Punctuated Equilibrium and the Validation of Macroevolutionary Theory 825
symbol for hundreds of similar cases representing a characteristic mixture of
satisfaction and frustration. These authors documented a sequence of three
successional species of oligopygoid echinoids from the Upper Eocene Ocala
Limestone of Florida. The two stratigraphic transitions are abrupt, and therefore
literally punctuational. But available evidence cannot distinguish among the
mutually contradictory explanations for such passages: gradualism, with transitions
representing stratigraphic gaps; rapid anagenesis for a variety of plausible reasons
including population bottlenecks or substantial environmental change; punctuated
equilibrium based on allopatric speciation elsewhere (or unresolvably in situ, given
coarse stratigraphic preservation), and migration of new species to the ancestral
range. Hence, frustration. (Moreover, as this pattern represents the most frequent
situation in most ordinary sequences of fossils, we can readily understand why the
testing of punctuational claims within the theory of punctuated equilibrium
requires selection of cases—fortunately numerous enough in toto, however modest
in relative frequency—with unusual richness in both spatial and temporal
resolution.)
At the same time, however, we gain satisfaction in eminent testability for the
set of claims representing the second key concept of stasis. Any species, if well
represented throughout a considerable vertical span marking the hundreds of
thousands to millions of years for an average duration, can be reliably assessed for
stasis vs. anagenetic gradualism by criteria outlined previously (pp. 765-774).
McKinney and Jones (1983) compiled excellent evidence for stasis in each of their
three species—the basis, after all, for using these taxa in establishing biozones for
this section. (As argued on pp. 751-752, biostratigraphers have always used criteria
of stasis and overlapping range zones in their practical work on the relative dating
of strata.) McKinney and Jones conclude (1983, p. 21): "These observations
suggest there is little chance of species misidentification due to ontogenetic or
phylogenetic effects when using this lineage for biostratigraphic purposes."
Smith and Paul (1985) studied vertical variation of the irregular echinoid
Discoides subucula in a remarkably complete and well-resolved sequence of Upper
Cretaceous sands. The species occurred throughout 8.6 m of section, apparently
representing continuous sedimentation within one ammonite zone spanning less
than 2 million years. The authors were able to sample meter by meter through a
section with an interesting inferred environmental history: "The sediment that was
then being deposited changed from clean, well-washed sand to a very muddy sand,
and so one might expect to find evidence of phyletic gradualism in response to
these changes" (1985, p. 36).
Smith and Paul did measure a steady change in shape towards a more conical
form, a common response of irregular echinoids to muddy environments. But such
an alteration can be ecophenotypically induced during ontogeny, and the authors
see no reason to attribute this single modification to genetically based evolution
(while not, of course, disproving the possibility of such genuine gradualism).
Otherwise, stasis prevails throughout the section: "In other, more important
characters, D. subucula remains morphologically static and shows no evidence of
phyletic gradualism" (1985, p. 29).