Punctuated Equilibrium and the Validation of Macroevolutionary Theory 839
in part—shifting means resulting from selective removal of disadvantaged flatter
specimens. But we also need to understand the potentiating condition established
by an initial (and geologically rapid) expansion in the range of variation. What
mechanisms underlie such change in a variational spectrum? Evolution can't
anticipate future needs for altered means, so the enlarged range can only be
exaptive for the subsequent trend. What, then, lies behind such rarely documented
(but eminently testable) expansions and contractions of variational ranges?
THE PUNCTUATIONS OF PUNCTUATED EQUILIBRIUM: TEMPO
AND MODE IN THE ORIGIN OF PALEOSPECIES
Stasis is data, and potentially documentable in any well-sampled series of
persistently abundant fossils spanning a requisite range of time—i.e., most of the
duration of an average species in a given taxon, ranging roughly from a million
years or so for such rapidly evolving forms (or perhaps just more closely
scrutinized, or richer in visibly complex characters) as ammonites and mammals to
an average of 5 to 10 million years for "conventional" marine invertebrates. But
punctuation may only record an absence of intermediary data. Thus, as noted
several times before, the second word of our theory stands more open to general
test than the first, and this operational constraint inevitably skews the relative
abundances of published information.
If punctuational claims were truly untestable, or subject to empirical
documentation only in the rarest of special circumstances, then the entire theory
would be severely compromised. Fortunately, many cases at the upper end of a
spectrum in richness of data (both in abundance of specimens and fine-scale
temporal resolution) offer adequate materials for distinguishing the causes and
modalities of punctuation. The testable cases do not nearly approach a majority of
available species, but neither do they stand out as preciously unusual. Thus, we
face a situation no different from most experimental testing in science, especially
when we cannot construct ideal conditions in a laboratory, and must use nature's
own "experiments" instead. That is, we must pick and choose cases with adequate
information for resolution, and without inherent biases that falsely presuppose one
solution over others. But experimentalists in "hard science" seek the same unusual
resolvability when they "improve" upon the ordinary situations of nature by
establishing fixed, simplified, and measurable circumstances in a laboratory.
Natural historians proceed no differently, and with no more artificiality or rarity of
acceptable conditions for testing—except that we must ask nature to set the
controls (and must therefore live by her whims rather than our manipulations. In
this sense, the naturalist's tactic of "choosing spots" selectively corresponds with
the experimentalist's strategy of establishing controls in laboratories).
The testing of punctuations has generated two primary themes of research: the
establishment of criteria for distinguishing among the potential causes of