806 THE STRUCTURE OF EVOLUTIONARY THEORY
equilibrium follows thereby. The second argument denies the common assumption
that high-frequency records uniquely complete geological evidence—and that
gradualism will therefore prevail whenever the fossil record becomes good enough
to preserve its true domination (with a high frequency for punctuated equilibrium
then construed, by Darwin's original argument, as the artifact of a gappy record).
This second argument maintains that, while foram data may be more complete on
average, the best metazoan examples of punctuated equilibrium have been
validated with excellent samples from admittedly rarer but equally complete
geological sequences, thus precluding the explanation of punctuated equilibrium as
artifactual.
The third argument also grants the reality of higher-relative frequency for
gradualism in forams, but argues against extrapolation to larger multicellular
organisms on grounds of genuine difference in evolutionary mode, based on
important biological distinctions between these single-celled creatures of the
oceanic plankton and sexually reproducing metazoan species that, however
parochially, have served as the basis for most of our evolutionary theory and, in
any case, form the bulk of the known fossil record.
This third argument should not be viewed as special pleading by partisans, but
as a positive opportunity for developing hypotheses about the importance (or
insignificance) of punctuated equilibrium based on the correlation between
differences in frequency and distinctive biological properties of various taxonomic
groups—particularly in features related to speciation, the presumed evolutionary
basis of punctuated equilibrium. Planktonic forams, with their asexuality, their
small size and rapid turnover of generations, their unicellularity, their vast
populations, and their geographic links to water masses, display maximal
difference from most metazoans, and may therefore be especially suited for
helping us to understand, by contrast, the prevailing mechanisms of evolution in
multicellular and sexually reproducing organisms. The general nature of these
differences does indeed point to a set of factors tied to the definition and division
of populations, therefore granting plausibility to the claim that so-called "species"
of planktonic forams should show more gradualism than metazoan taxa, while
punctuated equilibrium may prevail in sexually reproducing multicellular species.
The subject deserves much more attention and rigor, but to sketch a few suggested
factors:
(1) Population characteristics. We conventionally name Linnaean species of
asexual protistans, but even if adequately stable "packages" of form or genetic
distinctness exist in sufficiently extended domains of space and time to merit a
vernacular designation as "populations," what comparison do such entities bear
with species of sexually reproducing multicellular organisms? (Needless to say, I
raise no new issue here, but only recycle the perennial question of "the species
problem" in asexual organisms.) Punctuated equilibrium posits a link of observed
evolutionary rates to properties of branching speciation in populations. I don't even
know how to think about such issues in planktonic forams, where vast populations
may be coextensive with entire oceanic water masses, and where numbers must run
into untold billions