1040 THE STRUCTURE OF EVOLUTIONARY THEORY
examples of "wiping the slate clean" may explain the origin of some major groups
(see Mooi, 1990, on sand dollars among echinoids), but McKinney (1999) rightly
points out that an even more powerful, and almost surely more frequent,
heterochronic boost to the origin of novelties may lie in the potential for what I
previously called the "mix and match" of characters produced by varying modes of
heterochrony in different features and complexes within the same organism.
As a poor and parochial surrogate for adequate review of an immense literature,
I limit myself to two examples from my own research that have enlightened me about
the evolutionary implications of positive constraint in this allometric and
heterochronic form.
The two structural themes of internally set channels and ease of
transformation as potentially synergistic with functional causality
by natural selection: increasing shell stability in the Gryphaea
heterochronocline
In quantitative studies of fossil invertebrates, no case has commanded nearly so much
attention as the evolution of coiled Jurassic oysters of the genus Gryphaea in the
British Isles (see Trueman, 1922 for the classic statement). I collected only the major
papers of this debate into a full book (Gould, 1980f), while a volume of equal extent
has been published since then, leading to what I regard, with obvious self-serving
bias, as a genuine solution in Jones and Gould (1999). I will not discuss earlier errors
and struggles (see Gould, 1972, for a compendium), and will simply note a consensus
reached by the 1970'sā that the complete lower Jurassic sequence from Gryphaea
incurva to Gryphaea gigantea features a basic trend in a set of phyletically correlated
characters, including substantial increase in body size, decrease in coiling, and
increasing relative width of the valves.
These trends, at least in a descriptive sense, certainly seem to embody the
heterochronic result of paedomorphosis, as all sustained changes in form led to
progressive juvenilization in adult phenotypes of later phylogenetic stages (Fig. 10-
1). The strong allometry of increased coiling through ontogeny permits an easy
identification of this trend, as larval shells cement briefly to a hard object, with the
young organism then breaking free and coiling throughout life on a muddy substrate.
Juvenile lower valves (after breaking their initial cementation) therefore begin growth
as relatively flat, and then coil progressively throughout life. The phyletic trend to
flattening strongly resembles a progressive excursion to earlier and less coiled stages
of ontogeny.
But this descriptive consensus remained stymied by a common technical
problem in heterochronic studies within paleontology. The causal distinctions within
heterochrony can only be specified with reference to the chronological age of
specimens, and few fossils record the months and years of their growth in a
recoverable manner (see discussion of this dilemma in McKinney and McNamara,
1991; and Jones and Gould, 1999). Without information about the age of specimens,
we could not tell whether increasing body size simply