We combined molecular and morphological characters in a copepod taxon for which obtaining a sufficiently high number of characters that evolve at different rates is a challenge. Few molecular markers are known to resolve evolutionary relationships in the copepods, and thus there is potential for morphology to contribute substantially to phylogenetic reconstruction. We used a morphology based tree of the entire Mesocyclops genus to guide our taxon sampling of 10 species for molecular and combined analyses. Morphology including polymorphic characters, 18S rDNA, and ITS2 sequences were analyzed using parsimony, ML, and Bayesian methods. Strong similarities among topologies were observed regardless of the character type or algorithm, with higher levels of support obtained in combined data analyses. In combined analyses Old World species formed a monophyletic group and New World species formed a paraphyletic group in this freshwater, predominantly (sub)tropical genus. Mesocyclops darwini was the single taxon whose relationships showed conflict among the previous reconstructions using only morphological characters and the tree inferred from the combined data set. Support for these alternative positions of M. darwini were compared using constraint tests, with the result supporting monophyly of Old World taxa.
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