772 THE STRUCTURE OF EVOLUTIONARY THEORY
plane, but be unable to perform any fine scale analysis in the absence of methods
for dating individual shells. That is, we would be unable to discover whether the
unusual morphological range represented a temporal transition or a standing
population with enhanced variation. But Goodfriend and I could date the individual
shells by amino acid racemization for all specimens, keyed to radiocarbon dates for
a smaller set of marker shells. We found an excellent correlation between
measured age and multivariate morphometric position on the continuum between
ancestral C. excelsior and descendant C. rubicundum (see Fig. 9-4). The transition
lasted between 15,000 and 20,000 years—a good average value for a punctuational
event, and a fact that we could ascertain only because the individual specimens of a
single bedding plane could be chemically dated independently of their
morphology.
We can therefore define stasis and punctuation in operational terms, with
stasis available for test in almost any species with a good fossil record, but
punctuation requiring an unusual density of information, and therefore not
routinely testable, but requiring a search for appropriate cases (not an unusual
situation in sciences of natural history, where nature sets the experiments, and
scientists must therefore seek cases with adequate data). The third key issue of
relative frequency may be easier to operationalize—as one need only tabulate cases
pro and con within well-documented faunas—but remains harder to define.
As the most important ground rule, the theory of punctuated equilibrium
makes a claim about dominating pattern, or relative frequency, not just an assertion
9 - 4. Another way to dissect a punctuation by obtaining absolute age dates for all specimens on a
bedding plane, and thus obtaining temporal distinctions within the compression. The ancestral
and high-spired Cerion excelsior, over no more than 15,000 to 20,000 years (well within the
range of punctuational dynamics), hybridizes with invading Cerion rubicundum, with gradual
fading out of all morphometric influence from the unusually shaped ancestor.