780 THE STRUCTURE OF EVOLUTIONARY THEORY
originate in three circumstances that virtually guarantee a punctuational expression
in the fossil record: (1) they arise rapidly (usually instantaneously) in geological
time, and they originate both (2) in a small geographic region (the peripheral
isolate), and (3) elsewhere (beyond the borders of the parental range that provides
the exclusive source for standard paleontological collections). The "sudden"
entrance of a daughter species into strata previously occupied by parents usually
represents the inward migration of a peripheral isolate, now "promoted" by
reproductive isolation to full separation, not the origin of a new species in situ.
Eldredge and I have often been asked what we think of sympatric speciation,
or of various models, like polyploidy, for rapid origin even in human time. We do
not mean to be evasive or obscure in our assertions of agnosticism. (I am intensely
interested in the literature on speciation, and I would love to know the relative
frequencies of these other models vs. classical Mayrian peripatry. But this
important issue does not strongly impact punctuated equilibrium, and surely cannot
be resolved by paleontological data.) Punctuated equilibrium simply requires that
any asserted mechanism of speciation, whatever its mode or style, be sufficiently
rapid and localized to appear as punctuation when scaled into geological time. If I
understand them correctly, most alternative models to peripatry generally operate
even more rapidly than the conventional Mayrian mode that we invoked to anchor
our theory—as obviously true for polyploidy, and also for most versions of
sympatric speciation (if only because the constant threat of dilution by gene flow
from surrounding parentals can best be overcome by rapid achievement of
reproductive isolation in ecological time). Therefore, punctuated equilibrium can
only gain strength if these alternative mechanisms become validated at meaningful
relative frequencies. (The faster the better, one might say.) But punctuated
equilibrium does not require this boost—and we therefore remain agnostic—
because the most conventional form of Mayrian peripatry already yields the full set
of phenomena predicted by punctuated equilibrium when properly scaled into the
immensity of geological time. (Punctuated equilibrium, on the other hand, does not
maintain a similar agnosticism towards any putative mechanism of speciation that
conceives the process of splitting as no more rapid than imagined rates for the
gradual anagenesis of large central populations. Some models of so-called
"dumbbell allopatry"—or the splitting of a parental population into two effectively
equal moieties, with subsequent anagenesis in each—do construe speciation as
consequently slow in geological expression, and therefore do threaten punctuated
equilibrium. But I do not think that such models enjoy much support among
biologists, especially for operation at a high relative frequency.)
Geological time can be both a wonder and a snare because we grasp the idea
in our heads (all scientists know how many zeroes follow the one in expressing
millions or billions), but we face a primal, and fundamentally psychological,
difficulty in trying to incorporate this central concept into the guts of our intuition.
We can lose information in upward scaling when glacial slowness in human
history becomes a passing and unresolvable geological