GEOGRAPHIC RANGE, TAXONOMIC STRUCTURE, AND GLOBAL DIVERSITY PATTERNS IN MARINE BIVALVES

The taxonomic structure of clades, often measured as the ratio of species to genera (S/G), does not vary randomly on a global scale. Most taxonomic groups exhibit a hollow curve distribution, wherein few taxa are species-rich while most are species-poor. Explanations for this uneven distribution include, but are not limited to, (1) differential diversification among clades, with ecological or life-history traits promoting greater richness in some branches, and (2) clade longevity, with the age of a clade determining the number of species it has accumulated. At the regional scale, S/G ratios are diversity-dependent, such that the null expectation is a decrease in S/G ratios with declining species richness. Early workers interpreted decreasing S/G ratios in terms of competitive exclusion among congeners, but many results did not differ significantly from null models, and some analyses found more congeners than expected by chance.

Here, we analyze within-clade trends in S/G for shelf-depth marine bivalves among latitudinal bins and climate zones. S/G ratios generally decline from the tropics to the poles, but polar S/G ratios exceed the null expectation given the small number of species in this zone. When genera are analyzed individually, a strong, significant correlation exists between the latitudinal range of a genus and its species richness. Genera ranging from the tropics to the poles have systematically more species, both globally and in each climate zone and latitudinal bin throughout its range, than do climatic endemics. This suggests that genera preferentially expand their ranges through speciation, indicating a link between the two processes, and that range-expanders tend to speciate more prolifically within each zone than do taxa endemic to that zone. Higher-than-expected S/G ratios in the poles result from the predominance there of genera with large latitudinal ranges. A significant correlation between genus age and species richness breaks down when geographic range is accounted for by restricting the analysis to climatic endemics or genera with similar latitudinal ranges. This negates the primary control of clade longevity on species richness, and suggests that range expansion, fundamentally coupled with speciation, produces the skewed distribution of species among genera.