Historical Constraints and the Evolution of Development 1079
level under consideration—see Roth, 1991; Wagner, 1989; Bolker and Raff, 1996.
Wing of bats and birds are, after all, convergent as wings, but homologous as
forearms).
At the level of an overt phenotypic structure under explicit consideration,
parallelism denies homology and asserts independent origin. But, at the level of the
generators for the overt feature—the genes regulating its architecture, and the
developmental pathways defining its construction—parallelism affirms homology as
the concept's fundamental meaning and raison d'etre, and the basis for its
dichotomous contrast with convergence as alternatives within the more inclusive
category of homoplasy. Thus, parallelism does require independent regimes of
similar selection, but the resulting phenotypic likenesses must also be channeled from
within by homologous generators.
(In an odd sense, one might view this old issue of differences between
parallelism and convergence as a grand foreshadowing for an important debate that
evolutionary biologists have only recently clarified in their minds—but that might
have achieved earlier resolution had we all remembered this older discussion: the
recognition that cladistic gene-trees do not correspond entirely with organism-trees.
The capacity for parallelism rests upon organismal branching before gene branching.
Continuing the argument, one might also view the first steps in the opposite mode of
gene branching before organism branching as a molecular representation of Owen's
old concept of serial homology. Paralogs within one organism are serial homologs;
different paralogs in two organisms are general homologs; only orthologs in two
organisms are special homologs, the heart of the modern concept of pure homology,
or Lankester's homogeny—see p. 1071.)
Framed this way, the maddening complexities and counterclaims of the
literature gain immediate clarification. One must then ask why the distinction
between parallelism and convergence has bred so much conceptual trouble in the
past. In particular, the two terms have often been purposefully combined (and
demoted) to merely descriptive names for stages in a continuum. The terms will then
only designate the trivial geometric difference between features evolved
independently in two lines that remain at about the same distance in overall
phenotype (parallelism) and lineages that become more similar as a consequence of
their separate evolution of such functionally comparable features (convergence). One
can only wonder, then, why biologists ever bothered to devise explicit terms for mere
geometric waystations in a continuum with no interesting causal distinctions. Yet
Haas, for example, defended this descriptive and geometric meaning, while his
coauthor G. G. Simpson demurred (in Haas and Simpson, 1946). And Willey (1911),
in a first book entirely dedicated to the subject (and title) of "Convergence in
Evolution," also denied a meaningful distinction in choosing his single term to
encompass the entire subject of separately evolved similarities. Willey wrote (1911,
p. xi): "I have used the word convergence in a wide sense ... The [traditional]
definitions leave us in the dark as to what degrees of relationship would entitle a
given case to be classed as one of parallelism or of convergence."