Historical Constraints and the Evolution of Development 1147
gardens, but have almost never been reported as natural populations"—thus giving
special interest to this case of viability in nature. (They also mention the intriguing
historical footnote that Linnaeus himself found a peloric Linaria (with normal upper
petals replaced by spurred lower petals), apparently growing in abundance within its
habitat, but probably seed sterile, and therefore arising as a vegetatively propagated
clone.)
Ford and Gottlieb outline a reasonable scenario for the promotion of this fecund,
and naturally growing, homeotic form to specific status (1992, p. 673):
The absence of deleterious pleiotropy or fitness-reducing epistatic interactions
in bicalyx suggests that mutations with extensive morphological consequences
can be successfully accommodated by plant developmental systems. If such
mutants were to become associated with chromosomal rearrangements
reducing the fertility of hybrids between them and their progenitors, a process
that has occurred repeatedly in Clarkia, the new population would probably be
accorded species status (p. 673). Bicalyx demonstrates that a large
morphological difference governed by a simple genetic change can become
established in a natural plant population (p. 671).Since bicalyx presumably establishes its large and homeotic effect through the
developmental channel of the ABC system or some homolog (see previous discussion
on pp. 1063-1065), this case also illustrates the common linkage between internal
channels as positive constraints and potential speed of phyletic movement down the
channel—as Galton first proposed in his model of the polyhedron (see pp. 342-351).
In a zoological example of the same linkage, Fitch (1997, p. 166) documents a
"topological constraint" limiting the number and positions, and channeling the
potential directions of evolutionary change, for rays in the male tail of the nematode
C. elegans. Not only are the directions of potential change both limited and most
plausibly attained by saltation, but the transitions can also be generated by single
point mutations. Of this interesting correlation between constraint and saltation, Fitch
concludes (1997, pp. 166-167): "Because single genetic changes can be postulated for
some of the evolutionary change in the male tail, I predict that many evolutionary
changes in morphology will have resulted mainly from changes at single loci...
Because the power of selection is limited by variation, such developmental
constraints could cause significant bias in the evolution of form."
Movement four, Recapitulation and Summary: Early rules and the
inhomogeneous population of morphospace: Dobzhansky's
landscape as primarily structural and historical, not functional
and immediate.
BILATERIAN HISTORY AS TOP-DOWN BY TINKERING OF AN INITIAL SET
OF RULES, NOT BOTTOM-UP BY ADDING INCREMENTS OF COMPLEXITY.
As a common pattern in the history of science, great and unexpected theoretical
discoveries often elicit fairly conservative theoretical interpretations