Molecular Distance and Morphological Divergence in Cauloramphus (Cheilostomata: Calloporidae)

Molecular phylogenetic analysis of cytochrome c oxidase I (COI) sequences in the calloporid genus Cauloramphus was used to examine (1) the correlation between COI genetic distance and morphological divergence in selected characters; (2) relative levels of intra- and inter-population COI genetic divergence; and (3) the utility of COI in discriminating species and species groups. The phylogeny includes representatives of 15 previously described morphospecies and five unidentified taxa. Kimura 2-parameter (K2P) distances within local populations of five morphospecies ranged from 0.00% to 3.01%. For three morphospecies, K2P distances ranged from 0.50% to 11.0% between populations separated geographically by 750–4,500 km, with no correlation between genetic distance and geographical separation; we identified at least one putative cryptic species. The phylogeny detected at least three undescribed morphospecies; two other specimens not identified to species prior to the analysis emerged as divergent populations of, or sister taxa to, previously described or newly detected morphospecies. Our results indicate that in Cauloramphus, and perhaps in many other cheilostomes, there is a necessary bias in application of the morphological species concept. Lack of detectable morphological differences between geographically separate populations says little about genetic distance between them, and operationally they must be considered as populations within a single morphospecies. On the other hand, geographically separate populations exhibiting overt differences in the form of, or non-overlapping differences in the ranges of, one or more characters indicate substantial genetic divergence and probable reproductive isolation, validating application of the morphological species concept as a proxy for the biological species concept in these cases.