1076 THE STRUCTURE OF EVOLUTIONARY THEORY
and not just to lump these two principles together by their single common property of
specifying an independent origin for similar features in separate lineages. I believe
that the history and logic of debate about the meaning of parallelism provide our best
path to understanding this important revision in evolutionary theory.
THE TERMINOLOGICAL ORIGIN AND DEBATE ABOUT THE MEANING
AND UTILITY OF PARALLELISM. After struggling through dense paragraphs and
conceptual thickets, G. G. Simpson (in Haas and Simpson, 1946, p. 325) finally
conceded that traditional confusion about the evolutionary meaning of similarity
rested upon a logical dilemma, not an absence of empirical data for resolution of a
factual issue. Why, to state the dilemma succinctly, does parallelism resist easy fitting
into a coherent conceptual structure for the terminology of evolutionary similarity? In
particular, why, when parallelism comfortably joins convergence to establish a
coherent larger category (called homoplasy) for similar structures evolved
independently, have so many good biologists, from the first formulation of the
concept until today, continued to "feel in their bones" that something about
parallelism veers off towards the supposedly opposite category of homology? Why,
to use a vernacular expression often invoked in this discussion (as by Patterson,
1988), does parallelism seem to occupy a "gray zone" between the clear homology of
evident retention by common descent, and the clear homoplasy of convergence by
selective production of strikingly similar structures (in both form and function) from
entirely different points of origin (the "cup coral" shape of rudistid bivalves,
prorichthofeniid brachiopods and rugosan corals, for example)?
I have used Simpson's insight to construct the enlarged chart presented as Table
10 - 1. In an incisive footnote, explicating his differences with coauthor Otto Haas,
Simpson makes the logical point that, although homology and homoplasy do cohere
as dichotomous opposites encompassing all cases, they bear to each other the odd
relationship of a positive claim (A) contrasted with an absence thereof (not-A).
Nothing in logic forbids such a taxonomy, but scientists, maintaining a deeply
engrained (if unconscious) preference for classification by causes, feel discomfited,
in a way that they may not even be able to articulate, about a scheme that contrasts a
positive assertion (homology as descent from common ancestry) with its descriptive
absence (homoplasy as similarity not by descent from the same structure in a
common ancestor). Simpson wrote (in Haas and Simpson, 1946, p. 325):
Homology, as we agree, is best defined as similarity interpreted as due to
common ancestry. Homoplasy, as we also agree, is best defined as similarity
(or as including any process leading to similarity) that is not explicitly
interpreted as due to common ancestry. Both terms rather than being purely
descriptive... express an opinion, one positive and one negative. Homology
expresses an opinion as to how the similarity arose. Homoplasy