1276 THE STRUCTURE OF EVOLUTIONARY THEORY
Thus, Darwinians have always argued that mutational raw material must be
generated by a process other than organismal selection, and must be "random" (in the
crucial sense of undirected towards adaptive states) with respect to realized pathways
of evolutionary change. Traits that confer evolvability upon species-individuals, but
arise by selection upon organisms, provide a precise analog at the species level to the
classical role of mutation at the organismal level. Because these traits of species
evolvability arise by a different process (organismal selection), unrelated to the
selective needs of species, they may emerge at the species level as "random" raw
material, potentially utilizable as traits for species selection.
The phenotypic effects of mutations are, in exactly the same manner, spandrels
at the organismal level—that is, nonadaptive and automatic manifestations at a higher
level of different kinds of causes acting directly at a lower level. The exaptation of a
small and beneficial subset of these spandrels virtually defines the process of natural
selection. Why else do we so commonly refer to the theory of natural selection as an
interplay of "chance" (for the spandrels of raw material in mutational variation) and
"necessity" (for the locally predictable directions of selection towards adaptation).
Similarly, species selection operates by exapting emergent spandrels from causal
processes acting upon organisms.
- The acknowledgment of structural components as joint causes and specifiers,
along with natural selection, for the directions of evolutionary change: (In the most
radical version, these structural inputs operate as positive constraints generated as
consequences of features with nonadaptive origins, thus precluding a purely
functionalist or adaptationist account for both the origins, and the channels of
subsequent change, of organismal traits.) To summarize this major claim of the
preceding section: if important components of evolvability must emerge as spandrels
of natural selection on other features (for they cannot be fashioned as direct products
of a process that cannot explicitly make "things" for potential future benefits), and if
these spandrels serve as exaptable raw material for higher-level processes of change,
then we will need to describe macroevolution at least partly in the language of
channeling by historical and structural constraint (often based upon features with
nonadaptive origins), and not entirely in conventional functionalist terms of selective
modelling to current environments, potentiated by an effectively unfettered capacity
of mutational raw material to provide the wherewithal for evolutionary movement in
any immediately adaptive direction. - The conventional mechanisms of microevolution cannot, in their
extrapolation through geological immensity, fully explain the causes and patterns of
evolutionary change at larger scales: I have argued throughout this book that the
biological components of nonextrapolation lie within critiques of the first two legs of
the tripod—and I have therefore invoked the external "disobedience" of the
geological stage to represent this theme as a surrogate from the domain of another
relevant subject (particularly for paleontologists like myself). Similarly, for this case
of evolvability, species level selection on the first leg, and nonadaptive spandrels on
the second leg, identify the major barriers