The Structure of Evolutionary Theory

824 THE STRUCTURE OF EVOLUTIONARY THEORY

Such studies are pursued, in other words, to resolve patterns within
Australopithecus afarensis, or among species in the genus Miohippus, not to
adjudicate general issues in evolutionary theory.
Nonetheless, compendia of such studies do provide a "feel" for generalities of
data in admittedly non-randomized samples, and they do establish archives of
intriguing and well-documented cases both for pedagogical illustration, and simply
for the general delight that all naturalists take in cases well treated and
conclusively resolved. I shall therefore discuss this mode of documentation as
practiced for two categories central to punctuated equilibrium: patterns of
gradualism or stasis within unbranched taxa (part B of this section), and tempos
and modes of branching events in the fossil record (part C). Part D will then treat
the more decisive theme of relative frequencies.

THE EQUILIBRIUM IN PUNCTUATED EQUILIBRIUM:

QUANTITATIVELY DOCUMENTED PATTERNS OF STASIS IN

UNBRANCHED SEGMENTS OF LINEAGES

As previously discussed (see pp. 758-765), the main contribution of punctuated
equilibrium to this topic lies in constructing the theoretical space that made such
research a valid and recognized subject at all. When paleontologists equated
evolution with gradual change, the well-known stasis of most lineages only
flaunted a supposed absence of desired information, and could not be
conceptualized as a positive topic for test and study. By representing stasis as an
active, interesting, and predictable feature of most lineages most of the time,
punctuated equilibrium converted an unconceptualized negative to an intriguing,
and highly charged positive, thereby forging a field of study.
Nonetheless, we cannot argue that a proven predominance of stasis within
lineages can establish the theory of punctuated equilibrium by itself. Punctuated
equilibrium implies and requires such stasis, but remains, primarily, a theory about
characteristic tempos and modes of branching events, and the primary patterning of
phyletic change by differential birth and death of species.
Stasis has emerged from the closet of disappointment and consequent non-
recording. At the very least, paleontologists now write, and editors of journals now
accept, papers dedicated to the rigorous documentation of stasis in particular
cases—so skeptics, and scientists unfamiliar with the fossil record, need not accept
on faith the assurances of experienced paleontologists about predominant stasis in
fossil morphospecies (see pp. 752-755). Moreover, stasis has also become a subject
of substantial theoretical interest (see pp. 874— 885), if only as a formerly
unexpected result now documented at far too high a frequency for resolution as an
anticipated outcome within random systems (Paul, 1985); stasis must therefore be
actively maintained. In any case, paleontologists are now free to publish papers
with such titles as: " Cosomys primus: a case for stasis" (Lich, 1990), and
"Apparent prolonged evolutionary stasis in the middle Eocene hoofed mammal
Hyopsodus" (West, 1979).
The study of McKinney and Jones (1983) may be taken as a standard and