1330 THE STRUCTURE OF EVOLUTIONARY THEORY
the evolution of mechanisms for dormancy proved adaptive at the first tier in aiding
survival during high-latitude months of darkness and in times of low silica between
episodes of upwelling; whereas the same feature may have favored the persistence of
diatom species during a prolonged period of darkness imposed by a global dust cloud
excavated in bolide impact, the key ingredient advocated by many researchers for the
killing scenario of the K-T event.
But adjacent tiers may also act in an orthogonal manner, with invocation of the
"random" model at the third tier as an obvious general case—for truly random
differential survival must run orthogonal to deterministic Darwinian reasons for
evolving the clade's distinctive traits at the first tier, or equally deterministic (but
different) reasons for establishing the defining features of cladal trends by punctuated
equilibrium at the second tier. Finally, the evident possibility of opposing reasons at
adjacent tiers has sparked our interest in catastrophic mass extinction from the start—
with dinosaurian superiority over mammals maintained at the first and second tiers
throughout Mesozoic times, and mammalian success then achieved by the "different
rules" model at the third tier of differential passage through mass extinction,
putatively based upon the very features that marked the first and second tier failures
of mammals for 130 million previous years.
Although limited space and personal competence prevent my proceeding beyond
this sketchy and cartoonish model of three tiers, I suspect that future work will
identify several inhomogeneities and subtiers, particularly between the history of
independent and individual clades at the second tier and the coordinated impact of
environments upon entire biotas at the third tier. Several intermediate modes and
processes, affecting groups of species during times of unusual externalities in
particular geographic regions, but not global biotas at catastrophic moments, must
"intervene" between the pure influence of punctuated equilibrium upon a single clade
and the full impact of worldwide catastrophe upon a global biota. For example, the
model of coordinated stasis (discussed on pp. 916-922) argues that cladal trends do
not always maintain the implied freedom of punctuated equilibrium to proceed
independently, and in an unconstrained manner by differential success in the
generation of new species at the second tier, but will often be subject to a form of
community stasis that must first be broken by disruptions smaller than mass
extinction, and resident within its own tier. Similarly, between the first and second
tier, ordinary anagenesis often cannot "push through," even to the point of disruption
by punctuated equilibrium at the second tier, because the ecological communities that
set the anagenetic regime for a single species become disrupted on a scale of
hundreds of thousands of years by the climatic fluctuations of Milankovitch cycles
that break up communities and quickly disperse their elements into new arrangements
in different places.
To conclude this section with a historical example of the profound distinction
between traditional untiered views of selective domination smoothly scaled up to all
times and magnitudes, and the alternative nonfractal concept of a much broader range
of potential outcomes engendered by interaction among the characteristic modes and
processes at different tiers, consider