Divergence and speciation as viewed from an insect hybrid zone

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Abstract
Hybridism is examined both from Darwin's point of view and in light of current views on the species concept. It seems that many of the relevant concepts of population genetics and speciation are inherent in primitive molecular evolution and may be useful in explaining divergent evolution from the earliest life forms. When divergent genomes meet they may hybridize and, in many cases, form a hybrid zone. Work on insect hybrid zones is used to exemplify the factors and processes involved. Many such zones are the result of postglacial secondary contact and involve some form of hybrid unfitness. They usually comprise multigenic differences and each gene may have different properties. Two grasshoppers, Podisma pedestris and Chorthippus parallelus, are used as case studies. In the case of P. pedestris, as a result of vegetation analysis, it is possible to be fairly sure that the zone is not environmentally determined but rather the result of heterozygous disadvantage at many loci. In the other species, in which two divergent mate recognition systems meet, there is no evidence of their reinforcement toward speciation, despite the F1 hybrids showing severe testis dysfunction and providing another example of Haldane's rule. A scenario for the postglacial contact and hybridization is proferred in an attempt to explain differences between disjunct sections of the zone. It is argued that gene flow occurs over only very small distances in hybrid zones and that the zones themselves will not move far because they fall in low density traps. Compound hybrid zones may be formed by cycles of range contraction and expansion and the substructure of a species will be determined by its life style. Hybrid zones provide a demonstration that divergence occurs in various characters at different rates and divergence brings with it hybrid disadvantage and frequency dependence. Speciation is seen as some way down this path, but occasional transfers of genetic material may still occur.

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