Structural Constraints, Spandrels, and Exaptation 1275
on Darwinian individuals at several nested levels: Future flexibilities cannot be
targets of conventional organismal selection; nor can such an attribute enter any
calculation of the differential organismal fitness that fuels Darwin's mechanism. But
this conclusion does not imply that attributes of evolvability must remain uninvolved
as agents in any form of selection—for the most commonly cited components of
evolvability can act as positive traits to enhance the fitness of species-individuals in
selection at the species level. Just consider the two most widely recognized boosts to
evolvability at the species level: the propensity of some species to generate relatively
more daughter species, and the resistance to extinction conferred upon some species
by such organismal features as generalized anatomy and wide ecological tolerance.
These two properties represent the primary analogs, at the species level, of the two
cardinal attributes—birth rates and death rates—that virtually define the fitness of
organisms in the calculus of Darwinian benefits for traits that must ultimately express
their selective advantages by correlation with enhanced birth or retarded death of
organisms. Organismal selection cannot craft propensities for speciation or
resistances to extinction in any direct way, but these properties act as primary
features in the higher-level process of species selection, and therefore figure
prominently and directly in the calculus of selective advantage under hierarchical
models.
But a potential problem still remains. Species selection can certainly utilize and
maintain these important traits of species-individuals, but species selection cannot
always fashion such traits, especially when they emerge into this higher level as
spandrels of selection upon organisms—as must occur when emergent fitness at the
species level resides in the higher-level expression of organismal traits that can only
originate by conventional organismal selection (see Lloyd and Gould, 1993; Gould
and Lloyd, 1999 for an analysis of the logic of this issue). For example, in the case
cited above, the enhanced resistance of a species to extinction emerges as a
consequence of such organismal traits as generalized anatomy and broad
physiological tolerance.
But when we probe further, and draw the appropriate analogy to the level we
understand best, we recognize that ability to utilize, combined with inability to
fashion, represents a norm within Darwinian theory, not a distinctive anomaly
provoked by evolvability at the species level. Ordinary natural selection doesn't
manufacture its variational raw material either. Darwinians have always understood
that their theory's most quirky, most original, and most brilliant intellectual "move"
explains how a process that creates nothing directly can, nonetheless, operate on raw
material of different origin to become a "creative" force in the construction of novel
and useful features. Indeed, I devoted much of Chapter 2 to illustrating how Darwin's
contemporaries rarely grasped this intensely paradoxical, but defining, property of
natural selection's genuine creativity, based only upon its power to enhance or
eliminate, but not to craft variation in a direct and dedicated manner. And I pointed
out that the isotropy (or undirectedness) of variational raw material acts as a
prerequisite for granting natural selection this potentially creative role.