From Hopeful Monsters to Bolyerine Snakes?

Abstract

Two monotypic snake genera, Bolyeria and Casarea, comprise the Bolyerinae, a subfamily of the Boidae (pythons and boas). Bolyerines are known to have occurred on two or more of the most remote islands of the Mascarene assemblage and seem to be the only snakes indigenous to this region in Recent times. Today they either have reached, or are approaching, extinction on Round Island which, barely more than a mile across, is the last spot on earth from which living specimens have been collected. Bolyerines are distinct from all known amniotes, both living and fossil, in that the maxillary bone is divided into anterior and posterior sections by a movable joint. Such a modification has mechanical characteristics adaptive for feeding. It is difficult to devise a scheme whereby this adaptation evolved in smooth, uninterrupted fashion from an ancestral condition. Several possibilities are presented, all of which lead to the conclusion that the maxillary modification arose suddenly. The sudden appearance of a bizarre, adaptive type has certain implications in common with Goldschmidt's concept of the "hopeful monster." These are discussed. A definition of adaptive precision is proposed, and the selection values of crudely fashioned organic machinery-as might characterize a hopeful monster-is regarded in terms of simplified, hypothetical considerations.