1288 THE STRUCTURE OF EVOLUTIONARY THEORY
for the status of cross-level effects. First, cross-level effects are thereby identified as
things rather than potentials—that is, as miltons rather than franklins. (Differential
variation in range of song among subclades is a "thing" that may impose emergent
fitness at the species level at the same time as its generating adaptations in the form
of the bill operate in their ordinary Darwinian manner at the organismal level.)
Second, cross-level effects, as things, generate a potentially radical challenge to
conventional Darwinian concepts. Unused potentials, as argued several times
previously, remain fully within the adaptationist program as possible future uses of
features that arose as adaptations and will always be adaptations (albeit for a different
function in the future). But unused things begin as nonadaptations right at their origin
(whatever their future importance as exaptations)—and the demonstration of their
high relative frequency in the exaptive pool, and of their importance to the
evolutionary history of many lineages, would introduce a significant nonadaptationist
element into evolutionary theory.
So if we agree that cross-level effects are miltonic things and not franklinian
potentials, into what category of miltons (see Table 11-2) do they fall? If the previous
components of this argument prove acceptable, then this final question enjoys a
simple and unambiguous resolution: they are spandrels. The key property of
spandrels lies in their automatically consequential character as things necessarily
enjoined by other changes. Cross-level effects fit this definition in all ways, for they
arise in concert with the primary change, and as a necessary consequence thereof. But
they do manifest an interestingly different property from such conventional at-level
spandrels as the pendentives of San Marco or the cylindrical space that can be
exapted as a brooding chamber at the center of a gastropod shell: they express
themselves in a biological individual at a different level from the individual bearing
the feature that generated the effect.
As stated before, and as the heart of my argument, I believe that the designation
of cross-level effects as a second class of spandrels greatly increases the range and
importance of this concept—for cross-level spandrels are probably far more common,
and of far more frequent importance in potentiating the future direction of
evolutionary lineages, than the at-level spandrels that provoked the initial formulation
of this concept. As an almost naively evident defense of this claim, I would point out
that more cross-levels exist than focal levels (obviously, as focal levels must be
singular!), so cross-level spandrels have more "places to go" than at-level spandrels.
Moreover, any cross-level attribute holds greater potential, prima facie, for
manifestation as a quirky and oddly nonadaptive feature—for anything arising at one
level and injected into another must enter its new domain adventitiously and without
reference to the norms and needs of its adopted "home"; whereas an at-level spandrel
can only be tolerated if it meshes reasonably well with a design already established
for its kind of entity.
These attributes of cross-level spandrels embody their importance in revising
our usual understanding of evolution, particularly at the macroevolutionary level of
the species-individual. As argued above, at-level spandrels