Phylogenetic Relationships among Populations of Northern Swordtails (Xiphophorus) as Inferred from Allozyme Data

Twenty-nine populations of Xiphophorus fishes representing nine species of northern swordtails, one southern swordtail and a platyfish were assayed electrophoretically for allozyme variation. Phylogenetic relationships were inferred using parsimony and likeUhood analysis of gene frequency characters, as well as Fitch-Margoliash, minimum evolution and neighbor-joining analyses of genetic distances. The phylogenetic relationships among species that were well supported in all analyses included (1) monophyly of the northern swordtaUs, (2) the pygmaeus clade of Xiphophorus nigrensis, X. multilineatus, and X. pygmaeus, and (3) the clade of X. nigrensis and X. multilineatus. Of those species represented by more than one population, all analyses supported monophyly of X montezumae and weakly supported monophyly of X. nezahualcoyotl and X. hirchmanni. Only the distance analyses supported monophyly of X cortezi, and the support was weak. Finally, all analyses supported a clade including X nezahualcoyotl from the Rio Tamesi drainage and some populations from the Rio Panuco drainage, that is, nonmonophyly of the set of populations from the Rio Panuco drainage. Previously pubUshed trees based on morphology, behavior and randomly amplified DNAs were generally congruent with the optimal trees for the allozyme data and were not rejected by those data; in contrast, trees based on DNA sequences were more incongruent with the optimal trees for the allozyme data and were rejected by those data.