1270 THE STRUCTURE OF EVOLUTIONARY THEORY
Brenner then ribs my literary and terminological pretensions (and I accept his
criticism). But he also finds a resolution to the conceptual puzzles surrounding junk
DNA in recognizing that such amplified sequences, when they arise causally at the
gene level and then get propagated as effects to the organismal level, are nonadaptive
spandrels with great potential for later exaptation to utility. Therefore, their
designation as junk—that is, as currently useless, but harmless (as opposed to
garbage), and replete with potential future value—seems entirely appropriate, and I
belatedly embrace this term as a proper implication flowing from the definition and
meaning of spandrels:
The paper [Gould, 1997e] has an important message and I strongly urge my
readers at least to look at it, even if all the words in it can't be understood. I
offer this brief summary as a guide.
The term spandrel originates in architecture and is used to describe spaces
left over as a consequence of some other design decision, such as the triangles
that remain behind when a rectangular wall is pierced by an arched opening.
No self-respecting architect would simply leave such spaces, especially in a
grand cathedral with a rich patron. Instead they would be decorated, as is the
case of the four pendentives under the dome of San Marco in Venice, which
are decorated with the four evangelists. This example is a good one, because
the historical sequence of events is known. The spandrels are the consequence
of a structural design decision; a by-product of placing a dome on rounded
arches; three centuries later, mosaicists decorated these spaces. Thus spandrels
are not primary adaptations but, because they can have later uses, they become
in Gould's terminology, exaptations.The Exaptive Pool: The Proper Conceptual Formula
and Ground of Evolvability
RESOLVING THE PARADOX OF EVOLVABILITY AND DEFINING
THE EXAPTIVE POOLConventional Darwinian organismal selection adapts creatures to their immediate
local environments—a process of specialization, usually operating to produce
particular contrivances that reduce organismal flexibility for future evolution to
radically altered conditions, especially when adaptation leads to simplification and
loss of structures, as in extreme, but common, cases of internal parasitism. In a subset
of situations—especially emphasized by Darwin as the potential ground of general
"progress" in the history of life (see p. 467)—local adaptation may be achieved by a
generalized improvement in biomechanical design that might be construed as
promoting future prospects and flexibilities, rather than restricting phyletic options by
specialization. But few evolutionists would doubt that organismal selection leads far
more often to diminution of future prospects by specializations and losses than to
enlargement