HYBRID ORIGINS OF PLANT-SPECIES

The origin of new homoploid species via hybridization is theoretically difficult because it requires the development of reproductive isolati on in sympatry. Nonetheless, this mode is often and carelessly used by botanists to account for the formation of species that are morphologi cally intermediate with respect to related congeners. Here, I review e xperimental, theoretical, and empirical studies of homoploid hybrid sp eciation to evaluate the feasibility, tempo, and frequency of this mod e. Theoretical models, simulation studies, and experimental syntheses of stabilized hybrid neospecies indicate that it is feasible, although evolutionary conditions are stringent. Hybrid speciation appears to b e promoted by rapid chromosomal evolution and the availability of a su itable hybrid habitat. A selfing breeding system may enhance establish ment of hybrid species, but this advantage appears to be counterbalanc ed by lower rates of natural hybridization among selfing taxa. Simulat ion studies and crossing experiments also suggest that hybrid speciati on can be rapid-a prediction confirmed by the congruence observed betw een the genomes of early generation hybrids and ancient hybrid species . The frequency of this mode is less clear. Only eight natural example s in plants have been rigorously documented, suggesting that it may be rare. However, hybridization rates are highest in small or peripheral populations, and hybridization may be important as a stimulus for the genetic or chromosomal reorganization envisioned in founder effect an d saltational models of speciation.