The monarch butterfly,Danaus plexippus,is famous for its spectacular annual migration and recent worldwide dispersal.However,we know little about the genetic basis of these hallmark traits.We uncover the history of the monarchs evolutionary origin and global dispersal,characterize the genes and pathways associated with migratory behavior by denovo assembling the monarch genome and resequencing 101 Danaus genomes from around the globe.The results rewrite our previous understanding,showing that D.plexippus was ancestrally migratory and dispersed out of North America to occupy its broad distribution.We find the strongest signatures of selection associated with migration are flight muscle function.