Natural hybridisation and fitness

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Summary: Arnold, M. L., Kentner, E. K., Johnston, J. A., Cornman, S. & Bouck, A. C.: Natural hybridisation and fitness. – Taxon 50: 93–104. 2001. – ISSN 0040‐0262.There are several inferences that can be made from studies of the fitness of hybrid plants and animals. First, the fitness of hybrids varies. However, it is not possible to make a priori predictions concerning the relative fitness of a given hybrid genotype, or a series of genotypes. For example, plant hybrids are not more likely to have elevated fitness than animal hybrids. The variation in fitness for hybrids ranges from the highest fitness relative to parental genotypes to the lowest. The variation can be due to different environments or age classes. Furthermore, notwithstanding the paradigm that derived from the Neo‐Darwinian synthesis, some hybrid genotypes demonstrate elevated fitness relative to their parents.To test the evolutionary importance of hybridisation in a given species complex, it is critical that findings from greenhouse/population cage experiments be tested in the field. This latter statement is easier for a botanist to argue, but even with plants, the field experimentation needed for a rigorous test of fitness (i.e., reciprocal transplants) is at the best risky, and costly in time and resources. Yet, these are the types of studies needed to test predictions concerning the likeliest circumstances under which hybridisation will be promoted.The scientific literature over the past decade has seen a change in the tenor of papers describing natural hybridisation. Ten years ago, papers more often than not reported the use of natural hybridisation as a tool to understand divergent evolution. More recently, a majority of studies have discussed the evolutionary impact of natural hybridisation. We look forward to a continued increase in the frequency of studies that assume this process to have evolutionary importance per se.