Historical Constraints and the Evolution of Development 1081
evolved structures) needed to distinguish parallelism from the purely adaptational
phenomenon of convergence. But evo-devo has become an active field, while the
subject of parallelism has been catapulted from a periphery of forced inattention (as a
clearly defined but non-operational concept) into the center of evolutionary studies,
largely because biologists have now developed criteria for distinguishing the internal
constraints of parallelism from the purely selective basis of convergence.
In short, more than a century after recognizing the important conceptual
distinction, we can finally resolve actual cases by assessing the different
contributions to homoplastic similarity made by constraining channels based on
homologous generators and directing pathways based on common regimes of
selection. I shall present the evidence of best cases in the next section, but will first
close this section on conceptual and terminological analysis by citing five
chronological episodes in the history of evolutionary debate about parallelism. These
linked episodes all exemplify a crucial argument about the importance to general
evolutionary theory of current research on the genetics of development: Despite all
subsequent confusion and denigration, the concept of parallelism arose as a causal
claim about channels of constraint vs. purely functionalist explanations rooted in
natural selection (or some other adaptationist mechanism, as NeoLamarckism
remained popular in the early years of this debate) for the evolution of homoplastic
resemblance.
The interesting literature on parallelism (as opposed to some of the meaningless
wrangling over terminology) never lost this theoretical context throughout a century
of research and commentary. The delay in resolution, and the prolongation of
theoretical discussion, did not reflect any lack of clarity on the part of evolutionists,
especially as explicated by G. G. Simpson, who understood and promoted the concept
of parallelism and its potentially radical implications for Darwinian theory. Rather,
the persisting frustration about parallelism primarily recorded the inability of
geneticists and developmental biologists to identify the generators posited as the basis
of "latent" or "underlying" homology in the evolution of homoplastic structures
deemed parallel rather than convergent. This bolted door of stymied practice has now
been unlocked, and we have crossed a threshold into a period of amazingly fruitful
research on parallelism in particular, and on the role of developmental constraint
based on deep homology in general, for establishing the markedly nonrandom
clumping of actual organisms within life's potential morphospace.
THE ORIGIN OF THE TERM "PARALLELISM." Interestingly, this term first entered
evolutionary theory with an entirely different meaning—but for another concept,
indeed a far stronger version, of internal channels as major determinants of trends in
the history of life: the theory of recapitulation in embryology. In preevolutionary
versions, Agassiz had spoken of a "threefold parallel" of embryological, taxonomic,
and paleontological series within larger types. The American paleontologist and
evolutionary theorist E. D. Cooe then formalized an evolutionary version of the "law
of parallelism"