936 THE STRUCTURE OF EVOLUTIONARY THEORY
nature of change itself across the varying scales of nature's evolutionary
construction.
PUNCTUATION ABOVE THE SPECIES LEVEL. Punctuated equilibrium stands
on an "isthmus of a middle state" (to quote Alexander Pope out of specific context,
but in proper structural analogy, see page 680)—a speciational bridge linking the
microevolutionary history of discrete populations with the macroevolutionary
waxing and waning of clades through geological time. I believe that the prevalence
of punctuational change on the bridge itself (punctuated equilibrium sensu stricto),
combined with a strong case for punctuational dynamics in Darwinian processes
stripped to a lean and clean minimality in microevolution (see preceding section),
behooves us to consider a generalization across all scales, by suggesting an
examination of larger realms beyond the bridge of speciation. I shall therefore
discuss potentially instructive examples (not comprehensive statistical generalities,
a worthy goal not nearly in current reach) at three expanding levels: (1)
consequences of accumulated events of ordinary speciation within the history of
individual clades; (2) the origin of phenotypically complex and extensive
evolutionary novelties; and (3) the history of biotas in ecological and evolutionary
time.
STASIS ANALOGS: TRENDING AND NON-TRENDING IN THE GEOLOGICAL HISTORY
OF CLADES. Do we find cladal patterns, generated by different causal mechanisms,
that might be sufficiently "homologous" (see pp. 928-931) with punctuated
equilibrium to warrant comparison based on a common deep structure? (I am, of
course, not considering or reiterating here (see full discussion, pp. 886-893) the
most important and direct impact of punctuated equilibrium upon cladal
histories—its ability to explain trends as the differential success of species rather
than the extrapolated result of adaptive anagenesis. This section treats other scales
and causes of change with meaningful structural parallels to punctuated
equilibrium at the species level.) Possible parallels for the punctuational aspect will
be treated in subsequent chapters—rapid origin of extensive evolutionary novelties
for cladal beginnings (Chapter 10), and patterns of mass extinction for endings
(Chapter 12)—but we should also consider the almost entirely neglected analogs of
stasis at the cladal level.
An apt comparison for clades may be made to philosophical and sociological
reasons for the previous failure of evolutionary biology to study, or even to
acknowledge, the phenomenon of stasis as the predominant feature of phyletic
history in a large majority of species. Stasis, construed as absence of evolution,
once designated a negative result unworthy of a category, or even a name. In a
similar way (and I cast no stones from a sinless state, for I have followed this
tradition myself throughout my career), the study of trends has consumed nearly all
research on the history of clades. And why not? Trends tell stories, and evolution is
a narrative science. Western tradition, if not universal human nature itself, has
always favored directional tales of conquest and valor (with Darwinian analogs of
competition and adaptation), while experiencing