Punctuated Equilibrium and the Validation of Macroevolutionary Theory 767
Stasis does not mean "rock stability" or utter invariance of average values for
all traits through time. In the macroevolutionary context of punctuated equilibrium,
we need to know, above all, whether or not morphological change tends to
accumulate through the geological lifetime of a species and, if so, what part of the
average difference between an ancestral and descendant species can be attributed
to incremental change of the ancestor during its anagenetic history. Punctuated
equilibrium makes the strong claim that, in most cases, effectively no change
accumulates at all. A species, at its last appearance before extinction, does not
differ systematically from the anatomy of its initial entry into the fossil record,
usually several million years before.
Of course we recognize that mean values will fluctuate through time. After
all, measured means would vary even if true population values remained utterly
constant—which they do not. And, with enough samples in a vertical sequence,
some must include mean values (for some characters) outside conventional bounds
of statistically insignificant difference from means for the oldest sample. Such
fluctuation also implies that the final population will not be identical with the
initial sample.
In operational terms, therefore, we need to set criteria for permissible
fluctuation in average values through time. Two issues must be resolved: the
amount of allowable difference between beginning and ending samples of a
species, and the range of permissible fluctuation through time. Since we wish to
test a hypothesis that little or no change accumulates by anagenesis during the
history of most species, and since we have no statistical right to expect that (under
this hypothesis) the last samples will be identical with the first, we should predict
either that (i) the final samples shall not differ statistically, by some conventionally
chosen criterion, from the initial forms; and at very least (ii) that the final samples
shall not generally lie outside the range of fluctuation observed during the history
of the species. (If final samples tend to lie outside the envelope of fluctuation for
most of the species's history, then anagenesis has occurred.)
For the permissible range of fluctuation, we should, ideally, look to the extent
of geographic variation among contemporary populations within the species, or its
closest living relative. If the temporal range of variation stays within the spatial
range for any one time, then the species has remained in stasis. Obviously, we
cannot apply this optimal criterion for groups long extinct, but a variety of proxies
should be available, including comparison of a full temporal range with the known
geographic variation of a well-documented and widespread nearest living relative.
Studies of stasis in Neogene species can often use the optimal criterion because the
actual species, or at least some very close relatives, are often still extant. In the
most elegant documentation of stasis for an entire fauna of molluscan species,
Stanley and Yang (1987) used this best criterion to find that temporal fluctuation
remained within the range of modern geographic variation for the same species.
They could therefore affirm stasis in the most biologically convincing manner.
Since stasis is data, but punctuation generally records an unresolvable
transition when assessed by the usual expression of fossil data in geological time,