Why are Magnoliaceae so diverse in the tropics? Disentangling the roles of diversification and time-for-speciation effects

Abstract

Magnoliaceae, a typical boreotropical relict lineage, shows striking species richness in tropical regions, making it an important model for testing the time-for-speciation and diversification rate hypotheses for present-day diversity patterns. Here, we reconstructed a time-calibrated phylogeny using plastomes from 123 species, representing Liriodendron and all 15 sections of Magnolia, and investigated its colonization and diversification history. Our results reveal that Magnoliaceae likely experienced peak extinction during the mid-Eocene, accompanied by a range contraction from high latitudes to the amphi-Pacific tropics, followed by the rise of tropical clades with rapid diversification. Phylogenetic generalized least squares analysis demonstrates that diversification rate explains clade-level diversity variation more strongly than time for speciation. Tropical regions, such as South America and Southeast Asia, with high Magnoliaceae diversity consistently show elevated diversification rates and shorter time for speciation. These results indicate that higher diversification rate, rather than longer time for speciation, explains the high diversity of Magnoliaceae in tropical clades and regions. Our findings not only shed light on the evolutionary history of Magnoliaceae but also provide important insights into the broader processes that shape tropical biodiversity.

Keywords

diversification rates / diversity patterns / extinction / historical biogeography / magnoliaceae / time for speciation

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2025 Institute of Botany, Chinese Academy of Sciences.