Historical Constraints and the Evolution of Development 1051
If a constraint engendered by an allometric channel in ontogeny can so control the
regional pattern of geographic variation in an important species of such a well-
studied genus, then we cannot deny a major role to this positive mode of evolutionary
change by developmental constraint.
THE APTIVE TRIANGLE AND THE SECOND POSITIVE MEANING:
CONSTRAINT AS A THEORY-BOUND TERM FOR PATTERNS AND
DIRECTIONS NOT BUILT EXCLUSIVELY (OR SOMETIMES EVEN AT
ALL) BY NATURAL SELECTIONThe model of the aptive triangle
In a classic line of contemporary American literature, W. P. Kinsella writes of a
midwestern farmer so beguiled by the legend of the great baseball hero Shoeless Joe
Jackson that he constructs a stadium in his wheatfield because he heard a voice
saying to him: "if you build it, he will come." I often feel that many modern
evolutionary biologists unconsciously obey a similar mantra in their approach to the
phenotypic features of organisms: "if it works well, then natural selection made it."
In two historical discussion of this book's first part—my analysis of Darwin's
fateful words at the end of chapter 6 of the Origin of Species (pp. 251-260), and my
presentation of "Galton's Polyhedron" as the most effective formalist or structuralist
metaphor for illustrating missing alternatives in schemes of evolutionary causality
that consider natural selection as the only mechanism of change (pp. 342-351)—I
presented triangular models of causal poles for the origin of phenotypic features: a
representation well suited for portraying alternatives and complements to natural
selection as the causal basis of organic form.
Let me now propose a slightly different triangular model with the same three
poles, but now representing only organismal features that "work well" both in the
classical sense of good biomechanical design, and the technical meaning of
conferring fitness upon organisms in their interaction with environments—in other
words, to the features that biological terminology, and ordinary vernacular usage, call
"adaptations," but that I would rather designate as "aptations" (see Gould and Vrba,
1982), a more general term that acknowledges their current utility while remaining
agnostic about their source of origin. I will therefore designate this model (modified
from Seilacher, 1970, and ultimately traceable to Galton's Polyhydron) as "the aptive
triangle" (though I will submit to standard "loose" (or sensu lato) usage, and usually
refer to the features plotted upon this diagram by the only term that current language
recognizes—namely, adaptations).
The basic diagram (Fig. 10-10, presented before, in part, as Fig. 4-6) designates
three vertices as idealized end members and also recognizes, of course, that almost
any actual feature will plot either along an edge (influenced by two vertices), or,
more frequently, in the triangle's interior (where all three end members contribute).
This mode of ternary plotting has been used most frequently by petrologists for
depicting the composition of actual rocks as