concentrated at events of speciation themselves. Subsequent work in evolutionary
biology has not confirmed any a priori preference for concentration in such
episodes. Futuyama's incisive macroevolutionary argument—that realized change
will not become geologically stabilized and conserved unless such change can be
"tied up" in the unalienable individuality of a new species—offers a far richer, far
more interesting, and theoretically justified rationale for correlating episodes of
evolutionary change with speciation.
- Section III presents a wide-ranging discussion of why proposed empirical
refutations of punctuated equilibrium either do not hold in fact, or do not bear the
logical weight claimed in their presentation. Refutations for single cases are often
valid, but do not challenge the general hypothesis because we anticipate a low
relative frequency for gradualism, and these cases may reside in this minor
category. Claims for predominant gradualism in the entire clade of planktonic
forams may hold as exceptional (although, even here, the majority of lineages
remain unstudied, in large part because they seem, at least subjectively, to remain
in stasis, and have therefore not attracted the attention of traditional researchers,
who wish to study evolution, but then equate evolution with gradualism).
However, in these asexual forms with vast populations, gradualism at this level
may just represent the expected higher-level expression of punctuational clone
selection, as Lenski has affirmed in the most thorough study of evolution in a
modern bacterial species—and just as gradual cladal trends in multicellular
lineages emerge as the expected consequences of sequential punctuated
equilibrium at the species level (trends as stairsteps rather than inclined planes, so
to speak). Claims for genetic gradualism do not challenge punctuated equilibrium,
and may well be anticipated as the proper expression at the genie level (especially
given the high relative frequency of random nucleotide substitutions) of
morphological stasis in the phenotypic history of species. Punctuated equilibrium
has done well in tests of conformity with general models, particularly in the
conclusion that extensive polytomy in cladistic models may arise not only (as
usually interpreted) from insufficient data to resolve a sequence of close
dichotomies, but also as the expectation of punctuated equilibrium for successive
branching of daughter species from an unchanged parental form in stasis. In fact,
the frequency of polytomy vs. dichotomy may be used as a test for the relative
frequency of punctuated equilibrium in well resolved cladograms—a test well
passed in data presented by Wagner and Erwin. - Section IV then summarizes the data on empirical affirmations of
punctuated equilibrium, first on documented patterns of stasis in unbranched
lineages; second on punctuational cladogenesis affirmed by the criterion of
ancestral survival; third on predominant relative frequencies for punctuated
equilibrium in entire biotas (with particularly impressive affirmations by Hallam,
Kelley, and Stanley and Yang for mollusks; and by Prothero and Heaton for
Oligocene Big Badlands mammals, where a study of all taxa yielded 177 species
that followed the expectations of punctuated equilibrium and three cases of
potential gradualism, only one significant); fourth on predominant relative
frequencies for punctuated equilibrium in entire clades, with empha-
78 THE STRUCTURE OF EVOLUTIONARY THEORY