Natural hybridization: How low can you go and still be important?

This paper examines two assumptions that have formed the basis for much of the past and present work on hybrid zones. These assumptions derive from th e observation that crosses between genetically divergent individuals (e. g. , from different subspecies, species, etc.) often give rise to genotypes th at are less fertile or less viable than those produced from crosses between genetically similar individuals. The first assumption is that natural hybr idization will not affect the evolutionary history of the hybridizing forms because there is a low probability of producing novel genotypes with highe r relative fitness. The second viewpoint is more extreme in that it assumes that all hybrid genotypes will be less fit. Even if rare gene flow does oc cur it will thus not contribute to patterns of diversification or adaptatio n because the hybrids will always be selected against. Examples from both p lant and animal hybridization are discussed that are not consistent with th ese assumptions. Numerous instances of natural hybridization are used to de monstrate that extremely low fertility or viability of early-generation hyb rids (e.g., F-1, F-2, B-1) does not necessarily prevent extensive gene flow and the establishment of new evolutionary lineages. In addition, it is dem onstrated that various hybrid genotypes have equivalent or higher fitness t han their parents in certain habitats.