The Structure of Evolutionary Theory

518 THE STRUCTURE OF EVOLUTIONARY THEORY

Subsidiary orthogenetic restriction is probably frequent, but we are not yet

able to be sure in most cases whether a limitation of variation as actually

found in a group is due to a limitation in the supply of mutations or to

selection, or to other causes. It is, however, certain that some mutational

effects recur regularly in some allied species, and probable that this

phenomenon is widespread (1942, p. 524).

Huxley also cites overcoiling in Gryphaea (Trueman, 1922)—the classic case
of his time, though since invalidated (Gould, 1972)—as a primary puzzlement and
most promising example for "dominant orthogenesis":

We must provisionally face an explanation in terms of orthogenesis—i.e. of

evolution predetermined to proceed within certain narrow limits,

irrespective of selective disadvantage except where this leads to total

extinction. It should be noted that, even if the existence of orthogenesis in

this cause [sic, for case] be confirmed, it appears to be a rare and

exceptional phenomenon, and that we have no inkling of any mechanism by

which it may be brought about. It is a description, not an explanation.

Indeed its existence runs counter to fundamental selectionist principles

(1942, p. 509).

Despite his general commitment to adaptation, Huxley also granted some
importance (beyond mere existence) to Wright's genetic drift in the formation of
species with small population sizes (p. 58). He even extended the power of this
non-adaptational force to the origin of generic differences, though not beyond: "It
may be presumed, on somewhat indirect evidence, that 'useless' non-adaptive
differences due to isolation of small groups may be enlarged by the addition of
further differences of the same sort to give generic distinction, though it seems
probable that differences of family or higher rank are always or almost always
essentially adaptive in nature" (1942, p. 44).
Thus, the early synthesis, in the view of both its founders and its namegiver,
reinstated Darwinism as the centerpiece of evolutionary theory by rejecting any
substantial role for the full spate of previously popular alternatives. (I should say
"instated," for Darwinism had never before attained majority appeal as a
mechanism, even during Darwin's lifetime.) But the early synthesists, with Fisher's
exception, also left a few facets intact on Galton's polyhedron. Their interest lay in
showing that our increasing knowledge of the Mendelian world could establish
natural selection as the primary cause of evolutionary change, not in staking a
claim for Darwinian exclusivity.

Synthesis as Hardening

THE LATER GOAL OF EXALTING SELECTION'S POWER

Evolutionists have generally depicted the second phase of the Synthesis as a
gathering of traditional subdisciplines under an umbrella constructed during