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 el ectrophoretically for allozyme variation. Phylogenetic relationships were i nferred using parsimony and likelihood analysis of gene frequency character s, as well as Fitch-Margoliash, minimum evolution and neighbor-joining anal yses of genetic distances. The phylogenetic relationships among species tha t were well supported in all analyses included (1) monophyly of the norther n swordtails, (2) the pygmaeus clade of Xiphophorus nigrensis, X. multiline atus, and X. pygmaeus, and (3) the dade of X. nigrensis and X. multilineatu s. Of those species represented by more than one population, all analyses s upported monophyly of X. montezumae and weakly supported monophyly of X. ne zahualcoyotl and X, birchmanni. only the distance analyses supported monoph yly 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 published trees bas ed on morphology, behavior and randomly amplified DNAs were generally congr uent 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 da ta.