Data from: Unifying latitudinal gradients in range size and richness across marine and terrestrial systems

Many marine and terrestrial clades show similar latitudinal gradients in
species richness, but opposite gradients in range size—on land, ranges are
the smallest in the tropics, whereas in the sea, ranges are the largest in
the tropics. Therefore, richness gradients in marine and terrestrial
systems do not arise from a shared latitudinal arrangement of species
range sizes. Comparing terrestrial birds and marine bivalves, we find that
gradients in range size are concordant at the level of genera. Here, both
groups show a nested pattern in which narrow-ranging genera are confined
to the tropics and broad-ranging genera extend across much of the
gradient. We find that (i) genus range size and its variation with
latitude is closely associated with per-genus species richness and (ii)
broad-ranging genera contain more species both within and outside of the
tropics when compared with tropical- or temperate-only genera.
Within-genus species diversification thus promotes genus expansion to
novel latitudes. Despite underlying differences in the species range-size
gradients, species-rich genera are more likely to produce a descendant
that extends its range relative to the ancestor's range. These
results unify species richness gradients with those of genera, implying
that birds and bivalves share similar latitudinal dynamics in net species
diversification.