The phylogenetic underpinnings for spatial patterns of morphological disparity: analyses using strombid gastropods

Attempting to understand the relationship between morphological and taxonomic diversification has become a central concern of both paleobiology and ecology. Ordinating species in morphospace provides a visual representation of trends in form. In morphospace analysis, each axis of a graph represents a quantitative measure of a morphological trait or combination of traits found in a group of closely related organisms. How species fill open morphospace provide some insight into the history of adaptive radiations. Furthermore, if we map species\u27 positions in morphospace onto an independently derived phylogenetic hypothesis, we should be able to detect trends in morphological divergence along different lineages. The Strombidae, a family of shelled marine snails, shows a wide variety of forms among its 70 or so extant species, making it an ideal group for such analysis. A previous study has developed a well-defined morphospace for Strombus and Lambis, the two most species-rich and morphologically diverse genera in the Strombidae. Here I use portions of one nuclear (histone H3) and one mitochondrial (cytochrome oxidase I) genes to reconstruct the phylogeny of these two genera. I included 32 of the 50 extant species of Strombus, representing 10 of 11 extant subgenera and three of the nine species of Lambis representing two of three extant subgenera, in my phylogenetic analysis. Maximum likelihood bootstrap and Bayesian majority consensus phylogenetic analyses were performed on each gene individually and on both combined. The resultant phylogenetic tree suggests Lambis as a monophyletic radiation nested within the genus Strombus. This Lambis radiation seems to be driving exploration and colonization of morphospace along the first principal component axis. In addition, all New World species of Strombus are more closely related to each other than to congenerics from the Indo-West Pacific, causing polyphyletic subgenera (Tricornis and Lentigo). Lastly, while the total volume of morphospace filled by the New World radiation was equivalent to that seen among Old World species, the placement of that volume differed, being shifted to more flared forms that may reflect its shared ancestry with the strombid subgenus Euprotomus