Historical Constraints and the Evolution of Development 1135
be useful and interesting, must make distinctions and define categories of exclusion.
A rubric for all possible cases explains nothing—as Gilbert and Sullivan's Grand
Inquisitor Don Alhambra explained to the naively egalitarian Gondolier Kings of
Barataria: "When everyone is somebody, then no one's anybody."
It is certainly understandable, and probably psychologically inevitable, that
exciting discoveries tend to become overextended in the first flush of reformatory
application. Our recently acquired ability to identify genetic homologies in the
developmental pathways of homoplastic final structures has sometimes engendered a
misplaced enthusiasm for reinterpreting similarities previously ascribed to pure
convergence as examples of parallel evolution (defined as homoplastic results based
on homology of underlying generators).
But, as I have argued throughout this book, concepts only become interesting in
contexts set by the logic and the history of theoretical issues thus addressed. The
primary significance in recasting convergence as parallelism lies in the very different
implications of the two processes both within Darwinian theory and for the larger
question of the relative weights that should be assigned to internal structural
constraint and functional adaptation in populating the morphospace of life's history.
Pure convergence stands at a Darwinian functional extreme, where uncanny
similarities of phyletically distant taxa arise from entirely different starting points,
propelled by selective pressures alone, without any boost from internal channeling.
Parallelism, by contrast and with reversed evolutionary meaning, attributes the
identical result, at least in large part, to a homologous generating channel that guides
two independent sequences of selection down the same path from within.
Parallelism will only define an interesting antithesis to convergence if the underlying
homology prescribes a highly distinctive, detailed, and strongly determinative
channel of constraint—for only then will the homoplastic result owe its primary form
to the structure of the internal channel, and not to the functional processes of
adaptation acting from outside. But if, on the other hand, the underlying homology
only generates a simple immediate product, leading to a broad and non-specific range
of potential outcomes, the homology establishes no meaningful channel of internal
constraint, and makes no contribution to the revisionary power of this theme within
evolutionary theory—that is, to move away from an endpoint of pure Darwinian
functionalism towards a more comprehensive theory, enriched by contrasting
perspectives based on structural principles of internal channeling.
In the light of our burgeoning knowledge of genetic sequences and their actions,
homology of some sort or level will always be found in underlying generators of
similar end products—if only (however much the example becomes a reductio ad
absurdum) because all organisms share the same genetic code by common ancestry.
But no one would argue that we should redescribe a classical case of convergence as
parallelism simply because the markedly different developmental pathways of the
two adaptations both rest upon the action of genes made of DNA!
The analogy in the title of this subsection may help to clarify the central