Although hybridization and introgression have been considered important in generation of plant diversity, their role in evolutionary diversification of animals remains unclear. In this review, we reconsider the significance of introgressive hybridization in evolution and diversification of animals to determine if the generally negative assessment of these processes is warranted. Unlike the situation for plants, hybrid animal taxa appear to be relatively rare. This could, however, be due to negative attitudes toward hybridization and difficulty in detecting such forms. Hybridization has been responsible for instantaneous creation of several unique complexes of polyploid and unisexual animals. Allopolyploidy has allowed for diversification, whereas unisexual taxa have acted as conduits of gene exchange among related sexual species. Many instances of diploid, bisexual taxa of hybrid origin have been put forward, but few have been carefully tested. Changing attitudes toward hybrids and technological advances should allow for careful consideration of hypothesized hybrid taxa and will undoubtedly increase the number of known animal hybrid taxa.

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A diverse group of animals has adapted to caves and lost their eyes and pigmentation, but little is known about how these animals and their striking phenotypes have evolved. The teleost Astyanax mexicanus consists of an eyed epigean form (surface fish) and at least 29 different populations of eyeless hypogean forms (cavefish). Current alternative hypotheses suggest that adaptation to cave environments may have occurred either once or multiple times during the evolutionary history of this species. If the latter is true, the unique phenotypes of different cave-dwelling populations may result from convergence of form, and different genetic changes and developmental processes may have similar morphological consequences. Here we report an analysis of variation in the mitochondrial NADH dehydrogenase 2 (ND2) gene among different surface fish and cavefish populations. The results identify a minimum of two genetically distinctive cavefish lineages with similar eyeless phenotypes. The distinction between these divergent forms is supported by differences in the number of rib-bearing thoracic vertebrae in their axial skeletons. The geographic distribution of ND2 haplotypes is consistent with roles for multiple founder events and introgressive hybridization in the evolution of cave-related phenotypes. The existence of multiple genetic lineages makes A. mexicanus an excellent model to study convergence and the genes and developmental pathways involved in the evolution of the eye and pigment degeneration.

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