Structural Constraints, Spandrels, and Exaptation 1267
But the scope of nonadaptive side consequences would then be limited to overt
structures, physiologies and behaviors of bodies. That is, spandrels would be altered
bits and pieces (often substantial) of organic "stuff," molded as the propagated effects
of primarily adaptational changes wrought by selection upon other parts of the body.
But under the revised and expanded hierarchical theory (Chapter 8), where
selection works simultaneously on a nested hierarchy of biological individuals
(genes, cell lineages, organisms, demes, species, clades), the domain of spandrels
becomes much larger, and their importance to evolutionary theory expands
accordingly, and for an interesting reason that has not been adequately addressed in
the literature (see Gould and Lloyd, 1999), but will occupy most of the next and last
section of this chapter. The expansion of spandrels under a hierarchical theory of
selection establishes the most interesting and intricate union between the two central
themes of this book—the defense of hierarchical selection (as an extension and
alteration of Darwin's single-level organismal theory) on the first leg of the tripod of
essential components in Darwinian logic; and the centrality of structural constraint
(with non-adaptively originating spandrels as a primary constituent) for a rebalancing
of relevant themes, and as a correction to the overly functionalist mechanics of
selection on the second leg of the tripod (or branch of the tree—see Figure 1-4).
To epitomize the central argument: under a hierarchical theory of selection, any
novelty introduced for any reason (usually adaptational) at any level, must propagate
a series of effects to biological individuals at other levels of the hierarchy.
Duplication of genetic elements by direct selection at the gene level, for example,
propagates redundancies to the organismic level; any organismal adaptation at
Darwin's level propagates changes to the encompassing species-individual, as
expressed in such species traits as population size, geographic range, and coherence
among subparts (organisms). These propagated effects must be defined and treated as
spandrels. As "injections" from another level (where the initiating change probably
had an adaptational, or physically automatic, basis), these propagated effects cannot
be viewed as adaptations at the level under consideration. Moreover, because these
effects exist as true properties at the level under consideration—that is, as actual
"stuff" rather than unused potentials of features now operating in different ways—
they must be treated as initially nonadaptive "things" or spandrels, rather than as mere
potentialities of some hypothetical future utility. Therefore, under a hierarchical
model, spandrels include both the architectural side consequences of adaptational
changes at the level of their origin, and the large realm of effects propagated to other
levels as nonadaptive consequences of changes wrought for directly causal reasons.
This concept of cross-level spandrels neatly explicates a variety of phenomena
that have long been recognized as both true and essential, but that have remained
puzzling or anomalous under the conventional Darwinian rubric of a functionalist,
single-leveled theory of selection. For example, our canonical,