Structural Constraints, Spandrels, and Exaptation 1281
1979), with the pendentives of San Marco as my "holotype," I described only this
group—not for any good or principled reason, but simply because I had not
recognized the status of cross-level effects in a hierarchical theory of selection.
Obviously, I already regarded this restricted set of at-level architectural byproducts as
potentially significant in evolution. But the inclusion of cross-level effects as a
second category moves the concept of spandrels from an edge of interest to a center
of potential importance in evolutionary theory.
The second grouping of cross-level, or propagated, spandrels includes the
expressed effects upon biological individuals of changes introduced for a definite
reason (whether adaptational or not) at a different initiating level. I include such
cross-level effects under the rubric of miltonic spandrels, rather than franklinian
potentialities (as they have usually, if unconsciously, been regarded, when
conceptualized at all), because, like the architectural sequelae of my at-level group,
they are actual, initially unused, nonadaptive things that also arise as side
consequences—even though the side consequence in this second group of cross-level
spandrels are propagated effects to other levels (often with no expression at the level
of origin for the primary change that generated them), whereas the side consequences
in the first group of at-level spandrels are immediate mechanical correlates of a
primary change in the same individual, and therefore easier to spot and define.
I then divide the second subcategory of historical miltons into two groupings of
markedly different status:
- Features that lose an original utility without gaining a new function. In a first
group of "unemployments" or "manumissions," previously utilized features become
liberated from functional or selective control, and gain freedom to become exapted
for other uses. Currently nonadaptive as a historical result of their altered status, they
fall out of selective control and into the exaptive pool as actual items that must now
"stand and wait," but might serve again in an altered evolutionary future.
We have generally granted little evolutionary potential to such vestigial
unemployments because relatively quick reduction and loss—as in the standard
example of eyeless cavefishes—seems to follow as an inevitable injury of
degeneration added to the initial insult of unemployment. But such manumitted
miltons may be quite common, particularly at the gene level (where the process has
achieved greater recognition). Many multiply repeated evolutionary phenomena—the
deletion of larval forms in the evolution of direct development, or the shedding of
adult stages in progenetic lineages, for example—must leave a substantial number of
genes in such an "unemployed" state. Yet, as we also recognize, full unemployment
may occur only rarely because most genes function in more than one way. (This fact,
however, should be regarded as salutary for future exaptive potentials in keeping
partially "unemployed" genes in an active state of resistance to true operational
discombobulation by accumulation of neutral mutations.)
In the most fascinating confirmation of our literature (see p. 688 for more details
in a different context), Hendriks et al. (1987) sequenced the alphaA crystallin gene in
the blind mole rat Spalax ehrenbergi (which still grows a