The Genus Xiphophorus in Mexico and Central America

" The genus Xiphophorus is found from northeastern Mexico (Coahuila) for about 2200 Km as far as Honduras. There are 26 species, of which 21 occupy headwaters on the eastern slope of the Sierra Madre Oriental and continuing Cordillera to the southeast. Virtually ail the species in the headwaters occupy limited ranges, often in rivers traversing karst country that are separated from lowland streams by underground passages. Only the three forms in the coastal plain are more widely distributed. Nineteen taxa occur within 400 Km of the Mexican Trans Volcanic Belt, suggesting that the genus may have evolved in this region. In many localities two species are sympatric, but natural hybrids are only known from three or four sites. Four monophyletic groups have been identified: the northern platyfish and the northern swordtail groups, north of the Mexican Trans Volcanic Axis, and to the south the helleri and the clemenciae swordtail groups. The status of the three southern platyfish is still not resolved and the phylogenetic relationship of the different groups to each other is still not fully understood "