This review examines the alpha4/3 conotoxins as an example of molecular diversity in a class of compounds that have evolved in a group of closely related species in a single phylogenetic lineage. The species examined belong to Stephanoconus, a clade of Conus, a genus that contains 500-700 different species of carnivorous marine snails. We examine earlier work that describes the identification and characterization of alpha-ImI, the founding alpha4/3 toxin, and two other alpha4/3 toxins, alpha-ImII and alpha-RgIA. These three toxins all inhibit nicotinic acetylcholine receptors (nAChRs) belonging to a subset of nAChRs that are composed of only alpha subunits; they are, however, diverse in terms of the all-alpha subtype they preferentially antagonize and the receptor site that they bind to. We thus speculate that the alpha4/3 toxins may be a rich source of functionally diverse all-alpha subunit nAChR inhibitors. We review extensive work that has established a detailed model for alpha-ImI binding to one of its preferred nAChR subtypes (the alpha7 nAChR) and, by comparing the alpha-ImI, alpha-ImII and alpha-RgIA sequences demonstrate how structural features of alpha4/3 peptides that account for their diverse functional properties can be identified. This approach is extended to derive models of receptor-toxin binding that may account for the different subtype specificities of alpha4/3 peptides. We also speculate on how rational modification of alpha4/3 toxins may allow engineering of ligands with desired subtype specificities. The chemical diversity produced by the closely related animals in Stephanoconus is thus functionally differentiated, although structurally homologous.