Punctuated Equilibrium and the Validation of Macroevolutionary Theory 851
as devices for "dissecting" the punctuation by revealing an internal "fine structure"
with probative value for inferences about evolutionary causes. Three major modes
of dissection have been featured in the existing literature (although the theme has
not been organized in this manner before), each explicitly invoked as a tool for the
potential validation or refutation of punctuated equilibrium.
TIME. I discussed the operational definition of punctuation as scaled to periods of
stasis (see pp. 765-774). The obvious barrier to testing this primary requirement of
the theory lies in our inability to specify requisite information about time in
"standard" paleontological situations, where the duration of speciation lies beneath
the resolving power of our basic operational "moment"—the bedding plane.
Therefore, to achieve a proper dissection, we must search for unusual situations
that permit an adequate resolution of time in one of two manners dictated by the
logic of the problem: either by finding a way to date individual specimens
compressed on a single bedding plane, or by locating situations of unusually rapid
sedimentation, where a sequence of events usually collapsed onto a single bedding
plane can be expressed in true temporal order through a vertical sequence.
The first tactic can be applied only in highly unusual circumstances
effectively limited to nearly modern bedding planes with specimens that can be
dated individually, for the error bars associated with most radiometric techniques
exceed the entire duration of most bedding planes (except for isotopes with very
short half lives, which can then only be applied to Pleistocene or Holocene
specimens). However, in a recent example (discussed more fully on p. 771 and Fig.
9 - 4 ), Goodfriend and Gould (1996) traced a species transition by hybridization in
the land snail Cerion on the Bahamian island of Great Inagua. We found all
specimens jumbled together on a modern mudflat (a bedding-plane-to-be, if you
will), and we then used a combination of radiocarbon dating and amino acid
racemization to determine that the smooth and complete species transition
occupied 15,000 to 20,000 years—a reasonable figure for a punctuational event
(here compressed, as usual, into a single geological "moment," which we, thanks to
the rare combination of recent occurrence and availability of dating techniques,
were able to disaggregate and resolve).
In the far more common situation of sedimentation rates high enough to
spread the usual compressions of single bedding planes into resolvable vertical
sequences, assessments have been made in both relative and absolute terms. I
previously cited Fortey's (1985) conclusion in the relative mode (see p. 769), based
on calibrating punctuational origins against gradual transitions observed for other
taxa in the same strata (thus obviating the usual claim that literal punctuations
probably represent a geologically slow gradualism that extremely spotty
sedimentation cannot record). With this technique, Fortey's found about a 10:1
ratio for punctuated vs. gradual origins of species in Ordovician trilobites from
Spitzbergen.
The best examples in the more satisfactory absolute mode do not arise