Rapid in situ diversification rates in Rhamnaceae explain the parallel evolution of high diversity in temperate biomes from global to local scales

The macroevolutionary processes that have shaped biodiversity across the
temperate realm remain poorly understood and may have resulted from
evolutionary dynamics related to diversification rates, dispersal rates,
and colonization times, closely coupled with Cenozoic climate change. We
integrated phylogenomic, environmental ordination, and macroevolutionary
analyses for the cosmopolitan angiosperm family Rhamnaceae to disentangle
the evolutionary processes that have contributed to high species diversity
within and across temperate biomes. Our results show independent
colonization of environmentally similar but geographically separated
temperate regions mainly during the Oligocene, consistent with the global
expansion of temperate biomes. High global, regional, and local temperate
diversity was the result of high in situ diversification rates,
rather than high immigration rates or accumulation time, except for
Southern China, which was colonized much earlier than other regions. The
relatively common lineage dispersals out of temperate hotspots highlights
strong source-sink dynamics across the cosmopolitan distribution of
Rhamnaceae. The proliferation of temperate environments since the
Oligocene may have provided the ecological opportunity for rapid in situ
diversification of Rhamnaceae across the temperate realm. Our study
illustrates the importance of high in situ diversification rates for the
establishment of modern temperate biomes and biodiversity hotspots across
spatial scales.