Historical Constraints and the Evolution of Development 1067
constraints (and also designate their discovery as stunningly unexpected), while
continuing to claim that natural selection holds exclusive sway over evolutionary
change because deep homology only imposes limits upon styles and ranges of
developmental pathways, but cannot power any particular phyletic alteration. Natural
selection can still reign supreme as the pool cue of actual evolutionary motion.
But a formalist defender of positive constraint will reply that such unanticipated
deep homology also channels change in positive ways—and that the key to this
central argument resides in an old distinction that, unfortunately, cannot be matched
for both conceptual and terminological confusion, and for consequent failure of most
evolutionists to engage the issue seriously: namely, the differences in causal meaning
(not just in geometric pattern) between parallelism and convergence. The next section
shall treat the history and logic of this issue in detail, but I shall first present the
following basic formulation in relevant terms of balances between constraint and
selection:
Even the most committed adaptationist would not deny that the independent
evolution of similar phenotypic features (in both form and function) in two closely
related lineages may be facilitated by the presence, in both ancestors, of the same
genes and developmental pathways inherited from a recent common ancestor. (The
independently evolved features of these two lineages cannot be called homologous on
basic definitional grounds, but the features may still be built by homologous genes
and along homologous developmental pathways.)
For example, no adaptationist would be fazed by the suggestion that relative
increase in antler size within two separate cervid lineages undergoing phyletic
increase in body size occurred because both lineages retained an ancestral allometry
that may well be homologically pervasive within the Cervidae (Huxley, 1932, for this
classic case of positive allometry). Inherited constraint may set a preferred channel,
but selection must still guide any lineage into such an internally biased path. So a
functionalist may view such undeniably positive constraints as, at most, helpmeets or
facilitators of natural selection, while continuing to regard selection as a necessary
instigator, and therefore as the primary cause of change.
But—and now we come to the nub of the issue, and to the central role of
positive developmental constraint as a major challenge to selectionist orthodoxy—the
attribution of similar evolutionary changes in independent lineages to internal
constraint of homologous genes and developmental pathways, and not only to an
external impetus of common selective pressures, must be limited to very close
relatives still capable of maintaining substantial genetic identity as a consequence of
recent common ancestry. Mayr's characterization of selectionist orthodoxy comes
again to mind: distantly related lineages cannot be subject to such internal limitation
or channeling because the pervasive scrutiny and ruthless efficiency of natural
selection, operating on every feature over countless generations in geological
immensity, must have fractured any homological hold by underlying genes and
developmental pathways over the freedom of phenotypes to follow wherever
selection leads.