Precise but Fortuitous Convergence in Pleistocene Land Snails from Bermuda

Two convergences between subgenera of Bermudian land snails [P. (Poecilozonites) and P. (Gastrelasmus)] were initiated when a taxon of one subgenus evolved to the very different size range of the other. Entrance into this new size range produced or required a set of correlated changes in shape, color, and thickness that resulted in very precise convergences. Some of these ancillary changes were direct adaptations to the new size range: the large taxon of the generally small subgenus evolved a triangular dome; the rapid increase of height/width that produced a strong dome at the usual small sizes could not be maintained to this large size. Other changes were fortuitous consequences of growth and its interaction with the developmental rate: the small taxon of the generally large subgenus retained its color bands; the small subgenus does not usually produce bands-adult shells display the blotched pattern of an earlier ontogenetic stage common to the whole genus; but one taxon of the small subgenus grew more than the usual number of whorls, forming thereby both the high dome and the color bands that produced convergence. Convergence would seem awesome if we recognized: a) how many characters can be modified together as mechanical or developmental consequences of a primary adaptation; b) how limited are the adaptive solutions to common problems-in particular to altered size.