NATURAL HYBRIDIZATION: HOW LOW CAN YOU GO AND STILL BE IMPORTANT?

Abstract

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 the observation that crosses between genetically divergent individuals (e.g., from different subspecies, species, etc.) often give rise to genotypes that are less fertile or less viable than those produced from crosses between genetically similar individuals. The first assumption is that natural hybridization will not affect the evolutionary history of the hybridizing forms because there is a low probability of producing novel genotypes with higher 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 occur it will thus not contribute to patterns of diver- sification or adaptation because the hybrids will always be selected against. Examples from both plant and animal hybridization are discussed that are not consistent with these as- sumptions. Numerous instances of natural hybridization are used to demonstrate that ex- tremely low fertility or viability of early-generation hybrids (e.g., F1, F2, B1) does not necessarily prevent extensive gene flow and the establishment of new evolutionary lineages. In addition, it is demonstrated that various hybrid genotypes have equivalent or higher fitness than their parents in certain habitats.