1080 THE STRUCTURE OF EVOLUTIONARY THEORY
In my judgment, Wake (1991, pp. 543-544) has correctly explained why many
biologists blurred the theoretical difference between parallelism and convergence,
and then relegated the terms to descriptive waystations in a continuum of results for a
single causal process. When the subject of internal constraint faded to a periphery of
interest (or even of active denial) within the functionalist orthodoxy of selection's
overarching power and adaptation's empirical preeminence at the height of the
Modern Synthesis (see Chapter 7 on hardline versions of the Synthesis that peaked in
the late 1950's and 1960's), the conceptual distinction of parallelism as a
manifestation of internal channeling became uninteresting to most evolutionists (or,
in the worst effects of biasing by restrictive theories, even unperceivable). With the
defining feature of parallelism thus banished to a limbo of theoretical irrelevance,
biologists limited their concern to the support provided for adaptationist preferences
by the common feature of all homoplasies: the guiding power of independent
selective regimes, whether aided by homologous internal channels (parallelism) or
not (convergence), to fashion the same functional result in separate lineages. Wake
wrote (1991, pp. 543-544):
My central theme is the phenomenon of nondivergent evolutionary change
among lineages, including convergent morphological evolution, parallelism,
and some kinds of reversal—in other words, what phylogeneticists term
homoplasy... Convergence and parallelism often are considered to constitute
strong evidence of the functioning of natural selection. Patterson stated, "The
general explanation for convergence is functional adaptation to similar
environments" (1988, pp. 616-17), but I argue that alternatives must always be
considered. In recent years increasing attention has been given to the
possibility that parallelism is a manifestation of internal design constraints,
and so both functionalist and structuralist constructs predict its occurrence.As Wake's statement implies, two reasons—one "good" and the other "bad" in
the conventional, if simplistic, terms, usually applied to such assessments in
science—underlie this movement of parallelism to a periphery of limited interest, or
to conflation with convergence, a phenomenon of opposite theoretical import in
judging the differential weights of constraint and adaptation in the origin of
homoplastic similarities. Wake correctly identifies the "bad" reason, as an
overemphasis on functionalist themes that limited the scope of evolutionary theory
during the mid-century's height of enthusiasm for a "hardened" version of the
"Modern Synthesis." Phenomena like parallelism, defined by components of internal
constraint, did not elicit the attention of many evolutionists during this period.
But, as Wake recognizes in the last sentence of his statement, parallelism also
received limited attention for the eminently "good" reason that, however well defined
in a conceptual sense, the crucial distinction between parallelism and convergence
could not be cashed out in operational terms until recently—for biologists could not
identify the "homologies of underlying generators" (the shared genetic and
developmental bases of independently