688 THE STRUCTURE OF EVOLUTIONARY THEORY
downgrading, as marginal, a process potentially responsible for more than half of
all nucleotide substitutions—the supposed basis of evolution within a scientific
ethos centered on reductionist preferences? Only a lingering prejudice for viewing
organisms as a unique and intrinsic focal level could possibly generate such a
claim.
Yes, an organism might view the world of its own compatriots as stable
islands rising above an invisible sea—and choose to disregard random change
within this swirling ocean of underlying, constant activity. But (if I may pursue this
strained metaphor for a moment), any dynamic particle in the ocean could just as
well, and perhaps with more merit, view the islands as rare and insignificant
pedestals intruding into the truly fundamental substrate. May I just note the sterility
of such a subjective argument, and state that any process with so strong an impact
on change at any level cannot be unimportant in a world judged by relative
frequencies.
As an illustration of the importance (and separability) of hierarchical levels,
we may invoke balances produced by negative interaction among levels as a
measure for the indispensability of molecular neutrality in full explanations of
evolutionary phenomena. Just as a stable balance may arise by opposite forces of
selection at adjacent levels, different processes—in this case neutrality at one level
vs. selection at another—can also produce an intermediary result testifying to the
importance of both styles of change. In such cases, moreover, neutrality enjoys a
special heuristic advantage because random models yield general, quantitative
predictions, while selectionist explanations usually require knowledge of particular
circumstances that are much harder to decipher, and often impossible to quantify
(for lack of requisite historical information).
For example, Spalax ehrenbergi, a blind Near Eastern mole rat, develops a
rudimentary eye with an irregular lens that cannot focus an image. The eye is
covered by thick skin and hair, and the animal shows no neurological response to
powerful flashes of light (see p. 1282 for fuller discussion of this case in a different
context). As expected under the neutral theory, the major lens protein, A-
crystallin, evolves much faster in S. ehrenbergi than in other murine rodents with
normal vision (Hendricks et al., 1987)—nine amino acid replacements in a
sequence of 173, over 40 million years of evolutionary separation, whereas the
other nine rodents of this study show identical amino acid sequences, with no
alterations at all from the ancestral state. But this rate of change for Spalax
represents only 20 percent of the average for true pseudogenes, our best standard
for a maximal and purely neutral pace of evolution. At a rate of alteration too fast
for stabilizing selection, but too slow for pure neutrality, the results imply a
dynamic balance between molecular drift and weakened selective control at the
organismic level. (Suggestions for continued utility of a non-seeing lens include
possible function in adjusting physiology to seasonal cues from changing day
lengths, though we know no mechanism for perceiving such fluctuations without
vision, see Haim et al., 1983; and developmental constraint based on formation of
eyes as a necessary inducer of some later and fully functional feature in
embryology.)