Historical Constraints and the Evolution of Development 1069
the functional limits or mechanical constraint upon the human eye, one would
do well—as J. Z. Young and others have done—to study the octopus.But, as I shall discuss on pp. 1123-1132, one of the major discoveries of evo-
devo has revealed a deep genetic homology underlying and promoting the separate
evolution of lens eyes in cephalopods and vertebrates. The overt phenotypes do
record substantial convergence (for different body tissues build corresponding
structures in the two groups), but both phyla share key underlying genes and
developmental pathways as homologies, and the example has lost its former status as
the principal textbook case of natural selection's power to craft stunning similarities
from utterly disparate raw materials. Eyes of such strikingly similar design owe their
independent origin as much to genetic and developmental parallelism, based on
internal constraints of homologous genes and developmental pathways, as to
selection's capacity for iterating nearly identical adaptations from scratch by
convergence.
With this "one liner" of maximal force—evo-devo has reinterpreted several
textbook examples of convergence as consequences of substantial parallelism—we
can encapsulate the depth of theoretical disturbance introduced by this subject into
the heart of Darwinian theory. Our former best examples of full efficacy for the
functional force of natural selection only exist because internal constraints of
homologous genes and developmental pathways have kept fruitful channels of
change open and parallel, even in the most disparate and most genealogically distant
bilaterian phyla. The homological hold of historical constraint channels change at all
levels, even for the broadest patterning of morphospace, and not only for details of
parallel evolution in very closely related groups.
A terminological excursis on the meaning of parallelism
THE NINE FATEFUL LITTLE WORDS OF E. RAY LANKESTER. The
transforming power of this discovery upon evolutionary theory would stand out more
clearly if the key terms and concepts had not become so muddled in our literature,
and therefore so widely misunderstood or disregarded by modern researchers. (This
situation cannot validate the graybeard's perennial lament: "them young fellers just
don't keep up with the views of the older guys, like we did when we wuz gettin'
started." The concepts and terminology surrounding the origin and status of similar
structures in different lineages have inspired particular difficulty and unclear thinking
ever since Darwin, and even before. In their classic paper on the subject, still the best
treatment ever published, Haas and Simpson (1946) devoted the bulk of their long
text to the history of confusion over differences between parallelism and
convergence—with the two authors finally agreeing to disagree about the most
fruitful definitions, even as they resolved the conceptual confusions.)
We should begin by recalling a central distinction that we all know, and
probably all regard as refreshingly free from conceptual ambiguity: the difference
between homology and homoplasy. Homologous structures are similar