994 THE STRUCTURE OF EVOLUTIONARY THEORY
Steady flow or fits and starts—the division between these conceptions of
evolution has dominated the debate over evolutionary theory. The punctuated
equilibrium model has stimulated much research and drawn many adherents.
Some of its central notions have taken firm hold.
Even the most traditional Darwinians, for example, acknowledge that
punctuated equilibrium has become an important part of the picture of
evolution. Some species do little of evolutionary interest for millions of years
at a time. ...
But the debate continues to rage, because it concerns far more than speed
itself. At stake are the fundamental questions of evolution: when and why
does a creature change from one form to another? Is most evolution the slow,
unceasing accumulation of the small changes a geneticist sees in laboratory
fruit flies, or does it occur in episodes, when a small population, perhaps
isolated geographically, suddenly changes enough to give rise to a new
species?
Suddenly, in paleontological terms, can mean hundreds of thousands of
years... Proponents of punctuated equilibrium take pains to stress that such
events rely mainly on the Darwinian principles of natural selection among
individuals varying randomly from one another. Even so, to some biologists,
punctuated equilibrium seems like a resort to some process apart from the
usual rules—"mutations that appear to be magic," Dr. Maynard Smith said.
"They have argued that their results mean that evolution as seen on the
large scale is not just the summing up of small events," he said, "but a series
of quite special things that people like me"—population geneticists—"don't
see. We don't want to be written out of the script."The movement of scientific ideas into textbooks may provide our best insight
into social forces that direct the passage from maximal professional independence
into the most conservative of print genres. To be successful, textbooks must sell large
numbers of copies to audiences highly constrained by set curricula, teachers who
hesitate to revise courses and lessons substantially, and conservative communities
that shun scholastic novelty. These external reasons reinforce the internal propensities
of publishers who are happy to jazz up or dumb down, but not to innovate, and
authors who experience great pressure to follow the conventions of textbook cloning,
and not to depart from the standard takes, examples, illustrations, and sequences. Did
you ever see a high school biology textbook that doesn't start the evolution chapter
with Lamarck's errors, Darwin's truths, and giraffes' necks in that order?
In this context, I delight in the rapid passage of punctuated equilibrium from
professional debate to nearly obligatory treatment in the evolution chapter of biology
textbooks. I could put a cynical spin on this phenomenon, but prefer an interpretation,
in my admittedly partisan manner, based on the successful ontogeny of punctuated
equilibrium from a controversial idea to a firm item of natural knowledge, however
undecided the issues of relative frequency and importance remain.