Punctuated Equilibrium and the Validation of Macroevolutionary Theory 769
cannot be dissected); moreover, an observed punctuation often represents the even
less desirable circumstance of missing record (Darwin's classic argument from
imperfection), or only partial pattern (as when a punctuation in a single geological
section marks the first influx by migration of a species that originated earlier and
elsewhere). Since stasis, on the other hand, provides an active (and often excellent)
record of stability, empirical defenses of punctuated equilibrium have
understandably focussed on the more easily documentable claims for equilibrium,
and less frequently on more elusive predictions about punctuation. But we must not
conclude, as some authors have suggested, that punctuation therefore becomes
untestable or even impervious to documentation—and that the thesis of punctuated
equilibrium must therefore depend for its empirical support only upon the partial
data of stasis. The documentation of punctuation may be both more difficult and
less frequently possible, but many good cases have been affirmed and several
methods of rigorous testing have been developed.
In the first of two general methods, one may document the reality of a
punctuation (as opposed to interpretation as a Darwinian artifact based on gaps in
sedimentation) by finding cases of gradualism within a stratigraphic sequence
(which must then be sufficiently complete to record such an anagenetic transition),
and then documenting punctuational origins for other species in the same strata.
Using this technique for Ordovician trilobites from Spitzbergen, Fortey (1985)
found a ratio of about 10:1 for cases of punctuation compared with gradualism.
In a second, and more frequently employed, method, one searches explicitly
for rare stratigraphic situations, where sedimentation has been sufficiently rapid
and continuous to spread the usual results of a single bedding plane into a vertical
sequence of strata. Williamson (1981), for example, published a famous series of
studies on speciation of freshwater mollusks in African Pleistocene lakes. (These
articles provoked considerable debate (Fryer, Greenwood and Peake, 1983), and
Williamson died young before he could complete his work. However, in my
admittedly partisan judgment, Williamson more than adequately rebutted his critics
(1985, 1987).)
These African lakes form in rift valleys, where sedimentation rates are
unusually high because the rift-block foundations of the lake sink continuously,
and sediments can therefore accumulate above, without interruption. Thus, the
thousand-year duration of a speciation event may span several layers of foundering
sediment. With this unusual degree of resolution, Williamson was even able to
demonstrate a remarkable phenomenon in change of variability within a speciating
population—a pattern that appeared over and over again in several events of
speciation, and may therefore be viewed as potentially general (see Fig. 9-3):
Williamson found limited variation around parental mean values in the oldest
samples; intermediacy of mean values within speciating samples, but accompanied
by a greatly expanded range of variation (though still normal in distribution); and
subsequent "settling down" of variation to the reduced level of the ancestral
population, but now distributed around the altered mean value of the derived
species.