996 THE STRUCTURE OF EVOLUTIONARY THEORY
our worst mental habit of interpreting other ranges of size, or other domains of time,
in our own limited terms.
For example, Mettler, Gregg, and Schaffer's textbook on Population Genetics
and Evolution (1988, p. 304) states: "The punctuated equilibrium theory, on the other
hand, holds that sudden appearance is due to rapid selection, rather than rapid spread,
and that stasis results because evolutionary change occurs in large discrete jumps
rather than by a series of gene substitutions. There really are no gradual changes or
intermediate stages." In their volume on Sexual Selection for the prestigious Scientific
American series (1989, p. 83), Gould and Gould (no relation) write: "The proven
ability of selection to operate quickly in at least some cases, has led to the widely
publicized theory of punctuated evolution. According to the original version, no
intermediate forms are preserved simply because there are no halfway creatures in the
first place: new species come into being in single steps."
Turning to misscaling in the other direction, Wessells and Hopson (1988, pp.
1073 - 1074) equate punctuated equilibrium with the origin of new Bauplan and faunal
turnovers in mass extinction: "The central tenet of punctuated equilibrium is that a
lineage of organisms arises by some dramatic changes—say, the rapid acquisition of
body segmentation in annelids—after which there is a lengthy period with far fewer
radical changes taking place." They then write of two great evolutionary bursts in the
history of sea urchins (following the late Cambrian and Late Triassic mass
extinctions). "One might interpret this record to reflect two 'punctuations' in the
Ordovician and early Jurassic periods. And the 'equilibrium' times would be from the
Ordovician through the Triassic and, perhaps, from the Jurassic to today. This record
may be consistent with the punctuated equilibrium hypothesis."
Chaisson's ambitious textbook on nearly everything—Universe: An
Evolutionary Approach to Astronomy—equates punctuated equilibrium with faunal
turnovers in mass extinction. His section entitled punctuated equilibrium (1988, p.
481) begins by stating: "The fossil record of the history of life on Earth clearly
documents many periods of mass extinction." He then adds (p. 483): "Punctuated
equilibrium merely emphasizes that the rate of evolutionary change is not gradual.
Instead, the 'motor of evolution' occasionally speeds up during periods of dramatic
environmental change—such as cometary impacts, reversals of Earth's magnetism,
and the like. We might say that evolution is imperceptibly gradual most of the time
and shockingly sudden some of the time." But Chaisson's "imperceptibly gradual"
times—the intervals of so-called "normal" evolution between episodes of mass
extinction—build their incremental trends by stair steps based on the true rhythm of
punctuated equilibrium in rapid origin and subsequent stasis of individual species.
However, even in this maximally constrained and conservative world of
textbooks, some reform has emerged from punctuated equilibrium. Above all, the
debate on punctuated equilibrium prodded the authors of nearly all major textbooks
to include (often as entirely new sections) substantial and explicit material on
macroevolution—in contrast with the appalling absence