1260 THE STRUCTURE OF EVOLUTIONARY THEORY
did snails initially evolve their spiral coiling as part of an actively selected design
centered upon the direct advantages of protected eggs in a cigar-shaped central
space? We cannot use the first method of data from actual historical sequences to
resolve this question because we do not know whether the first coiled snails brooded
their eggs in an umbilical chamber. But the second method of cladistics and
comparative anatomy seems decisive in this case, however inferential: the cladogram
of gastropods includes thousands of species, all with umbilical spaces (often filled as
a solid columella and therefore unavailable for brooding) but only a very few with
umbilical brooding. Moreover, the umbilical brooders occupy only a few tips on
distinct and late-arising twigs of the cladogram, not a central position near the root of
the tree. We must therefore conclude—both from geometric logic (ineluctable
production of the umbilicus, given coiling of the shell) and from the distribution of
umbilical brooding on the cladogram—that the umbilical space arose as a spandrel
and then became coopted for later utility in a few lines of brooders.
In an equally evident example of an automatic side consequence generated as a
geometrical necessity, Megaloceros giganteus, the so-called "Irish Elk," elongated
the neural spines of its shoulder vertebrae for an immediately adaptive reason that
seems well documented in fossil evidence, and rigorously validated by biomechanical
analysis (Lister, 1994). These deer grew the largest antlers in the history of the
Cervidae (up to 35 kilograms in weight on a 2 kg skull). Many big herbivorous
mammals with heavy heads extend their neural spines to provide an increased area of
attachment for enlarged ligamentum nuchae muscles that hold up the head and
neck—a problem that probably affected Megaloceros more than any other deer.
The elongated spines are clearly adaptive (for internal insertion of larger
muscles), but the outward expression of these enlarged bones—a raised area at the
shoulders, covered with hair—probably existed as a nonadaptive spandrel at its initial
phyletic appearance, an inevitable consequence of the geometry of physical space.
And so this feature would have remained, as the vast majority of spandrels do, until
the species's demise—except for a coopted utility that converted the original spandrel
into an exaptation in this case. This raised area—literally a patch of skin spanning a
space enlarged for internal reasons—became enhanced, altered in shape to a more
prominent and localized hump, and festooned with distinctive stripes and colors, all
(presumably) for coopted utility in mating display. I freely admit that the exaptive
potential of such simple bumps and spaces must be limited, and may never exceed the
primary adaptation (which originally engendered the feature in question as a
spandrel) in evolutionary importance. (I also confess that I love this example largely
for humanistic reasons (see Gould, 1996b). Fatty humps and coat colors do not
fossilize, and Megaloceros became extinct many thousands of years ago. We only
know about the hump and its colors because our Cro-Magnon ancestors painted a few
of these animals on French and Spanish cave walls—see Figure 11-10.)
When we move from simple tubes and sheets (aspects of universal geometry,
even though evolved modes of growth in particular lineages must elicit