A developmental constraint in Cerion, with comments on the definition and interpretation of constraint in evolution

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 since 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. When whorls are large and final size is limited, adult shells must grow fewer whorls. In Cerion, this 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. 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. -from Author