1088 THE STRUCTURE OF EVOLUTIONARY THEORY
Parallelism has several theoretical bases that help one to understand and also
to recognize it. The structure of an ancestral group inevitably restricts the lines
of possible evolutionary change. That simple fact greatly increases the
probability that among the number of descendant lineages several or all will
follow one line. That probability will be further reinforced by natural selection
in a geographically expanding and actively speciating group if the ecologies
of diverse lineages remain similar in respect to the adaptations involved in the
parallelism. The degree of dependence on similar ecology resembles that of
convergence, but the retention of homologous characters from the relatively
near common ancestry usually distinguishes parallelism. The parallel lineages
(unlike those only convergent) furthermore start out with closely similar
coadapted genetic systems, and similar changes are more likely to keep the
system adequately coadapted.PARALLELISM AS A "GRAY ZONE" BETWEEN HOMOLOGY AND CONVERGENCE.
Despite Simpson's careful separations, and his stress on their theoretical importance,
many biologists ignored the important theoretical differences between these two
subcategories of homoplasy. If they recognized parallelism and convergence as
distinctive terms at all, they often could not state any rationale for the terminology
beyond the triviality of an abstract and formal geometric difference between parallel
and converging lines.
But thoughtful evolutionists continued to struggle with the "hybrid" character of
parallelism. Michener (1949), for example, in the finest technical application of the
concept, honored the causal (rather than geometric) distinction: "The potentiality for
similar changes, resulting in parallel characters, no doubt results from the fact that
related animals have homologous chromosomes and genes" (1949, p. 140).
The cladistic revolution in taxonomic practice also forced renewed attention to
the distinction, and to the "intermediate" status of parallelism in producing
homoplastic structures based on homologous generators—leading, for example, to
Saether's (1983) concept of "underlying synapomorphies," defined as "the capacity to
develop synapomorphy" or "close parallelism as a result of inherited factors within a
monophyletic group" (Saether, 1983, p. 343).
The acknowledgment of homologous generators actually led some taxonomists,
including such leaders as Mayr (1974), to include parallelism within a broader
definition of homology, while most researchers continued to rank parallelism as an
uncomfortable subcategory of homoplasy (Patterson, 1988), or as a "hybrid" notion
based on homoplastic origin from homologous generators (as in Saether, 1983).
Perhaps Patterson (1988, p. 619) put the matter best by writing: "In morphology, the
'gray zone' between homology and nonhomology concerns congruence—or inferred
common ancestry—and whether parallelism (which does invoke common ancestry)
should be included or excluded from homology."
THE OPERATIONAL RESCUE OF PARALLELISM BY EVO-DEVO. The