1282 THE STRUCTURE OF EVOLUTIONARY THEORY
lens in its vestigial eye, although the lens's irregular shape cannot focus an image, and
the eye remains buried under skin and hair in any case). They found that the blind
mole rat has accumulated mutational changes in alphaA crystallin at a rate far higher
than that observed in nine other rodents—but still (to emphasize the intended point)
at only 20 percent of the characteristic rate for truly neutral pseudogenes. The authors
conclude that although the alphaA crystallin gene emplaces substantial gene product
into a nonoperational lens (at least for vision), the gene must still serve some
function, supported by stabilizing selection, to resist the full neutral rate of change.
In another context on p. 1245,1 noted that, although the alphaA crystallin gene is
more specialized for generating lens protein than its paralog alphaB crystallin, its
product still appears in other organs in some mammalian species. Moreover, the
nonseeing eye of blind mole rats may continue to function in other ways. Haim et al.
(1983) show that blind mole rats still perceive light (and not by the obvious nonvisual
route of correlates to temperature) in regulating their circadian clocks to photoperiod.
Hendriks et al. (1987) argue that melatonin, secreted by the equally nonvisual retina,
may act as a prime circadian regulator—and they therefore suggest that the
developmental pathway leading to the eye and its lens, including the action of alphaA
crystallin, may be adaptively preserved because the eye also performs essential
nonvisual functions.
- Features introduced beneath selection's scrutiny. In the second group of
"insinuations," nonadaptive features may enter the exaptive pool by neutral drift. Just
as we have unfairly discounted the role of manumitted miltons, we have probably
underestimated the relative frequency of insinuated miltons, although for a different
reason. We have regarded neutral changes as both peripheral and rare (largely
restricted to tiny populations on the verge of elimination in any case) because we
have granted too much power to selective control—thus permitting our orthodoxy to
become self-fulfilling by circular argument. But the frequency of insinuated miltons
may actually be quite high, especially given the inevitability of their substantial
introduction via founder effects.
In a fascinating recent example (Tsutsui et al., 2000; Queller, 2000), the
Argentine ant Linepithema humile, "a superb invader of non-native habitats" (Queller,
2000, p. 519), has become firmly established in Mediterranean environments of
California, much to the detriment of several native species and to the distress of
humans. Queller notes (op. cit.): "We rarely understand why invading species
succeed, although a common advantage is that they leave their predators, parasites
and pathogens behind. Although this explanation could apply to Argentine ants, it
seems that the most serious enemies left behind were the warring clans of its own
species."
In Argentina, ants of different colonies fight, and these antagonisms, and the
resulting elimination of many colonies, keep the entire species at relatively low
population densities, thus leading to an ecosystem that includes many other equally
successful ant species. But, in California, Linepithema humile does not fragment into
mutually antagonistic colonies, but lives as a single