The Structure of Evolutionary Theory

(Michael S) #1

Structural Constraints, Spandrels, and Exaptation 1277


to explaining evolvability by extrapolated microevolutionary Darwinian processes.
Above all, and to move forward towards a clear and operational definition of
concepts, the crucial subject of evolvability requires a taxonomy for its numerous
modes, and their strengths and distributions. The key to an inclusive accounting lies
in the general notion of usable features (for promoting long-term diversification and
success) now unused. What kinds of attributes thus contribute to evolvability? What
encourages their production or augments their number (especially since natural
selection in the organismal mode cannot craft such features directly)? Who has more
of them and why?
Clearly, organisms and populations maintain what we might call a "fund" or
"pool" of potential utilities now doing something else, or at least doing no harm. I
propose that we designate this ground of evolvability as "The Exaptive Pool,"* and
that we try to establish a logical, interesting, and empirically workable taxonomy for
the various attributes in this fund of exaptable potential. The exaptive pool represents
the structural basis of evolvability, the potential vouchsafed to future episodes of
selection (at all levels) in a world of strongly polyhedral objects always and
ineluctably built with interesting corners and facets that facilitate and channel the
directions of evolutionary movement.


THE TAXONOMY OF THE EXAPTIVE POOL

Franklins and Miltons, or inherent potentials vs. available things
The American dime, the smallest and thinnest (but not the least valuable) of our
coins, can't purchase much these days, but still functions as legal tender, the primary
cause of its manufacture and useful persistence. American dimes have also, starting a
few years after his death near the end of World War II, borne a representation of
Franklin D. Roosevelt on their recto ("heads") side. As an unintended consequence of
their thinness, American dimes (and no other indigenous coins) also happen to fit
snugly into the operative groove on the head of a standard screw—and dimes
therefore work very well as adventitious screwdrivers. (I strongly suspect that
virtually every adult American has used a dime in this well-known supplementary
way, just as many of us know



  • Because exaptations are coopted utilities, and because the attributes in this pool of
    potential remain as yet uncoopted, some people might object to my designation of this
    ground of evolvability as "the exaptive pool." Shouldn't this fund be called the "preadaptive
    pool" (not that ugly and misleading word again!), or the "potaptive pool" (but why invent a
    neologism if a current term will suffice)? However, in a purely linguistic and etymological
    sense, "exaptive pool" represents proper usage, and does not convey any confusing
    implication of names applied before actions that justify the names. The suffix "ive" refers to
    "something that performs or tends towards or serves to accomplish an action" (per
    Webster's)—a fine description of attributes that bring organisms towards evolvability and
    serve to help them accomplish this happy state. Directives are meant to be followed, but
    perhaps not just yet. Active sockets will work when you plug in an appliance. Sedative pills
    will calm you down after you ingest them. Exaptive pools will supply their bearers with a
    leg up in the longterm sweepstakes of evolution.

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