Punctuated Equilibrium and the Validation of Macroevolutionary Theory 951
then the mechanics of this usual interpretation of escalation become elusive. The
pattern certainly exists—especially for Vermeij's (1977) classic case of increasing
strength and efficiency in crab claws matched by growing intricacy and
sophistication of adaptive defenses in molluscan shells. But how can such an arms
race operate if the full trend proceeds by stepping stones of punctuational
speciation for any increment, and not in the style of tit-for-tat anagenetic
escalation, based on immediate organismal competition and more familiar to us
through human models of "anything you can do, I can do better"—a point that
Vermeij himself recognizes and finds puzzling (1987)?
Miller, on the other hand, affirms the gradual trend to escalation in
biomechanical improvement—and I don't think that any party to this debate denies
the reality of the pattern (for we have been arguing about mechanisms)— but finds
the same unconventional (and punctuational), finer-scale theme upon "dissecting"
the full result into component causal units. Again, each step in escalation seems
episodic in each region, with the full trend thus rendered as a summation of
punctuational events. Miller writes (p. 1159): "Although the case for these kinds of
transitions over the sweep of the Phanerozoic is difficult to deny, the manner in
which they transpired over shorter intervals is less certain. There is little evidence
of gradual escalation through stratigraphic intervals at local or regional levels. The
introduction of escalated forms appears to have occurred episodically, in concert
with the broader class of changes in taxonomic composition discussed earlier,
which suggests a role for physical mediation."
Two general points provide a fitting close for this section:
- The probable generality of punctuation and stasis as a powerful—if not
predominant—style of change across all scales must lead us to reassess our
previous convictions about "important" and "interesting" phenomena in
evolutionary theory and the history of life. Kerr admits the potential generality, in
reporting punctuation at lower levels to complement Sheehan's similar claim for
the broadest scale. But he closes his report by writing (1994, p. 29): "Sheehan sees
these intervals as analogous to his longer, global EEU's reaffirming that stability—
as boring as it may be—is the evolutionary norm." But, to restate my mantra, and
to emphasize its implications for understanding the history of earthly life and the
psychology of human discovery, stasis is data—and data of such high generality,
such unanticipated occurrence, and such theoretical interest simply cannot be
boring. - The ubiquity—and the possibly canonical character—of punctuational
change at all scales, from the shortest trends of bacterial anagenesis in single clonal
lineages over weeks to months, to the broadest patterns of global waxing and
waning of biotas through the history of life in deep time, can only recall the
familiar tale, by now a cliche, of the Eastern sage who revealed the nature of the
cosmos to his disciple: the globe of the earth rests on the back of an elephant who
stands, in turn, on the back of a turtle. When asked by the disciple what one might
find under the turtle, from its feet to the ultimate source of being, the sage simply
replies: "it's turtles all the way down." I suspect that it is also punctuational change
all the way down, from Permian