There has been considerable debate about the role of hybrids in the evolutionary process. One question has involved the relative fitness of hybrid versus non-hybrid genotypes. For some, the assumption of lower hybrid fitness continues to be integral to their concept of species and speciation. In contrast, numerous workers have suggested that hybrid genotypes might demonstrate higher relative fitness under various environmental settings. Of particular importance in deciding between these opposing hypotheses are long-term analyses coupling ecological and genetic information. Although currently rare, such analyses have provided a test of the fitness of hybrid genotypes across generations and habitats and their role in adaptation and speciation. Here we discuss examples of these analyses applied to viruses, prokaryotes, plants and Darwin's Finches.
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