Punctuated Equilibrium and the Validation of Macroevolutionary Theory 857
one lineage, the famous oyster Gryphaea, showed a corresponding gradual change
in shape as well—a consequence of heterochronic linkage to phyletic variation in
size (see affirmation of Jones and Gould, 1999). Hallam concluded, in persuasive
support of punctuated equilibrium by the proper criterion of relative frequency, and
with explicit attention to important and potentially confounding issues of
geographic variation and missing data due to gaps in the geological record (1978,
p. 17):
The results of my analysis of 329 European Jurassic species provide, with
an important exception, overwhelming support for the punctuated equilibria
model. Species whose morphology appears to persist unchanged for long
periods are abruptly terminated usually with one or more species of the
same genus succeeding the older species with marked morphological
discontinuity. The species ranges are long compared with the ammonites
that allow fine stratigraphic subdivision and can be used to eliminate the
possibility of significant stratigraphic gaps in the rock succession.
Geographic variation within Europe is negligible, and more cursory
examination of data from other continents provides no encouragement for
the view that gradualistic events linking the "punctuated equilibria" in time
took place outside Europe.I trust, however, that Tony Hallam, one of my best friends in science, will not
think me fractious or ungrateful if I point out that he then devoted the empirical
content of his paper to documenting phyletic size increase in several species and,
especially, to tracing gradual evolutionary changes within Gryphaea—in other
words to the 1 lineage among 329 that illustrated phyletic gradualism. He
presented no morphometric data for the overwhelming majority of species that
remained in stasis throughout their existence. He wrote (1978, p. 17): "The
succeeding sections of this paper are devoted primarily to this aspect of phyletic
gradualism [size increase] and its implications in the broader context of
environmental control of speciation, starting with the detailed analysis of
Gryphaea."
The unconsciously imbibed power of gradualism thus remained so strong
during these early years of the punctuated equilibrium debate, that Hallam could
declare "overwhelming support for the punctuated equilibria model" as his primary
conclusion and the focus of his study—and then follow conventional practice in
applying morphometric methods only to rare examples of gradualism within his
sample, even though the predominant signal of stasis could be validated just as
rigorously by the same methods. For all the theoretical uncertainties that still
animate the punctuated equilibrium debate, at least we have made substantial
headway on this operational issue since the 1970's. Any similar study, done now,
would almost surely include the documentation of stasis.
Kelley (1983, 1984) studied all molluscan lineages with adequate samples
over sufficiently long ranges in one of the most famous and widely studied of all
fossil faunas: the Miocene deposits of the Chesapeake Group in Maryland
(Shattuck, 1904, for the classic statement; Schoonover, 1941, for