show Abstracthide AbstractMarine phytoplankton play an important ecological role underpinning food webs in the ocean and are responsible for about half of global primary productivity. Yet, however, little is known about how marine plankton species form. In particular, it is unclear how new plankton species originate and evolve in relatively homogenous habitats, such as in the open ocean, where no physical barriers to gene flow exist to promote allopatric speciation, which is the most common speciation scenario in terrestrial organisms. Also the unexpected diversity of plankton species have been analysed in multiple studies, but still remain obscure in terms of evolutionary genetic processes.