An interesting and different type of reproductive isolation is when a new species originates as the result of hybridization between two extant species. This phenomenon is not uncommon in fungi (Olson and Stenlid, 2002). A remarkable example is that of the rust fungus Melampsora × columbiana that emerged from hybridization of M. medusa, a parasite of the poplar tree Populus deltoides, and M. occidentalis’ parasite of the tree P. trichocarpa (Newcombe et al., 2000). This hybrid emerged in 1997 when a poplar hybrid resistant to the two parental rust species was widely grown in California, and the hybrid rust fungus was able to infect the newly cultivated hybrid poplar. Several other cases have been reported of hybrid speciation in