The evolution of discontinuous distribution: shell morphology of terrestrial gastropods.

The distribution of a phenotypic state is often discontinuous and dispersed. As an example of such a distribution, we focused on the shell shapes of terrestrial gastropods, which exhibit a bimodal distribution whereby species possess either a tall shell or a flat shell. Their shell shapes and the distribution were proposed to be related to the mechanics of shell stability on substrates with different inclinations. We estimated the shell stability and geometric constraints by using theoretical morphology (Okamoto model and Raup model) and analyzed the empirical distribution of shell shapes. The following conclusions were derived from the results of these estimations and analysis.
(1) The bimodality of shell shape in terrestrial gastropods was caused by the adaptation to the stability.
(2) There are adjustments in shell morphology to have a good stability from constrained shapes.
(3) Middle-spired gastropods have the ability as the generalist for a wide range of substrates.