A DEVELOPMENTAL CONSTRAINT INCERION, WITH COMMENTS ON THE DEFINITION AND INTERPRETATION OF CONSTRAINT IN EVOLUTION

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Abstract
Since orthodox evolutionary theory is functionalist, constraints attain their most important positive meaning as channels of change imposed by historical and formal determinants, rather than by immediate natural selection. Since ontogeny is the usual locus of expression for these determinants, developmental constraint is an appropriate, general term. A particular developmental constraint in Cerion, most variable of West Indian land snails, stands out for two reasons: 1) it is simply and inexorably defined as a consequence both of formal principles (coiling of tube about axis) and of historical contingencies in Cerion's invariant allometry of growth; 2) it is pervasive in its influence, underlying major patterns of variation in every Cerion study I have ever undertaken. I refer to this pattern as the “jigsaw constraint.” When whorls are large and final size is limited, adult shells must grow fewer whorls. In Cerion, this obvious fact is promoted from trivial to important because complex allometries impose a substantial set of further consequences for form upon this basic trade‐off of whorl size and whorl number. I show that this complex of consequences dominates patterns of natural variation in Cerion at all levels (among shells within samples, between samples in the geographic variation of single species, and between species in multitaxon faunas). It also sets patterns of hybridization between taxa. This paper is primarily a compendium of such examples. It is designed to illustrate the importance of this constraint by the fundamental criterion of relative frequency.