Structural Constraints, Spandrels, and Exaptation 1279
within structures now adapted to a different utility. When evolution then coopts the
structure for actual service in this formerly potential role, the franklin becomes an
exaptation. The entire process remains under the control of Darwinian selection and
adaptation throughout. Franklins do not vie with current utilities, and they cannot be
construed as "things" (whether adaptive or nonadaptive) waiting for a potential place
in the adaptive sun. Franklins are the inherent potentials that permit Darwinian
pathways, under the control of natural selection leading to adaptation at all times, to
undergo quirky and unpredictable shifts from one function to a qualitatively different
utility.
The attributes in the second major category of the exaptive pool are, by
important contrast, actual entities, pieces of stuff, material things that have become
parts of biological individuals for a variety of reasons (to be exemplified in the next
section), but that have no current use (and also cause no substantial harm, thereby
avoiding elimination by selection). Items in this second category of "available things"
can originate in several ways—as nonadaptive spandrels (the most important
subcategory, I shall argue), as previously useful structures that have become vestigial,
or as neutral features fortuitously introduced "beneath" the notice of selection.
I propose that we refer to these available but currently unused material organs
and attributes as "miltons" to honor one of the most famous lines in the history of
English poetry. John Milton ended his famous sonnet On His Blindness, written in
1652, by contrasting two styles of service to God: the frenetic activity of evangelists
and conquerors and the internal righteousness of people with more limited access to
worldly action:
... thousands at his bidding speed,
And post o'er land and ocean without rest;
They also serve who only stand and wait.
Miltons, in short, are actual things, presently without function, but holding within
their inherent "goodness" the rich seeds of potential future utility. Now, they only
stand and wait; tomorrow, they may be exapted as key innovations of great
evolutionary lineages.
Miltons constitute the radical counterpart to the conventionality of franklins
within the exaptive pool. Miltons break the exclusivity of the adaptationist program
by basing a large component of evolvability not upon the potential of already
functional (and adaptive) features to perform in other ways, but rather upon the
existence of a substantial array of truly nonadaptive features—unused things in
themselves rather than alternative potentials of features now functioning in other
ways (and regulated by natural selection at all times). If features with truly
nonadaptive origins occupy a substantial area in the full domain of evolvability, then
we must grant this structuralist (and nonadaptationist) theme a generous and
extensive space within the logic and mechanics of evolutionary theory. For this
reason, I have argued that spandrels—already the most important category of miltons,
but made far more significant by the inclusion, under their rubric, of cross-level
effects of features originating at other levels (see pp. 1286-1294 of this section)—win