Punctuated Equilibrium and the Validation of Macroevolutionary Theory 919
differ in proposed explanations for this common pattern. Brett and Baird identify
rapid environmental turnover as the trigger for collapse of incumbent faunas, but
tend to view the prolonged stability of each fauna as a consequence, at least in
large part, of internal ecological dynamics. New faunas come together largely by
migration of separate elements from other regions (rather than originating
primarily by local speciation in situ, as in Vrba's model), but then maintain
stability by ecological interactions. Vrba, on the other hand, tends to attribute both
fundamental aspects of the pattern— prolonged stasis and abrupt replacement—to
vicissitudes and stabilities of the physical environment. As noted above, she also
attributes the construction of new faunas to local speciation following
fragmentation of habitats induced by environmental change, whereas Brett and
Baird stress migration for the aggregation of new faunal associations. In Vrba's
view, rapid physical changes induce the turnover by (at least local) extinction, and
then also engender the subsequent stability as a propagated effect—because
interspecific interactions play little role in regulating faunal stability, which must
then arise as a basic expectation from punctuated equilibrium about the
independent behavior of individual species. Ivany (1996, p. 4) accurately describes
this aspect of Vrba's model: "stasis intervals between are in essence side
consequences requiring no additional explanation beyond that required to explain
stasis in individual lineages."
The developing debate in the paleontological literature has focussed upon two
issues of markedly different status. First, does the pattern actually exist— with
sufficiently crisp and operational definition in any single case, and with
sufficiently frequent occurrence among all cases—to warrant an assertion of
evolutionary generality (see numerous examples and discussion in the Ivany and
Schopf symposium, 1996, cited above)? I remain entirely optimistic on this point,
if only because the "type" example of the Hamilton Fauna seems well and
extensively documented.
However, and to confess a personal bias, my feelings of caution about
unmitigated endorsement also arise from a substantial worry under this heading. If
a capacity for individuation establishes the basis, or even just ranks as an important
criterion, for status as an evolutionary agent in a Darwinian world, then our logical
inability to render the faunas of coordinated stasis or turnover-pulse as coherent
individuals does cause me concern. I do recognize that higher units of ecological
hierarchies (see Eldredge, 1989) generally lack the coherence of individuals
defining most levels of the standard genealogical hierarchy—because ecological
associations cannot "hold" their component members as tightly as genealogical
individuals enfold their subparts. Species, at a high level of the genealogical
hierarchy, function as excellent Darwinian individuals because their subparts
(organisms) remain tightly bounded by potential for interbreeding within, and
prevention without. But ecological units like "faunas" must be constructed in a
more "leaky" manner, for I cannot imagine a force that could hold taxonomically
disparate forms together by ecological interaction with anything like the strength
that species can muster