Evolutionary outcomes of natural hybridization

Three main patterns of natural hybridization can be recognized: (1) occasional formation of hybrids (generally F1); (2) more or less stable hybrid zones of different width and shape, with differential penetration of ‘foreign’ alleles in the two parental genomes; (3) the onset of hybrid species, that combine the genomes of two parental species. A number of study cases are examined in plants and animals. Different evolutionary outcomes of hybridization are considered, including: breakdown of gene coadaptation in hybrid genomes; reinforcement of reproductive isolation by natural selection; introgressive evolution of populations; long-term evolution of hybrid species; reticulate evolution. The widespread occurrence of hybrid species in plants (e.g. about 50% of flowering plants) appears to be related to the general coexistence of sexual and vegetative reproduction. Asexual reproduction in hybrid plants can be considered as a ‘stand-by’ phase, until sexual reproduction is restored by chromosome doubling. Also in animals, hybrid species are common in those groups where asexual reproduction is present, allowing the transmission of the hybrid genome over generations; however, no cases of return to sexuality are so far known. In most animal groups, sexual reproduction is obligatory and true hybrid species are not found. On the contrary, stabilized recombinants by introgressive hybridization can be present and even successful, due to their enhanced genetic variability and heterosis.