1248 THE STRUCTURE OF EVOLUTIONARY THEORY
present regimes of natural selection were exapted from ancestral features with
nonadaptive origins—and were not built as direct adaptations for their current use (or
exapted from ancestral features with adaptive origins for different functions)—then
we cannot explain all pathways of evolutionary change under functionalist mechanics
of the theory of natural selection. Instead, we must allow that many important (and
currently adaptive) traits originated for nonadaptive reasons that cannot be attributed
to the direct action of natural selection at all and, moreover, cannot be inferred from
the exaptive utility of the trait in living species. Because the subject of evolutionary
biology must engage many crucial questions about the origins of features, and cannot
be confined to the study of current utilities and selective regimes, nonadaptationist
themes therefore assume an important role in any full account of life's history and the
mechanisms of evolutionary change.
The key to this expansion of evolutionary theory therefore lies in the category of
currently useful traits with nonadaptive origins—my rationale for prefacing this topic
with two sections to develop the prerequisites of the argument: one on Nietzsche's
principle of general discordance between bases of current utility and reasons for
historical origin, and another on the terminology of exaptation as a framework for
describing and appreciating the importance of currently useful structures coopted
from a different ancestral status, rather than directly evolved for their present
function. By introducing, in this final section, the theme of exaptations based upon
features with nonadaptive origins, I complete the chart of Table 11-1 (see p. 1233) by
recognizing two subcategories of exaptation: (1) Cooptations of features that
originated for different adaptive reasons—the principle of "quirky functional shift"
that enriches (with structuralist "flavor"), but does not challenge, the functionalist
control of evolution by natural selection, and that also establishes the ground of
contingency within the Darwinian world view. (2) Cooptations of features with
nonadaptive origins—the theoretically radical category that precludes any complete
explanation of evolution in adaptationist terms, and that provides a nonadaptationist
alternative for evolutionary inquiry about the origins of currently adaptive biological
features.
But if this argument is so simple to state, so airtight in logic, and at least
interesting (I would say profound, but not with everyone's initial approbation!) in its
implications for evolutionary theory, then why hasn't the category of currently useful
features with nonadaptive origins been perceived as more troublesome by orthodox
Darwinians? Why, to sharpen the paradox, did the concept not even receive a name in
conventional theory? This complex question embraces many dimensions, including
psychological and historical influences lying well beyond my professional
competence and the scope of this book. But, for the relevant dimension of the
structure of Darwinian theory, two strong reasons—both invalid in my view, with
their refutation as the primary aim of this section—have permitted most defenders of
strict natural selection to acknowledge the existence of nonadaptive features, but then
to relegate them to a periphery of rarity and impotence where they can exert no
effective role in setting pathways for the history of life, or in specifying principles