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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Analysis of mtDNA sequence variation (2,548 bp from ND2, cytb, and part of the control region) indicates that the genus Cyprinodon began diverging in the Late Miocene from a common ancestor with Megupsilon, a monotypic genus on the Mesa del Norte of Mexico. The geographic pattern of mtDNA variation, with estimates of divergence time, suggests that by the end of the Miocene Cyprinodon occurred from the Atlantic Coast and West Indies to near the western margin of North America via ancestral Rio Grande and Colorado River systems. Phylogeographic structure within the major mtDNA complexes supports a variety of hypotheses from geology and previous phylogenetic analyses for Late Neogene connections among basins in southwestern North America now separated by formidable barriers to dispersal. Comparison of the mtDNA tree with previous phylogenetic inferences from allozymes indicates that reticulate evolution involving divergent lineages probably was important in the history of Cyprinodon.El análisis de variació n de secuencias de ADNmt (ND2, cytb, y parte de la región reguladora; 2,548 pb) indica que el género Cyprinodon empezó a divergir en el mioceno tardío de un antepasado comú n con Megupsilon, un género monotípico de la Mesa del Norte de México. El patró n geográfico de la variació n de ADNmt, con estimadas de los tiempos divergencias, sugiere que al llegar al fin del mioceno Cyprinodon ocurrió desde la costa Atlántica y las Antillas hasta casi el margen oeste de Norteamérica por los sistemas pluviales antiguos del río Bravo y del río Colorado. La estructura filogeográfica entre los grupos principales de ADNmt apoya a una variedad de hipó tesis de la geología y de los análisis filogenéticos anteriores para conexiones del neogeno tardío entre cuencas del suroeste de Norteamérica que ahora están separadas por barreras imponentes contra la dispersió n. La comparació n del árbol de ADNmt con deducciones filogenéticos anteriores de alozimas indica que la evolució n reticulada que incluye líneas divergentes probablemente fue importante en la historia del Cyprinodon.

Morphological and genetic characters from cyprinid fishes of the genus Gila were examined to assess a hypothesized hybrid origin of Gila seminuda from the Virgin River, Arizona-Nevada-Utah. The presumed parents, Gia robusta robusta and Gba elegans, are clearly differentiated from one another based on morphology, allozymes, and mtDNA haplotypes. G. seminuda is morphologically intermediate and polymorphic at allozyme loci diagnic for the parental species. Restriction endonuclease analysis of mtDNA showed G. seminuda nearly identical to G. elegans. These