Genus age, provincial area and the taxonomic structure of marine faunas

Abstract

Species are unevenly distributed among genera within clades and regions, with most genera species-poor and few species-rich. At regional scales, this structure to taxonomic diversity is generated via speciation, extinction and geographical range dynamics. Here, we use a global database of extant marine bivalves to characterize the taxonomic structure of climate zones and provinces. Our analyses reveal a general, ZipfMandelbrot form to the distribution of species among genera, with faunas from similar climate zones exhibiting similar taxonomic structure. Provinces that contain older taxa and/or encompass larger areas are expected to be more species-rich. Although both median genus age and provincial area correlate with measures of taxonomic structure, these relationships are interdependent, nonlinear and driven primarily by contrasts between tropical and extra-tropical faunas. Provincial area and taxonomic structure are largely decoupled within climate zones. Counter to the expectation that genus age and species richness should positively covary, diverse and highly structured provincial faunas are dominated by young genera. The marked differences between tropical and temperate faunas suggest strong spatial variation in evolutionary rates and invasion frequencies. Such variation contradicts biogeographic models that scale taxonomic diversity to geographical area.

Convergence, divergence, and parallelism in marine biodiversity trends: Integrating present-day and fossil data

“…Further, the diversity differences among the regionally dominant families are not correlated with Pliocene−Recent extinction intensities, indicating that post-Pliocene speciation and immigration were also involved in shaping present-day biotas. Such a scenario is consistent with previous suggestions (3,40) that the milder temperature gradients along the W Atlantic owing to the northward flow of the Gulf Stream have been more conducive to northward expansion of species over the past 5 My (the E Pacific receives cool water via the California Current and localized upwelling, both potential barriers to the northward spread of species). This hypothesized contrast in the biotic dynamics of western and eastern boundaries of ocean basins, with a stronger flow of species and clades out of the tropics in the W Atlantic, may account for the rapid recovery of diversity in that region, even as in situ diversification rates in the Caribbean left its diversity lagging behind that in the tropical E Pacific.…”

“…This hypothesized contrast in the biotic dynamics of western and eastern boundaries of ocean basins, with a stronger flow of species and clades out of the tropics in the W Atlantic, may account for the rapid recovery of diversity in that region, even as in situ diversification rates in the Caribbean left its diversity lagging behind that in the tropical E Pacific. The oceanographic contrast and its biotic consequences also imply that diversity will be higher, and will more readily rebound following extinction, along the temperate margins of the W Pacific than along the margins of the E Atlantic, a prediction supported by the high diversities recorded in southern Japan and southeast Australia (40).…”

“…A similar set of changes along the narrow Panamanian shelf would affect a far smaller area at a time, and could be ameliorated by latitudinal shifts in geographic ranges of species, as commonly found in the fossil records along the continuous Pacific coastal shelf (37). Thus, in contrast to analyses of terrestrial diversity patterns (38,39), present-day habitat area is a poor predictor of large-scale marine diversity patterns within major climate zones (40), and, equally important, the tropical faunas from which OTT taxa emerged into temperate zones each had distinctive histories.…”

Evolutionary dynamics of taxonomic structure

“…As a complement to these approaches, it is worth exploring whether evolutionary rates can be inferred from the size-frequency distribution of living genera [3].…”

“…Biologists have focused attention on differences in diversification between clades [1] and on temporal changes in diversification within clades [2]. Taxonomic structure-how species within a clade are distributed among genera or higher taxonomic unitshas not been as widely studied [3,4], yet it has real potential to reveal the dynamics of diversification [3]. Here, I will outline one way to infer rates of species extinction and genus origination, based on the proportion of living genera with one species, two species, etc., i.e.…”