Defining and Revising the Structure of Evolutionary Theory 87
- I affirm the importance and high relative frequency of spandrels, and
therefore of nonadaptive origin, in evolutionary theory by two major arguments for
ubiquity. First, for intrinsic structural reasons, the number of potential spandrels
greatly increases as organisms and their traits become more complex. (The
spandrels of the human brain must greatly outnumber the immediately adaptive
reasons for increase in size; the spandrels of the cylindrical umbilical space of a
gastropod shell, by contrast, may be far more limited, although exaptive use as a
brooding chamber has been important in several lineages.) Second, under
hierarchical models of selection, features evolved for any reason at one level
generate automatic consequences at other levels—and these consequences can only
be classified as cross-level spandrels (since they are "injected into" the new level,
rather than actively evolved there). - The full classification of spandrels and modes of exaptation offers a
resolving taxonomy and solution—primarily through the key concept of the
"exaptive pool"—for the compelling and heretofore confusing (yet much
discussed) problem of "evolvability." Former confusion has centered upon the
apparent paradox that ordinary organismal selection, the supposed canonical
mechanism of evolutionary change, would seem (at least as its primary overt
effect) to restrict and limit future possibilities by specializing forms to complexities
of immediate environments, and therefore to act against an "evolvability" that
largely defines the future macroevolutionary prospects of any lineage. The solution
lies in recognizing that spandrels, although architecturally consequential, are not
doomed to a secondary or unimportant status thereby. Spandrels, and all other
forms of exaptive potential, define the ground of evolvability, and play as
important a role in macro-evolutionary potential as conventional adaptation does
for the immediacy of microevolutionary success. I emphasize the centrality of the
exaptive pool for solving the problem of evolvability by presenting a full
taxonomy of categories for the pool's richness, focusing on a primary distinction
between "franklins" (or inherent potentials of structures evolved for other adaptive
roles— that is, the classical Darwinian functional shifts that do not depart from
adaptationism), and "miltons" (or true nonadaptations, arising from several
sources, with spandrels as a primary category, and then available for later
cooptation from the exaptive pool—that is, the class of nonadaptive origins that
does challenge the dominant role of panadaptationism in evolutionary theory). - I argue that the concept of cross-level spandrels vastly increases the
range, power and importance of nonadaptation in evolution, and also unites the two
central themes of this book by showing how the hierarchically expanded theory of
selection also implies a greatly increased scope for non-adaptive structural
constraint as an important factor in the potentiation of macroevolution.
Chapter 12: Tiers of time and trials of extrapolationism- Darwin clearly recognized the threat of catastrophic mass extinction to the
extrapolationist and uniformitarian premises underlying his claim for full