[THE ROLE OF CONTACT HYBRIDIZATION AS A BARRIER TO GENE FLOW BETWEEN PARAPATRIC POPULATIONS]
Bush (1975) estimates that perhaps one half of all animal species have been formed by processes that do not fit the classical allopatric speciation model. And it is clear from this and other comparative data that parapatric or chromosomal speciation models potentially do fit many of these non-allopatric cases. If the chromosomal speciation and contact hybridization models presented above are valid for a significant proportion of this non-allopatric speciation, then it is likely that the evolutionary patterns and successes of lineages which have proliferated chromosomally have been profoundly influenced by this speciation. Clearly the models [presented in this paper] deserve especially rigorous testing, both through mathematical simulation to verify the logic and to set numerical limits on their parameters, and through detailed comparative studies of the population cytogenetics and evolution of additional natural groups of species to test the many phylogenetic predictions made by these models.