Systematics, biogeography, and patterns of diversification in a group of circumtropical marine gastropods (Genus: Nerita)

Understanding the factors that drive diversification, and contribute to species diversity patterns remains a central goal for evolutionary biologists. Here, I used a combination of molecular phylogenetic and comparative approaches to examine the systematic relationships, biogeographical history, and patterns of diversification in a circumtropical clade of intertidal gastropods (Genus: Nerita). First, I examined the biogeographical history of Nerita to determine how historical processes, namely speciation, extinction, and dispersal, have contributed to regional species diversity patterns across the marine tropics. A species-level phylogeny, constructed using mitochondrial and nuclear markers, was combined with geographical distributions and fossil evidence to reconstruct ancestral ranges, produce a time-calibrated chronogram, and estimate diversification rates. Analyses revealed two eastern Pacific + Atlantic (EPA) clades, each of which likely split from an Indo-West Pacific (IWP) sister clade prior to an early Miocene Tethys Seaway closure. While more recent diversification within each region appears to have been driven by vicariance and dispersal, inter-regional dispersal has been rare, and likely contributes little to diversity patterns. A regional comparison suggested that IWP clades have experienced, on average, higher rates of speciation. Fossil evidence indicates that the EPA likely witnessed greater extinction relative to the IWP. These findings suggested that regional differences in species diversity have been largely shaped by differential rates of speciation and extinction. Second, I investigated the relative contribution of geography and ecology on species diversification. Using the previously reconstructed Nerita phylogeny as a framework, I tested whether geographical patterns of speciation primarily correspond to allopatric or sympatric models, and whether habitat transitions played a major role in diversification. Patterns of geographical distribution revealed that speciation has predominately occurred under allopatric conditions; whereas habitat transitions have taken place throughout the history of the clade. Finally, I proposed a revised classification of Nerita, based on the previously reconstructed molecular phylogeny. While the systematic relationships did not depart radically from prior classification schemes, the phylogenetic results suggested a need for minor revisions. I discussed the reassignment of several species, and reviewed the molecular and morphological evidence in support of each subclade.