Structural Constraints, Spandrels, and Exaptation 1283
aggregate. Queller states (op. cit.): "They form a vast supercolony within which there
is little aggression and extensive interchange of both workers and queens. Such ants
are called unicolonial because a whole population effectively becomes one colony."
The Californian success of Linepithema humile does not arise from their prowess as
fighting individuals (for these relatively small ants do not seem especially gifted as
gladiators in combat by the myrmecine equivalent of mano a mano), but from the
sheer power of their numbers—"because of their ability to rapidly recruit legions of
troops from their network of nests. So peace with flanking nests generates advantages
in competition with other species" (Queller, 2000, p. 519).
Fighting among different colonies in native Argentina depends upon the ants's
ability to recognize degrees of genetic relationship as assessed by differences at seven
microsatellite loci of nuclear DNA. (Interestingly, in another form of exaptation—a
cross-level spandrel in this instance—the usefulness of these markers presumably
arises from the rapid and substantial variability thus imparted among populations, and
flowing upwards as an effect from the neutral status of most mutations in these
microsatellites.) Thus, in Argentina, ants tolerate conspecifics of closely related
colonies and fight with genealogically more distant conspecifics in colonies of greater
genetic disparity.
However, and unsurprisingly, the ancestors of the California invaders passed
through a genetic bottleneck in their initially small population, probably derived from
a single native colony. The California ants lost about two-thirds of the genetic
variability at microsatellite loci. "So all of the ants are genetically alike, and those
applying the old similarity-tolerance rule could be fooled into accepting everyone as
kin" (Queller, 2000, p. 519). Thus, as an example of an insinuated milton (a feature
introduced nonadaptatively by neutral drift) that has been exapted for marked, if
transient, utility, the success of Linepithema humile in California seems to rest upon a
loss of genetic variability that induced the ants to form a single supercolony operating
as a military phalanx (to continue the dubious tradition of anthropomorphic
description for behaviors of social insects) of remarkable power and efficiency.
Queller notes (p. 5190): "Paradoxically, the ecological success of the introduced
populations stems not from adaptation but from the loss of an adaptation—colony
recognition—due to genetic drift." But, then, if we recognized the hierarchical nature
of selection, and repressed our adaptationist biases, this situation might not appear so
paradoxical, and we might even search explicitly, encouraged by a conceptual reform
and expectation, for phenomena that may be very common in nature, but that elude
our notice because we find them anomalous, and do not recognize their existence
until they stare us in the face.
Finally, this case also suggests an interesting flip-side of potential longterm
disadvantages for such transient success—a near necessity in our hierarchical world
of potentially conflicting levels, lest victory at one position propagate throughout the
system to full and permanent triumph. (The scientifically astute Tennyson may have
experienced a more universal insight than we usually acknowledge in penning his
famous line in Morte d'Arthur (1842): "The old