88 THE STRUCTURE OF EVOLUTIONARY THEORY
explanation of macroevolutionary results by microevolutionary causes (and not as
a challenge to the efficacy of natural selection itself). Darwin therefore employed
his usual argument about the imperfection of geological records to "spread out"
apparent mass extinction over sufficient time for resolution by ordinary processes
working at maximal rates (and therefore only increasing the intensity of selection).
- The transition of the impact scenario (as a catastrophic trigger for the K-T
extinction) from apostasy at its proposal in 1980 to effective factuality (based on
the consilience of disparate evidence from iridium layers, shocked quartz and,
especially, the discovery of a crater of appropriate size and age at Chicxulub) has
reinstated the global paroxysms of classical catastrophism (in its genuinely
scientific form, not its dismissive Lyellian caricature) as a legitimate scientific
mechanism outside the Darwinian paradigm, but operating in conjunction with
Darwinian forces to generate the full pattern of life's history, and not, as previously
(and unhelpfully) formulated, as an exclusive alternative to disprove or to trivialize
Darwinian mechanisms. - If catastrophic causes and triggers for mass extinction prove to be general,
or at least predominant in relative frequency (and not just peculiar to the K-T
event), then this macroevolutionary phenomenon will challenge the crucial
extrapolationist premise of Darwinism by being more frequent, more rapid, more
intense and more different in effect than Darwinian biology (and Lyellian geology)
can allow. Under truly catastrophic models, two sets of reasons, inconsistent with
Darwinian extrapolationism by microevolutionary accumulation, become
potentially important agents of macroevolutionary patterning: effectively random
extinction (for clades of low N), and, more importantly, extinction under "different
rules" from reasons regulating the adaptive origin and success of autapomorphic
cladal features in normal times. - Catastrophic mass extinction, while breaking the extrapolationist credo,
may suggest an overly simplified and dichotomous macroevolutionary model
based on alternating regimes of "background" vs. "mass" extinction. Rather, we
should expand this insight about distinctive mechanisms at different scales into a
more general model of several rising tiers of time—with conventional Darwinian
microevolution dominating at the ecological tier of short times and intraspecific
dynamics; punctuated equilibrium dominating at the geological tier of phyletic
trends based on interspecific dynamics (with species arising in geological
moments, and then treated as stable "atoms," or basic units of macroevolution,
analogous to organisms in microevolution); and mass extinction (perhaps often
catastrophic) acting as a major force of overall macroevolutionary pattern in the
global history of relative waxing and waning of clades. (I also contrast this
preferred model of time's tiering with the other possible style of explanation, which
I reject but find interesting nonetheless, for denying full generality to smooth
Darwinian upward extrapolation from the lowest level—namely, an equally
smooth and monistic downward extrapolation from catastrophic mortality in mass
extinction to