1042 THE STRUCTURE OF EVOLUTIONARY THEORY
10 - 2. Determination of age of individual Gryphaea shells by oxygen isotope profiles across
annual growth increments. Solving this problem of sclerochronology allowed us to determine,
for the first time, the actual mode of heterochrony in Gryphaea. From Jones and Gould, 1999.10 - 3. Increase in shell size (measured as shell height) in the phyletic sequence G. arcuata to G.
gigantea. Although the shells augment markedly in size, this increase does not reflect longer
periods of growth as descendants are larger at each comparable age, and the average adult
descendant dies before reaching the final age of the average adult ancestor. From Gould,
2000e.(Fig. 10-4), using the allometric channel of Gryphaea's ontogeny to evolve a broader
and less coiled adult shell in later stages of the sequence.
When we combine this structural analysis of the evolutionary trend with a well-
documented scenario for its adaptive basis, the positive aspect of constraint as an
adjunct to selection stands forth in an unusually clear manner. The environmental
correlation of flat and cemented Ostrea with clear waters and hard substrates, and of
coiled and free-living Gryphaea with muddy substrates,