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| 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. | |
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