, and each ends at a mass extinction. The subsequent recovery periods for the
new units occupy only 3 to 8 million years.
This recent affirmation of a strongly punctuational character for change
(primarily extinction) at the highest level has led to a tendency, probably
overextended—and I blame myself, in part, for propagating the theme, see Gould,
1985a—for ascribing a dualistic character to the pulse of evolution, with
punctuations of mass extinction alternating with a more stately flow in "normal"
times between these macropulses. But this view may prove to be overly simplistic,
although not wrong. When we assess each level of change by its own appropriate
measuring rod (scaled to emphasize the relevant unit or units), all may be
punctuational. We must dismiss as irrelevant and misleading the fact that
punctuations at a small scale may "smooth out" to more gradual and continuous
trends when inappropriately measured at too large a scale to reveal the causal
mechanics, or even to identify the relevant unit, of change—a theme that I have
emphasized throughout this chapter, in such examples as punctuated bacterial
anagenesis, viewed as gradual when sampled too infrequently to note the steps of
mutational sweeps; and cladal trends, viewed as anagenetic when sampled too
broadly to discern the speciational jumps of punctuated equilibrium.
In a provocative work, Raup (1992) played devil's advocate by asking if all
extinctions at all levels, from single local populations to global faunas, might be
catastrophic—for he could not reject the "null hypothesis" of his "field of bullets"
model (random and catastrophic removal, triggered by "bolides" of various sizes
randomly shot towards the earth at frequencies inversely proportional to their size
and effect) in favor of the traditional Darwinian model of gradual declines
mediated by competitive inferiority in biotic interactions. I do not believe that such
extreme punctuationalism could rule so completely (see full discussion of this
argument in Chapter 12, pp. 1323-1326). But finer analysis of the most famous
cases of supposedly gradual, and biotically controlled, events may well require
such a punctuational reinterpretation. Most outstandingly, perhaps the two most
widely discussed and most generally accepted examples of geologically slow
global diversification—the Ordovician