selective pressure to prevent crosses between incipient species (Le Gac and Giraud, 2008), whereas when species live in sympatry there may be selection to avoid the cost of producing hybrids that would likely be unviable or sterile (i.e. postmating barriers as described below). In the saprophyte mushroom Amylostereum for instance, premating barriers are always complete between species from the same continent but species from different continents can be crossed experimentally (Boidin and Lanquetin, 1984). These examples