Monophyly and geography of the Río Pánuco Basin swordtails (Genus Xiphophorus) with descriptions of four new species

" Four new species ofthe genus Xiphophorus (Cyprinodontiformes: Poeciliidae) from the Rio Pcanuco basin ofeastern Mexico are described. They, together with the four previously described swordtail species of this area and one recently described subspecies (herein treated as a full species), form a monophyletic group that is most closely related to one of the southern swordtails, X. clemenciae. The monophyly of the Rio Panuco (northern) swordtails is supported by four morphological and five electrophoretic characters. Relationships within the group are examined by means of parsimony analysis of morphological and electrophoretic characters. Three clades are described: (1) a montezumae clade with X. montezumae and X. nezahualcoyotl n. sp. as sister species, and X. continens n. sp. related to that pair; (2) a cortezi clade with X. malinche n. sp. and X. birchmanni as sister species, and X. cortezi related to that pair; and (3) a pygmaeus clade, with X. nigrensis and X. multilineatus n. sp. as sister species, and X. pygmaeus related to that pair. Among the three clades, the cortezi and pygmaeus groups are found to be most closely related.

The geographic distribution of this group is examined in detail. Sister taxa are allopatric with respect to each other with the exception of sympatry between X. cortezi and X. birchmanni and between X. birchmanni and X. malinche. This situation is discussed, and the relationship of the phylogenetic branching pattern to current distribution is explored for possible correlations with the geological history ofthe region. Both east-west and north-south dichotomies are recognized and related to uplift in particular regions of the Sierra Madre Oriental and to the response of the major rivers to these uplifts "