Abstract: Species are distributed heterogeneously, and different regions have different numbers of species, producing species richness anomalies. More than 100 angiosperm genera demonstrate disjunct distributions in at least two of these regions: Europe, eastern North America, western North America, and East Asia, and commonly between East Asia and eastern North America. These regions have similar climates but usually exhibit species richness anomalies; however, the underlying causes of species richness anomalies in disjunct intercontinental regions remain poorly understood. In this study, we investigated the drivers of anomalies in ash tree (genus Fraxinus L.) species richness anomalies among disjunct intercontinental regions based on distribution data, macrofossil records, and corresponding evolutionary and environmental variables. Generalized linear regression and pathway model analyses incorporating environmental and evolutionary processes indicated that global cooling has contributed to the low species richness in Europe, whereas evolutionary divergence, shaped the distinct species distribution patterns in East Asia (which was identified as an evolutionary cradle) and North America (which was identified as an evolutionary museum). Environmental heterogenies and evolutionary divergence have resulted in a significant diversity anomaly between these regions. This study emphasizes the important role of evolution in the formation of species richness distribution patterns.

Key words: ash species, evolutionary process, intercontinental disjunct regions, species richness anomaly.