542 THE STRUCTURE OF EVOLUTIONARY THEORY
own empirical work increased his belief in the power of selection. In 1937, he
tended to attribute chromosomal inversion frequencies in natural populations of
Drosophila to genetic drift, but he then discovered that these frequencies fluctuate
in a regular and repeatable way from season to season, and he therefore decided
(with evident justice) that such systematic and iterated change must be adaptive.
But empirical discovery cannot supply the entire (or even, I think, the major)
reason for adaptationist hardening, for each favorable case can be matched by a
failure (often hedged or unacknowledged), and no adequate assessment of an
overall relative frequency has ever been achieved—to this day. Thus, any
judgment, in either direction, must represent the fashionable imposition of a few
well-documented cases upon an unstudied plethora. For example, A. J. Cain and
colleagues did win a major victory for adaptation by showing that banding-morph
frequencies in the land snail Cepaea, a former mainstay for claims about genetic
drift, reflected selection based upon visual predation by birds, and upon climatic
factors (Cain and Sheppard, 1950, 1952, 1954).
But Cain and his colleagues then recognized and named the outstanding
pattern of "area effects" (Cain and Currey, 1963a and b)—abrupt geographic
changes in banding-morph frequencies occurring with no perceptible alteration in
any environmental factor that might impose a selection pressure. In what can only
be labeled an article of faith, Cain attributed area effects to selection based upon
"cryptic [meaning truly unmeasured and unperceived by any investigator, not
merely subtle] environmental differences"—a remarkable affirmation of an a
priori preference based upon not finding the necessary empirical confirmation.
Good evidence has since been presented for a non-adaptive explanation of area
effects as historical remnants of previous patterns in land use, and not as an
outcome of current regimes in selection (Cameron et ah, 1980; see review of the
entire case in Gould and Woodruff, 1990). (Area effects rank as anomalies under
selectionist presuppositions— hence Cain's need to supply an orthodox
adaptationist explanation, even in the absence of required evidence. Under a
"legacy of history" explanation, such discordance of morphology with present
geography presents no anomaly and need not even receive a special name.)
If adaptationist hardening cannot be explained as simply and empirically
driven, we might turn to historical and sociological themes. Smocovitis (1996), as
previously mentioned (see p. 503), presents the intriguing thesis that renewed
optimism following the wreckage of World War II (including the hope inspired by
the newly constituted United Nations) launched a strong push for scientific
defenses of potential human improvement and evolutionary progress—an impetus
that became a semi-official movement spurred by positivistic theories of
knowledge proffered as antidotes for older irrationalisms. Smocovitis writes:
If selection had enough agency (and at the same time were a mechanical
principle) then all the more rapid and possible the "improvement" of
humans...