782 THE STRUCTURE OF EVOLUTIONARY THEORY
species (and thereby establishing the basis for an independent theoretical domain
of macroevolution).
As discussed in Chapter 8 (see pp. 648-652), punctuated equilibrium wins this
role by refuting Fisher's otherwise decisive argument for the impotence (despite
the undeniable existence) of species selection. So long as most new species arise
by branching (speciation) rather than by transformation (anagenesis), species can
be individuated by their uniquely personal duration, bounded by birth in branching
and death by extinction. But if anagenesis, fueled by Darwinian organismic
selection, operates to substantial effect during the lifetimes of most species, then,
by Fisher's argument, such micro-evolutionary transformation must overwhelm
species selection in building the overall pattern of macroevolutionary change—for
the number of organism-births must exceed species-births by several orders of
magnitude, and if every event of birthing, at each level, supplies effective variation
for evolutionary transformation, then the level of species can contribute virtually
nothing to the totality of change. But if stasis rules and anagenesis rarely occurs,
then speciation becomes the more effective level of evolutionary variation. And if
speciation unfolds in geological moments, then species in geological time match
organisms on our ordinary yearly scales in both distinctness and discreteness.
Thus, the pattern of punctuated equilibrium establishes species as effective
individuals and potential Darwinian agents in the mechanisms of macroevolution.
In summary, G. G. Simpson gave a singularly appropriate title to his epochal
1944 book that defined the potential of paleontology to devise insights about
evolutionary mechanisms: Tempo and Mode in Evolution. If we accept Simpson's
focus on tempo and mode as primary subjects, then punctuated equilibrium has
provoked substantial revisions of macroevolutionary theory and practice in both
domains.
Tempo and the significance of stasis
For tempo, punctuated equilibrium reverses our basic perspective. We must
abandon our concept of constant change operating within a sensible, stately range
of rates as the normal condition of an evolving entity. We must then reformulate
evolutionary change as a set of rare episodes, short in duration relative to periods
of stasis between. Stability becomes the normal state of a lineage, with change
recast as an infrequent and concentrated event that, nonetheless, renders phylogeny
as a set of summed episodes through time. The implications of this fundamental
shift resonate afar by impacting a set of issues ranging from the most immediately
practical to the most broadly philosophical (including, in the latter category, an
interesting consonance with the atomism and quantization invoked to define the
general intellectual movement known as "modernism"—as expressed in disparate
disciplines from Seurat's pointillism in art, to Schonberg's serial style in music; and
as opposed to the smooth continuationism favored by earlier mechanistic views of
causality). In a theme more immediately relevant to biology, the same shift
ineluctably places much greater emphasis upon chance and contingency, rather