1038 THE STRUCTURE OF EVOLUTIONARY THEORY
variation upon which selection operates, but it can result in discontinuities and
directionality in morphological transformations." For further discussion of the
positive meaning of constraint, see Riedl (1978), Gould (1980c, 1989a), and Wagner
(1988). Note also that both statements, cited just above, define the positive meaning
in explicit contrast with the more usual negative reading, while emphasizing the far
greater evolutionary interest of the positive sense.
The familiar and conceptually conjoined realms of allometry and heterochrony
define a locus classicus for positive constraints in providing a sensible link between
the two central themes of speed (for ease) and channeling (for direction). If we wish
to argue that biased channels of internally-set variation can aid natural selection or
any other functional theme in evolution, where could we find a better example than
ontogeny itself, especially when the course of life features substantial allometry
across a broad range of size, and often of environment as well (especially for
organisms with distinct phases of larva and adult, for example). After all, this
fundamental channel already generates a series of well-adapted stages each time an
organism grows to maturity, for all parts of the life cycle must "work" in the
Darwinian world of environmental interaction, or else the organism would not exist.
(See Chapter 5 for my historical discussion of orthogenesis, as advocated by Eimer,
Hyatt, and Whitman, for longstanding recognition of ontogenetic allometry as the
primary source of positively channeled constraints.)
If any of these phenotypes would benefit the organism at a different size or stage
of life, or if any different combination of characters (reachable by retuning the rates
of development among relevant features), might yield increased adaptation, then the
existing channel of ordinary ontogeny already holds the raw material in a particularly
effective state for evolutionary change. And the more pronounced the allometry, the
greater the potential extent of such realizable change.
If allometric ontogenies establish channels of positive constraint, then
heterochrony supplies a convenient and effective mechanism for evolutionary
utilization. By selective acceleration or retardation of single traits, small to large
complexes of correlated characters, or even entire phenotypic stages, heterochrony
can differentially extend or compress features across ontogenetic trajectories, and can
also "mix and match" the characteristics of several stages into a transformed
phenotype. (Contrary to a popular impression, for example, the evolutionary power of
progenesis does not lie in full "promotion" of a functioning larva to sexual maturity,
but rather in the almost invariable, and sometimes adventitiously beneficial,
combination of characters that progenesis yields—with some features "left behind" in
the early ontogenetic stages appropriate to the truncated age of sexual maturation, and
others accelerated to appear in a phenotypically more adult form through correlation
with the early achievement of sexual maturity—see Gould, 1977b.)
For these reasons, heterochrony has long been a favored concept among
evolutionists searching for mechanisms to accelerate evolutionary rates in complexes
of characters—for simple changes in "rate genes" (to use Gold- Schmidt's old phrase)