The Structure of Evolutionary Theory

portance of drift (both species drift and founder drift) vs. selection as a mechanism
of sorting at the species level, but not at the organismal level, where selection
predominates in standard formulations. Sixth, the identification of an intrinsically,
and probably unbreakable (in most cases), negative correlation between speciation
and extinction propensities as the primary constraint operating to prevent the
takeover of life by a few megaclades (which might dominate by enhancing
speciation while retarding extinction among constituent species—or perhaps the
Coleoptera have prevailed by this means). Seventh, the recognition that the
organismal level operates uniquely in securing the integrity of its individuals by
devices (physiological homeostasis among organs, and spatial bounding by an
external surface) that "clear out" both drive from below and drift at its own level as
mechanisms operating at high relative frequency—thus leaving selection in its
most dominant position at this level. Perhaps our Darwinian prejudice for
regarding selection as by far the most effective, or virtually the only important,
process of evolutionary change arises more from the parochialism of our
organismal focus (given our own personal residence in this category) than from
any universal characterization of all levels in evolution.

Chapter 9: Punctuated equilibrium and the validation of

Macroevolutionary Implications

1. The clear predominance of an empirical pattern of stasis and abrupt
   geological appearance as the history of most fossil species has always been
   acknowledged by paleontologists, and remains the standard testimony (as
   documented herein) of the best specialists in nearly every taxonomic group. In
   Darwinian traditions, this pattern has been attributed to imperfections of the
   geological record that impose this false signal upon the norm of a truly gradualistic
   history. Darwin's argument may work in principle for punctuational origin, but
   stasis is data and cannot be so encompassed.


2. This traditional argument from imperfection has stymied the study of
   evolution by paleontologists because the record's primary (and operational) signal
   has been dismissed as misleading, or as "no data." Punctuated equilibrium, while
   not denying imperfection, regards this signal as a basically accurate record of
   evolution's standard mode at the level of the origin of species. In particular, before
   the formulation of punctuated equilibrium, stasis had been read as an embarrassing
   indication of absence of evidence for the desired subject of study—that is, of data
   for evolution itself, falsely defined as gradual change—and this eminently testable,
   fully operational, and intellectually fascinating (and positive) subject of stasis had
   never been subjected to quantitative empirical study, a situation that has changed
   dramatically during the last 25 years.


3. The key empirical ingredients of punctuated equilibrium—punctuation,
   stasis, and their relative frequencies—can be made testable and defined
   operationally. The theory only refers to the origin and development of species in
   geological time, and must not be misconstrued (as so often done) as a claim for
   true saltation at a lower organismal level, or for catastrophic mass extinc-
   76 THE STRUCTURE OF EVOLUTIONARY THEORY