1174 THE STRUCTURE OF EVOLUTIONARY THEORY
selection may always do the actual pushing, but if internal channels—set by history,
and grafted into the genetic and developmental architecture of current organisms—
designate a limited set of possible pathways as conduits for selection's pushing, then
these internal constraints can surely claim equal weight with natural selection in any
full account of the causes of any particular evolutionary change.
But if the challenge posed by historical constraint to traditional Darwinian
functionalism does not lie in an argument about nonadaptive origins, then how can
this category of constraint rectify and expand evolutionary theory beyond the
narrowness imposed by overly adaptationist versions of Darwinism favored during
the heyday of the Modern Synthesis (see Chapter 7)?
In describing my basic framework of argument, I asserted (see pp. 1055-1057)
that the challenge of historical constraint resides in a "metaquestion" about the role of
adaptation in establishing the dumpiness of occupied morphospace, not in a direct
inquiry about the adaptive status of each evolutionary novelty considered one-by-one.
In short, I argued that the markedly inhomogeneous occupation of morphospace—
surely one of the cardinal, most theoretically important, and most viscerally
fascinating aspects of life's history on earth—must be explained largely by the limits
and channels of historical constraint, and not by the traditional mapping of organisms
upon the clumped and nonrandom distribution of adaptive peaks in our current
ecological landscapes. In other words, the inhomogeneous occupation of
morphospace largely records the influence of structural rules and regularities
emerging "from the inside" of inherited genetic and developmental systems of
organisms, and does not only (or even primarily) reflect the action of functional
principles realized by the mechanism of natural selection imposed "from the outside."
In a recent article, Arthur and Farrow (1999, p. 183) pose the key issue in much
the same terms: "Why do animal take the forms they do, and not others? Why ... are
all land vertebrates 'tetrapods'—except for cases of secondary loss, for example
snakes—while none have six, eight, or many legs? Why is the situation precisely
reversed for land arthropods? In general, why are certain areas of multicellular
morphospace densely populated with many representative species, while other areas,
apparently characterizing viable designs, are unoccupied by any extant or extinct
animals?" Then, although I would label their distinctions as overly dichotomized and
too mutually exclusive (for I seek a fusion of structural and functional influences),
Arthur and Farrow also pose the alternatives (1999, p. 183) in much the same manner
followed here:
There are two very different answers to these questions, representing two
opposing schools of thought on the relative importance of natural selection
and developmental constraint in determining the actual distribution of
morphologies that we observe ... One is the "pan-selectionist" view that
variation is potentially available in all directions from any given phyletic
starting-point, and that selection determines which subset