Types of hybrid zones

Three types of hybrid zones may be recognized from their origin and evolutionary significance: Mayr's zones of primary intergradation between geographical races; secondary overlap zones following a more or less prolonged isolation; and narrow overlappings of parapatric populations differing in their karyotype. This third type may be the expression of stasipatric or of invasive patterns of chromosomal speciation. For some mammals and grasshoppers with parapatric hybrid zones it has been possible to identify the number and kind of chromosomal rearrangements involved and to estimate the time of fission between incipient species and the geographical pattern of speciation. The narrowness of the parapatric hybrid zones is due to the low vagility of the organisms concerned and/or to the lowered viability or fertility of the hybrids. The future of the hybrid zones seems to be quite undetermined and possibly very different in the different instances.