Structural Constraints, Spandrels, and Exaptation 1237
can easily be separated, and their named distinction helps us to understand the
probable sequence of evolutionary events. As Arnold states (p. 139): "there is
subsequent, presumably adaptive, modification improving the initial exaptation, the
foramen becoming larger and more rounded."
Interestingly, and as further confirmation of the primarily exaptive nature of
these vacuities, crevice-dwelling cordylid lizards, representing another separate
evolutionary entrance into this habitat, shift their eyes medially into the
interpterygoid vacuity, rather than downward into the suborbital foramen. Arnold
argues (see Figure 11-7 on these dual routes to exaptation in crevice-dwelling lizards)
that cordylids may have utilized this alternative strategy because this group happens
to possess a large interpterygoid space (as lacertids and scincids do not) that can
accommodate the eye without the secondary modification required to "house" eyes in
the suborbital space. Of this sidewards exaptation of cordylids, Arnold writes (p.
140): "This again turns out to be an exaptation, for examination of the phylogeny of
cordylids shows that expansion of the interpterygoid vacuity evolves before crevice
use, although after the origin of the suborbital foramen. The vacuity may have been
utilized instead of the foramen because, being large, it provided immediate housing
for a large portion of the eye, whereas the foramen would only have been able to
provide this after some modification, as in lacertids and scincids."
In his richest and most extensive analysis, Arnold then discusses six separate
evolutionary innovations of "sand-diving" among lizards (quick entry into aeolian
dunes to escape predators). He finds all six to be equivalent, both in efficacy and as
solutions to the same functional requirement. "In no instance," he writes (p. 156), "is
there evidence that the different methods employed reflect different mechanical
problems." In his combination of functional and cladistic analysis, he interprets the
mechanisms used for sand-