The Structure of Evolutionary Theory

1294 THE STRUCTURE OF EVOLUTIONARY THEORY

is, they enter the exaptive pool as miltonic things rather than franklinian potentials.
Moreover, as constraints of this type, they add a structuralist and nonadaptationist
component to the workings of evolution—thus strengthening the tripod on leg two by
including aspects of this formerly rejected mode of causation among the totality of
devices that generate creative change in evolution. On a more specific note, if cross-
level spandrels maintain an important relative frequency among the components of
evolutionary change, then these automatic expressions at other levels—introduced
separately from, and simultaneously with, the primary changes that generate them at
a different focal level—may largely control the possibilities and directions of
evolution from a structural "inside," rather than only from the functional "outside" of
natural selection.
Thus, if the positive structural constraints of spandrels—particularly in their
cross-level mode as effects propagated to various levels of the evolutionary
hierarchy—can help to explain the phenomenon of evolvability and the parsing of
categories in the exaptive pool, then reforms on legs one and two of the essential
Darwinian tripod will also illustrate why the extrapolationist premise of the third leg
cannot suffice to explain evolution either. For macro-evolution cannot simply be
scaled-up from microevolutionary mechanics if the phenomenology of this larger
scale depends as much upon the potentials of evolvability as upon the impositions of
selection, and if the exaptive pool promotes evolvability largely by the later utility at
the same level (or simultaneous exaptive use at other levels) of spandrels that
originate for nonadaptive reasons. The explanation of macroevolution requires
structuralist and hierarchical inputs from various scales, and cannot be fully rendered
as an extension of organismal adaptation, smoothly scaled up through the immensity
of geological time.

A CLOSING COMMENT TO RESOLVE THE MACROEVOLUTIONARY

PARADOX THAT CONSTRAINT ENSURES FLEXIBILITY WHEREAS

SELECTION CRAFTS RESTRICTION

In closing this section by reiterating the opening argument (p. 1270) in another
context, I should extend my previous statement on the bounded independence of
macroevolution to stress the positive theme of interesting differences, and not only a
negative claim for the necessary limitation of any explanation based upon pure
extrapolation from microevolutionary mechanics. For the failure of
microevolutionary extrapolation resides in something far deeper than mere
insufficiency. Rather, and thus operating to intensify the explanatory gap, a cardinal
feature of microevolution works directly against the potentials for macroevolution
defined by the exaptive pool—thereby requiring that macroevolution proceed by
actively overcoming this microevolutionary limitation, and not only by "adding
value" to its mere insufficiency.
Darwinian evolutionists have known this all along in their heart of hearts, and
have tended to escape the resulting paradox by a leap of faith into the enabling power
of geological time to accomplish anything by accumulation