Environmental filtering, not dispersal history, explains global patterns of phylogenetic turnover in seed plants at deep evolutionary timescales.

Environmental filtering and dispersal history limit plant distributions and affect biogeographical patterns, but how their relative importance varies across evolutionary timescales is unresolved. Phylogenetic beta diversity quantifies dissimilarity in evolutionary relatedness among assemblages and might help resolve the ecological and biogeographical mechanisms structuring biodiversity. Here, we examined the effects of environmental dissimilarity and geographical distance on phylogenetic and taxonomic turnover for ~270,000 seed plant species globally and across evolutionary timescales. We calculated past and present dispersal barriers using palaeogeographical reconstructions and calculated geographical linear and least-cost distances, accounting for dispersal over water, mountains or areas with unsuitable climates. Environmental dissimilarity and geographical distance jointly explained most of the deviance in taxonomic (up to 86.4%) and phylogenetic turnover (65.6%). While environmental dissimilarity consistently showed strongly positive effects, the effect of geographical distance on phylogenetic turnover was less pronounced further back in evolutionary time. Past physiogeographical barriers explained a relatively low amount of the variation across all timescales, with a slight peak at intermediate timescales (20-50 Myr BP). Our results suggest that while old lineages have generally dispersed widely, the imprint of environmental filtering on range expansion persists, providing insights into biogeographical and evolutionary processes underlying global biodiversity patterns.