HOMOLOGY AND HOMOPLASY IN CONUS (NEOGASTROPODA) SHELL CHARACTERS: A TEST OF PHYLOGENETIC SIGNAL USING MOLECULAR AND MORPHOLOGICAL DATA

The extent to which phylogenetic signal is preserved in gastropod shell characters is a contentious issue that has been underexplored analytically. This topic is of particular concern to paleontologists who seek to address evolutionary questions within a phylogenetic context but only have access to hard-part material. Some workers argue that shell characters should not be included in cladistic analyses because they are highly influenced by selective pressures, and are therefore beset by homoplasy (similarity not due to common ancestry, but rather parallelism or convergence). Others contend that shell characters are not necessarily any more homoplasious than other aspects of morphology and should not be rejected a priori from cladistic analyses.

The genus Conus, which has a fossil record extending from the Eocene to the Recent, was used to investigate the amount of phylogenetic information preserved in species-level gastropod shell characters. Two molecular sequences (16S rRNA and calmodulin) from over 60 extant Conus species and three outgroup taxa were downloaded from GenBank, aligned, and combined into a single matrix. This matrix was combined with a morphological matrix consisting of several classes of shell characters that are commonly preserved in Conus fossils, including coloration patterns, shell ornamentation features, and several aspects of shell shape. These shell characters were scored using literature sources and museum specimens. The molecular + morphological data matrix was subjected to a heuristic tree search under equal-weights parsimony, from which a strict consensus of all most parsimonious trees was generated. The topology of this tree was nearly identical to that generated using only the molecular data.

Most Conus shell characters show multiple origins on the consensus tree and large amounts of homoplasy, as evidenced by relatively low ci and ri values. This result has negative implications for the determination of phylogenetic relationships among fossil Conus species because, when analyzed on their own, shell characters may produce trees that are poorly resolved and/or misleading. However, identification of homoplasious characters permits other avenues of research, particularly investigation of repeating patterns of evolutionary change and the factors underlying such change.