The likelihood of homoploid hybrid speciation

New species may be formed through hybridization and without an increase in ploidy. The challenge is for hybrid derivatives to escape the homogenizing effects of gene flow from parental species. The mechanisms hypothesized to underlie this process were modelled using a computer simulation. The model is of recombinational speciation, in which chromosomal rearrangements betwe en parental species result in poor fertility of F-1 hybrids, but through re combination, novel homozygous types are formed that have restored fertility . In simulations, stable populations bearing the recombinant karyotypes ori ginated frequently and were maintained when the fertility of F-1 hybrids wa s high. However, this high rate of origination was offset by low genetic is olation, and lower F-1 hybrid fertility increased the evolutionary independ ence of derived populations. In addition, simulations showed that ecologica l and spatial isolation were required to achieve substantial reproductive i solation of incipient species. In the model, the opportunity for ecological isolation arose as a result of adaptation to extreme habitats not occupied by parental species, and any form of spatial isolation (e.g. founder event s) contributed to genetic isolation. Our results confirmed the importance o f the combination of factors that had been emphasized in verbal models and illustrate the trade-off between the frequency at which hybrid species aris e and the genetic integrity of incipient species.