Accelerated speciation in temperate Cannabaceae: the role of pre‐adaptation, ecological opportunities and niche divergence

Aim: The macroevolutionary processes underlying biome transitions and the geographical distribution of species diversity and ecological traits remain poorly understood in most clades. Here, we study tropical-to-temperate transitions in Cannabaceae (the hemp family) and hypothesize that pre-adaptation (i.e., deciduous leaves) and ecological opportunities in temperate habitats have led to niche divergence and increased speciation rates.

Location: Global.

Time Period: Late Cretaceous–Cenozoic.

Major Taxa Studied: Cannabaceae.

Methods: We combined a well-resolved, dated phylogenetic tree of Cannabaceae (including about 70% of species diversity) with leaf duration type (deciduous/semi-deciduous/evergreen), occurrence and environmental data. We employed phylogenetic comparative methods to test for correlated evolution between deciduousness and tropical-to-temperate transitions, and to infer speciation and niche evolution rates in tropical, temperate and widespread lineages.

Results: Our results support a model of correlated evolution, illustrating that deciduous lineages exhibited a higher rate of transition from tropical to temperate regions than evergreen lineages. Furthermore, temperate lineages, particularly combined with a hidden state, experienced higher speciation rates than tropical lineages. Finally, we detected higher rates of niche divergence in temperate than in tropical lineages, especially along climatic axes related to temperature range and seasonal precipitation.

Main Conclusions: Our results suggest that deciduousness acted as a pre-adaptive trait that facilitated transitions from tropical to temperate environments in Cannabaceae. The expansion of the temperate biome since the Oligocene may have provided ecological opportunities for deciduous taxa, driving speciation via niche divergence along temperature or seasonal precipitation niche axes. Our study highlights multidimensional niche evolution as a key mechanism in shaping the evolutionary dynamics of temperate lineages.