|The effects of regional diversity on diversification remain
controversial. The classic hypothesis that diversification decelerates
as regional diversity increases has been recently revived. Yet,
there is little geographic evidence for slower diversification across regions
of high diversity, and diversity is often thought to promote diversification
through its effects on ecological divergence and speciation.
Here, we use the newest phylogeny for mammals (4,990 species) and
two different methods to test the effects of regional diversity on diversification.
We find that regions of high diversity are dominated by
expanding clades that are far from their estimated carrying capacities.
Regions of low diversity host clades that are small and mostly saturated.
These results were supported across mammals and their six largest
orders. They were corroborated by the two methods when controlling
for clade relatedness, clade nestedness, and clade size. Together, these
results reject the hypothesis that high geographic concentration ofmammals
effectively suppresses their further diversification. Instead, highly
diverse regions (especially the tropics) seem to act as the engine of