Punctuated Equilibrium and the Validation of Macroevolutionary Theory 893
speciation rates" (p. 514), whereas the origin of major apomorphies of larger
groups do not correlate so clearly with numbers of speciation events, but rather
with their magnitude (with such rare events favored in certain times and
environments).
In documenting a speciational basis as a "null hypothesis" of sorts for
identifying evolutionary patterns arising by other routes, McKinney et al. (1998)
compared the differential successes of cheilostome and cyclostome bryozoan
clades through time. In a fascinating discovery, they noted that, in times of joint
decline for both clades (late Cretaceous and post-Paleocene after a Danian spike in
diversification), "the relative skeletal mass of cyclostomes declined much more
precipitously than did relative species richness" (p. 808). The authors could
therefore identify an important trend by standardizing species numbers: "There is a
long-term trend for the average cheilostome species to generate a progressively
greater skeletal mass than the average cyclostome species. This could result from a
gradual trend toward relatively larger colony sizes within cheilostomes, a greater
number of colonies per cheilostome species, or both" (pp. 808-809).
In a similar vein, but at the larger scale of the phylum's initial Ordovician
radiation, Anstey and Pachut (1995) found no relationship between number of
speciation events and the establishment of defining apomorphies among major
subgroups at the base of the clade. They write (1995, pp. 262-263): "The
morphologies recognized as higher taxa of bryozoans were not built up through a
gradual accumulation of species differences but appear to have diverged very
rapidly in the initial radiation of the phylum... The processes producing the major
branching events and familial apomorphies, therefore, apparently were not driven
by speciation, and likewise could not have resulted from species selection or
species sorting."
THE SPECIATIONAL REFORMULATION OF MACROEVOLUTION. Beyond
the immediate expansion of explanations for the signature phenomenon of trends,
the recasting of macroevolution as a discourse about the differential fates of stable
species (treated as Darwinian individuals) carries extensive implications for
rethinking both the pageant of life's history and the causes of stability and change
in geological time.
I am particularly grateful that Ernst Mayr (1992, p. 48)—the doyen of late
20th century evolutionary biologists, and the inspirer of punctuated equilibrium
through his views on peripatric speciation (though scarcely an avid supporter of
our developed theory)—has identified the required speciational reformulation of
macroevolution as the principal component of "what had not been recognized [in
evolutionary theory] before" Eldredge and I codified punctuated equilibrium. Mayr
continues, stressing the species as the macroevolutionary analog of the organism
considered as the "atom" of microevolution:
It was generally recognized that regular variational evolution in the
Darwinian sense takes place at the level of the individual and population,