Reproductive isolation is the basis of the Bio logical Species Definition and can be a driving force of speciation. Theoretical studies have pro vided models of how reproductive isolation can arise within individual species. Genetic tests of these models are limited to populations in which reproductive isolation is present but not complete. Here, reproductive isolation in the Elgans-Group of the nematode genus Caenor habditis is reviewed. Pre-mating barriers, assor tative fertilization and post-zygotic barriers all have been observed in this clade. In some combinations of species, fertile F1 hybrids can be obtained. There fore, the Elegans-Group of Caenorhabditis is poised to become an important experimental system for the study of reproductive isolation.
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