Structural Constraints, Spandrels, and Exaptation 1293
nonsuppression acts as a source of evolutionary potential by permitting species to
draw upon a wider pool of features than organisms can access.
By not suppressing this evolutionary churning from within, the species-
individual gains enormous flexibility in remaining open to help from below,
expressed as exaptive effects that confer emergent fitness. Rather than viewing this
nonsuppression of aid from other levels, with the accompanying failure to build many
active adaptations at its own level, as a sign of wimpy weakness for the species—
construed as a "poor organism" in the implication of most traditional thought—we
should rather interpret these allometrically driven properties as cardinal strengths, and
recognize the species as a "rich-but-different" Darwinian individual. The species, in
this view, acts as a shelter or arbor that holds itself fast by active utilization of the
properties that build its well-defined individuality. By fostering internal change, and
thereby gaining a large supply of upwardly cascading exaptive effects, species use the
features of all contained lower-level individuals through the manifestation of their
effects on the shelter itself. The species, through its own distinctive features of
individuality, and requiring neither indulgence nor apologia from human
understanding, will continue to operate as a powerful agent in Darwin's world
whether or not we parochial organisms, limited by our visceral feelings and traditions
of language, choose to expand our view and recognize the sources of evolutionary
potency at distant scales of nature's hierarchy.
In conclusion, and to reiterate my rationale for placing so much emphasis on
cross-level spandrels in evolution, this single theme, more than any other in this
book, unites and exemplifies the weaknesses on all three legs of the tripod of
essential postulates in conventional Darwinian logic, while also pointing the way
towards revisions that will expand and strengthen these three supports to produce an
improved, and more comprehensive, evolutionary theory by retaining its Darwinian
basis in the expanded form of a fully hierarchical theory, while adding, to its
preserved selectionist core, several aids and flavors from alternative traditions of
structuralist thought.
Thus, the hierarchical theory of selection (for strengthening and expansion of the
first leg) greatly augments the role of spandrels by adding the cross-level category as
more potent in numbers and more various in potential results than the traditional at-
level category. For at-level spandrels, in the usual Darwinian account of selection as a
uniquely organismal process, must remain confined to structural byproducts and
space fillers within a context of integral adaptation of the body. But if selection works
simultaneously at several hierarchically ordered levels of biological individuality,
then the domain of spandrels expands to include any enjoined expression (upon
Darwinian individuals at other levels) of changes causally introduced at a focal
level— for these injected and adventitious expressions must originate nonadaptively
(and "randomly" in our usual loose parlance) relative to their causal reasons of origin
at the focal level of their construction.
By the same token, spandrels become structural constraints rather than direct
adaptations (or even alternative potentials of direct adaptations)—that