Structural Constraints, Spandrels, and Exaptation 1291
thereby achieved in encountering a favorable spandrel that can then be exapted in the
canonical organismal process of natural selection. (This phenomenon gains its
extreme expression in bacterial evolution, where the rarity of favorable mutants
hardly curtails rapid evolution because populations are so large, and generations so
short, that highly infrequent exaptable effects occur often enough to drive substantial
evolutionary change by sheer brevity of the waiting time between favorable
injections, even in the absence of any mechanism of recombination to spread the
benefits among individuals.)
Thus, the organismal level can usually well afford this carnage of generally
deleterious mutational effects in order to win its fuel of positive variants for natural
selection. Indeed, Darwin's world offers the organism no other choice —for natural
selection makes nothing directly and can only operate creatively in the generation of
evolutionary change if some other process supplies enough undirected variation to
power its odd mode of negative construction by elimination. One might even
"applaud" this inevitable property of cross-level spandrels as "just the thing" that
natural selection needs to become nature's potent driver of evolution despite its
weakness, its dependencies, and its peculiar style of operation. Three interrelated
facts establish and undergird natural selection's capacity to power organismal
evolution in the face of these limitations: (1) natural selection requires "random" fuel
undirected towards adaptive states (lest such an internal force for automatic organic
good overwhelm the weak power of selection to produce similar results in a
characteristically slow, gradual and roundabout way); (2) the fuel supplied by
phenotypic effects of mutation expresses itself as cross-level spandrels injected from
the gene level into organismal phenotypes; and (3) cross-level spandrels manifest the
required property of noncorrelation with benefits at their new level of injection.
Would nature be Darwinian at all, absent these interesting properties of cross-level
spandrels that must supply the fuel of natural selection—thus establishing the
category of "chance" in the duality of "chance" (effects of cross-level spandrels
manifested as mutational phenotypes) and "necessity" (direct action of natural
selection for adaptation to local environments) that we generally cite as an epitome of
the Darwinian mechanism and worldview? If we just remember that the phenotypic
expressions of mutations are cross-level spandrels, we will hold an important key for
unlocking the curiosities of Darwinism.
- I don't wish to imply that species-individuals can only be weakened, given
their generally small population sizes within their clades, and their slow cycling
times, by the nonadaptive (and perhaps usually inadaptive) character of most effects
that supply a component of emergent fitness to species selection (Gould and Lloyd,
1999), and that arise as cross-level spandrels injected upwardly from the organismal
level below. On the contrary, although species may often suffer (in terms of their
ability to overcome in-adaptive spandrels) by their small population numbers and
slow cycling times—whereas organisms vanquish this impediment by large
populations and quick cycling—species also, and in "trade-off," obtain a tremendous
"leg up" over the organismal level by expressing an inherent "allometric" property