Historical Constraints and the Evolution of Development 1053
features arose initially by natural selection in the ancestral line, then their ultimate
origin remains functional—and natural selection, as previously noted, represents the
only known (for Darwin, the only effective) cause of functional change. Finally, at
the structural vertex, Darwin allowed that features not arising for functional reasons,
but only coopted for current utility, must be admitted as genuine exceptions to the
principle that adaptive features can only originate for functional reasons (with natural
selection as the only known and sufficiently powerful functional mechanism). But he
then demoted this class of real exceptions by the standard argument in studies of
natural history: he claimed, invoking the classical justifications of "sequelae" and
"nooks and crannies" (see p. 1249), that currently adaptive features with nonadaptive
structural origins must, by their rarity, reach only an insignificant relative frequency
among evolved traits of organisms.
The impeccable logic of this formulation can help critics by clarifying how any
potential argument against this hegemony of natural selection must proceed. At the
functional vertex, one would have to identify other important mechanisms in addition
to natural selection—and none have been proposed, at least to the satisfaction of this
author (although the argument for "a little bit of bacterial Lamarckism"—as I like to
characterize the controversial claims of Cairns et al. (1988)—may have some merit in
a limited domain).
At the historical vertex, one would have to reject the contention that
constraining homologies of inheritance, and the resulting heterogeneous clumping of
species in organic morphospace, record the consequences of natural selection in
constructing the novel traits of ancestral forms, followed by the continuing control of
selection upon subsequent patterns of phyletic change in descendant lineages—an
argument that I will advance in Section II of this chapter. Finally, at the structural
vertex, one would have to counter Darwin's argument by asserting that he greatly
underestimated the relative frequencies of these admitted exceptions to natural
selection for the origin of currently functional features—a claim that I will advance in
Chapter 11. Thus, the form of Chapters 10 and 11, and my argument for the
importance of structural constraints at high relative frequency in the origin of
currently adaptive organismal characters, will center upon recent arguments for a
reconceptualization of the historical vertex, and for a reevaluation of relative
frequency at the structural vertex of the aptive triangle.
Distinguishing and sharpening the two great questions
THE STRUCTURAL VERTEX. I shall begin with the simple and more direct
question posed from the structural vertex: does the existence of current adaptations
necessarily imply their functional origin at all (either as a direct response to current
environments, or by inheritance of traits with a functionalist origin at their ancestral
inception)? How, in other words, can good Darwinian design (aptations in my
favored restriction, adaptations in the vernacular) arise by processes that do not
involve functional adaptation?
Since this category is defined negatively—to designate causes of functional
characters not evolved by functionalist mechanisms—the structural vertex