Evolutionary legacies structure the geography of seagrass traits across the world's oceans

Summary Traits modulate species' ability to track shifts in climate, yet the extent to which traits have been shaped by the contemporary environment and/or historical processes remains poorly understood. Here, we fill this gap for the world's seagrasses, habitat‐forming species that provide critical ecosystem services. We compiled information on geographic ranges, traits, and a time‐calibrated phylogeny for 57 extant seagrasses, spanning > 100° of latitude. We explored variation in seagrasses trait compositions and richness across marine realms, ecoregions, and latitude, and then used additive partitioning of beta diversity to decouple the role of evolutionary and taxonomic relatedness from that of environmental similarity in explaining trait similarity among marine ecoregions globally. Across tropical and temperate marine realms, seagrasses retain a core set of traits, despite a nearly 4‐fold difference in species richness. Temperate species occupied larger volumes of the trait space, while tropical seagrasses occupied lower volumes. Latitude alone was a poor predictor of seagrasses' trait compositions, which were primarily explained by the degree of evolutionary relatedness, while contemporary environmental variation contributed negligibly. By decomposing patterns of alpha and beta diversity, we highlight the overarching importance of historical events and shared ancestry in structuring the geography of seagrass traits across the world's oceans.