Niche evolution and asymmetric diversification in elm species (Ulmaceae)

Global patterns of species diversity are highly heterogeneous, and even within the same evolutionary lineage, species richness can differ markedly. Understanding the mechanisms underlying such disparities remains a central question in evolutionary biology and biogeography. Here, we investigate the evolutionary drivers of asymmetric diversity and distribution patterns in Ulmaceae by integrating molecular phylogenetics, fossil evidence, climatic niche analyses, and biogeographic reconstructions. We reconstructed a time-calibrated, species-level phylogeny encompassing 86.21% of extant Ulmaceae species and incorporating 61 fossil taxa. Phylogenetic analyses supported the division of Ulmaceae into a temperate clade and a tropical clade, which differ substantially in species richness, climatic niche characteristics, and geographic distributions. The temperate clade exhibited strong climatic niche conservatism combined with a broader niche breadth, enabling repeated intercontinental dispersal and geographic expansion across the Northern Hemisphere since the Late Cretaceous. In contrast, the tropical clade shows more constrained niche evolution and limited dispersal capacity, remaining largely restricted to low-latitude tropical regions and exhibiting lower species richness. Fossil-integrated analyses suggest that Ulmaceae originated in northern temperate regions during the Late Cretaceous and experienced extensive intercontinental exchanges during the Paleogene, particularly via the Bering and North Atlantic land bridges. Diversification-rate analyses indicated that net diversification rates in both clades remained relatively stable through time. This pattern suggested that long-term differences in species richness were better explained by early climatic niche divergence and contrasting biogeographic histories, rather than by temporal shifts in diversification rates. Our study highlights the critical role of fossil integration in revealing deep-time evolutionary processes that shape modern plant diversity under long-term climatic change, and provides a framework applicable to many other extant lineages with similar distribution patterns.