1078 THE STRUCTURE OF EVOLUTIONARY THEORY
The flavor of parallelism, however, lies in the gray zone of the A-B type
classification that our traditions favor (see Table 10-1). In a descriptive sense,
parallelism surely ranks within not-A by definition. But when we assess parallelism's
relationship to the B category of our alternative scheme, we run right into the
aforementioned logical conundrum. Parallelism partakes of B in its invocation of
similar selective regimes to produce homoplastic structures from two separate
starting points lacking the structure. But parallelism also includes too much A-ness
(that is, claims for genuine homology of some sort) to rank as pure B.
The logical solution, had the issue been properly formulated, is not, and never
has been, particularly arcane or difficult. Parallelism lies in a gray zone as a
consequence of its different status within two conceptual schemes that do not parse
nature in exactly the same way, but that we tend to conflate (because both capture
important properties of phyletic change) when we consider the meaning of similarity
in evolution. Parallelism includes aspects of both constraint and independent
selection—not as a wishy-washy mixture in one grand pluralistic glop of all-things-
for-all-people, but in rigorously different parsings for different levels of consideration
(again, as evident in Table 10—1. Several authors have stressed the dependency of
these terms on the hierarchical
Table 10-1. Evolutionary Similarities in Different Lineages Classified by Two Logical Types
(A vs. B or A vs. Not-A) and by Two Criteria (in Realized Structures or in Underlying
Generators of the Structures