Evolution and diversification within the intertidal brown macroalgae Fucus spiralis/F. vesiculosus species complex in the North Atlantic

► Three distinct genetic entities of F. spiralis were recovered. ► Mid- Fsp-Low and high-intertidal Fsp-High co-occurred with F. vesiculosus. ► Fsp-South allopatric in the south and did not co-occur with F. vesiculosus. ► Fsp-South hybridized with F. vesiculosus in sympatry to produce Fsp-Low. ► Fsp-High throughout N Atlantic stem from a glacial refugium near Brittany. ► F. vesiculosus from NE to W Atlantic stem from glacial refugium in SW Ireland.