Historical Constraints and the Evolution of Development 1075
internal constraints, and exclusive focus on a common external context of
adaptation), homoplasy loses its former common ground with homology (in positing
historical hold—whether of structures in homogeny, or of genes, pathways and
potential in homoplasy—as the shared causal basis of similar structures in two
lineages). Moreover, this expanded category of homoplasy now includes only one
universal feature to define its own coherence: independent origin, in both parallelism
and convergence. This feature also places homoplasy into antithesis with homology
(common ancestry vs. independent origin).
Convergence did join parallelism to build an expanded category of homoplasy,
thus setting the opposition that continues to define these terms as an exhaustive and
dichotomous division today. Ironically, as a final point, Lankester himself—in a
logical inconsistency within his own paper— spawned this dramatic shift in his own
intended parsings by adding those "nine fateful little words" (of my title to this
subsection) to the end of one statement in his original article. On page 41, as he tries
to distinguish his newly formulated concept of homoplasy from the older notion of
analogy, he presents (in this single passage) such a broad definition of homoplasy (in
the midst of a "generous" attempt to show that analogy must be construed as broader
still, and therefore not synonymous with homoplasy no matter how far we extend the
concept) that he actually includes independent evolution by convergence—not a
subcategory of homology by any stretch of the imagination! —in those nine words at
the end (presented in italics below, but not in Lankester's original): "Homoplasy
includes all cases of close resemblances of form which are not traceable to
homogeny, all details of agreement not homogenous, in structures which are broadly
homogenous, as well as in structures having no genetic affinity."
Once pure convergence had been added to homoplasy via these nine fateful little
words, the linkage of homoplasy to homology could no longer be defended—and the
two concepts moved from their initial union to their current antithesis.
I recount this story at some length because I know no better way to illustrate the
central tension and conceptual confusion within the concept of homoplasy.
Parallelism and convergence do share the common descriptive feature of defining an
independent origin for similar structures in two lineages. But in causal terms,
particularly for assessing the relative weights of formal vs. functional factors in
evolutionary change, the conceptual difference could not be more important—for
parallelism marks the formal influence of internal constraint, while convergence
reflects the functional operation of natural selection upon two substrates different
enough to exclude internal factors as influences upon the resulting similarity.
This recognition of internal channeling as the root cause of parallelism— the
principal basis for ascribing evolutionary change, and not only limitation, to historical
constraint—lies at the heart of evo-devo's theoretical novelty and importance to the
Darwinian worldview. This context behooves us to formulate, and to clarify, the
causal distinction between parallelism and convergence—