Historical Constraints and the Evolution of Development 1045
Heterochrony now seals the case and intensifies the joint benefit. Neoteny often
operates through a correlation of juvenile form with rapid growth rates of the young
organism. If these rapid rates can be prolonged into later ontogeny, then juvenile
form can also be "promoted" to the adult stage—and the adult shell will also increase
substantially in size (at any age in common with ancestors). Thus, as an automatic
consequence of heterochronic correlations, working within a pre-set allometric
channel of ancestral ontogeny, all three adaptive desiderata for greater shell stability
can evolve in tandem as consequences of a single focus of selection—that is, for
prolongation of rapid juvenile growth rates. By thus linking all the valued characters,
and evoking their common expression by one basic developmental change, positive
constraints work synergistically with natural selection to produce an apparently
complex set of adaptive changes with relative ease and speed.
Ontogenetically channeled allometric constraint as a primary
basis of expressed evolutionary variation: the full geographic and
morphological range of Cerion uva
Since snail shells preserve a complete record of ontogeny in a unitary and rigid
structure that generally cannot be modified after initial formation (at least in exterior
expression, whereas the shell interior can often be altered through secondary
resorption and deposition by appressing soft tissues), this taxon presents unusual
opportunities for the study of developmental constraints. Evolution in any character,
whether caused by selection or not, must automatically elicit a suite of correlated
responses throughout such an integral and integrated structure.
However, the isometric growth model of the logarithmic spiral, so often
assumed to apply to the actual growth of most mollusks will greatly limit the
expression of such constraints based on "correlations of growth" (Darwin's own
phrase), because such a logarithmic shell does not change its shape through ontogeny
(D'Arcy Thompson, 1917), and heterochrony therefore loses its power to alter the
form of descendants by general retardation or acceleration—for the juvenile shell
looks just like a scaled-down adult, and global paedomorphosis, for example, would
therefore exert no effect upon form. But when shells grow with pronounced
allometry, then positive channels of constraint attain great potential for influencing or
directing the evolution of phenotypes (as expressed in a complex, rigid structure,
preserving a complete and unaltered record of ontogeny, where any change must
elicit a cascade of correlated consequences, and where strong allometries establish a
rich playing field for effective heterochrony).
In fact, molluscan shells rarely grow as idealized logarithmic spirals, and nearly
all forms, even such prototypes of supposed isometry as the gastropod genus
Turritella, actually grow with measurable allometry (Andrews, 1971). Moreover,
Vermeij (1980) and Kemp and Bertness (1984) have presented strong theoretical
arguments for regarding an allometry of doming (relative increase of height to width)
as a predictable consequence of general modes of