Structural Constraints, Spandrels, and Exaptation 1285
explaining the phenomena encompassed by the taxonomy. (Thus, as Ernst Mayr has
so forcefully argued throughout his career, taxonomies are theories about the basis of
order, not boring and neutral hatracks, pigeonholes or stamp albums for
accommodating the objective and uniquely arranged items of nature.)
For the exaptive pool, I can imagine three fairly obvious candidates for
fundamenta (and I am sure that other possibilities have eluded me). My choice of
franklins vs. miltons reflects my convictions and arguments about the most
appropriate theoretical context for understanding the evolutionary meaning and
importance of the exaptive pool.
First, suppose I were working within a more orthodox context that assumed
effective control over evolution by natural selection. Suppose further that, within this
context, I still appreciated the importance of specifying the currently unused
attributes of organisms that might contribute to future success. From such a
standpoint, I would probably be tempted to choose a fundamentum that drew a
primary distinction between the ineluctable unused attributes that arise even when
natural selection works in its "leanest and meanest" mode of optimization, and a
second groups of unused attributes that arise either from the cessation or the
weakness (or at least the non-exclusivity) of natural selection. In such a taxonomic
system, I would place all franklins together with all miltonic spandrels in my first
category of ineluctable attributes of pure selection (inherent potentials of features
optimized by selection for other functions, and inevitable side consequences of
similarly optimizing selection). I would then devote my second category to the other
two groupings of miltons—manumissions and insinuations—arising as historical
results of selection's relinquished control: features falling out of its purview for the
first group, and features insinuated beneath its notice in the second group.
In a second fundamentum that might also follow from stronger selectionist
commitments, I might decide to make a primary division between attributes that
originated either as direct adaptations or as inherencies of direct adaptations for the
first group, and attributes with truly nonadaptive origins for a second group. In this
case, I would place miltonic manumissions and franklins in the first category (as
adaptations that become unemployed, and as additional potentials of features that
arose as direct adaptations). I would then unite miltonic spandrels and insinuations
into a second category of attributes originating as nonadaptations—spandrels as
features that are nonadaptive in their own isolated selves (whatever the adaptive
status of other features that generated them as side consequences), and insinuations as
nonadaptive features that originated beneath the notice and malleability of selection.
I have chosen a quite different third alternative—inherent potentials (franklins)
vs. available things (miltons)—because I wish to emphasize the structuralist and
nonadaptionist components of evolutionary theory that have, in Darwinian traditions,
been downplayed or ignored. This fundamentum stresses the primary difference
between consequences of purely adaptationist mechanisms applied to directly
adaptive features (the franklins,