Hendrickson, B.; Patterson, C. M.; King, N.; Doucet, B. I.; Hernandez, F.; Kooyers, N. J.

A key challenge in invasion biology is dissecting the roles of colonization history, admixture, and selection on invasion success. However, it is often difficult to determine the sources and locations of introduction events as well as the timing and magnitude of gene flow and selection using genomics datasets from contemporary samples. We disentangle the evolutionary processes acting during the North American invasion of *Trifolium repens* (white clover) using low-coverage whole genome sequencing from 513 herbarium specimens originally collected between 1838-2018. Introduction history paralleled European colonization patterns. The earliest collected samples had predominantly Belgian and British ancestry, with increasing French and Spanish contributions over time in the Mid-Atlantic and South. Genomic diversity increased significantly through time likely driven by admixture between independently introduced lineages. In lowest latitude regions, there is a strong shift in dominant ancestry with limited admixture (i.e., lineage turnover). Putatively adaptive loci exhibited protracted shifts in allele frequency during the invasion, consistent with contemporary clines. Our study demonstrates the power of herbarium genomics to provide spatiotemporal insight into recent evolutionary history. We find a complex invasion history that includes multiple introductions, lineage turnover, and admixture that increased genetic diversity and likely facilitated adaptation in this now cosmopolitan plant.